Showing 1–50 of 115 results for author: Le, D

Machine learned potential for defected single layer hexagonal boron nitride

Authors: John Janisch, Duy Le, Talat S. Rahman

Abstract: Development of machine learned interatomic potentials (MLIP) is critical for performing reliable simulations of materials at length and time scales that are comparable to those in the laboratory. We present here a MLIP suitable for simulations of the temperature dependent structure and dynamics of single layer hexagonal boron nitride (h-BN) with defects and grain boundaries, developed using a stri… ▽ More Development of machine learned interatomic potentials (MLIP) is critical for performing reliable simulations of materials at length and time scales that are comparable to those in the laboratory. We present here a MLIP suitable for simulations of the temperature dependent structure and dynamics of single layer hexagonal boron nitride (h-BN) with defects and grain boundaries, developed using a strictly local equivariant deep neural network as formulated in the Allegro code. The training dataset consisted of about 30,000 images of h-BN with and without point defects generated with ab-initio molecular dynamics simulations, based on density functional theory (DFT), at 500, 1000, and 1500K. The developed MLIP predicts potential energies and forces with a mean absolute error (MAE) of 4 meV/atom and 60 meV/Angstrom , respectively. It also reproduces phonon dispersion curves and density of vibrational states of pristine bulk h-BN that are comparable with that obtained from density functional theory-based calculations. Molecular dynamics simulations of the motion of the 4|8 grain boundary unit in h-BN shows the first step to have an activation barrier ~2.2 eV, indicating immobility of the grain boundary. Moving the grain boundary units past the first shows much lower activation barriers of ~0.42eV, suggesting a facile motion of the grain boundary once the first movement is stimulated. These simulations yield a scaled mobility of 1.739*10^(-11) m^3/Js for a temperature of 1500K which, given the inherent differences in the set-ups, is not too far from the experimental value of 1.36*10^(-9) m^3/Js. The ability to predict grain boundary mobility within reasonable agreement with experiment demonstrates the robustness of the MLIP and its suitability for reliable simulations of defect structures and dynamics in single layer h-BN. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Comments: 24 Pages, 11 Figures, 1 Table. Submitted to The Journal of Chemical Physics

Phononic Casimir Effect in Planar Materials

Authors: Pablo Rodriguez-Lopez, Dai-Nam Le, Lilia M. Woods

Abstract: The Phononic Casimir effect between planar objects is investigated by deriving a formalism from the quantum partition function of the system following multiscattering approach. This fluctuation-induced coupling is mediated by phonons modeled as an effective elastic medium. We find that excitations with three types of polarizations arise from the resolved boundary conditions, however the coupling i… ▽ More The Phononic Casimir effect between planar objects is investigated by deriving a formalism from the quantum partition function of the system following multiscattering approach. This fluctuation-induced coupling is mediated by phonons modeled as an effective elastic medium. We find that excitations with three types of polarizations arise from the resolved boundary conditions, however the coupling is dominated by only one of these degrees of freedom due to exponential suppression effects in the other two. The obtained scaling laws and dependence on materials properties and temperature suggest effective pathways of interaction control. Scenarios of materials combinations are envisioned where the Phononic Casimir effect is of similar order as the standard Casimir interaction mediated by electromagnetic fluctuations. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 15 pages, 5 figures

Magnetic and phononic dynamics in the two-ladder quantum magnet (C5H9NH3)2CuBr4

Authors: J. Philippe, F. Elson, T. Arh, S. Sanz, M. Metzelaars, D. W. Tam, O. K. Forslund, O. Shliakhtun, C. Jiang, J. Lass, M. D. Le, J. Ollivier, P. Bouillot, T. Giamarchi, M. Bartkowiak, D. G. Mazzone, P. Kögerler, M. Månsson, A. M. Läuchli, Y. Sassa, M. Janoschek, B. Normand, G. Simutis

Abstract: In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Can such an interplay occur in a structurally soft system with comparable elastic and magnetic energy scales? Here we study the metal-organic material (C5H9NH3)2CuBr… ▽ More In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Can such an interplay occur in a structurally soft system with comparable elastic and magnetic energy scales? Here we study the metal-organic material (C5H9NH3)2CuBr4 (Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single-crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, $Δ$ b = 0.41 meV and $Δ$ a = 0.55 meV, which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains a highly localized mode at an anomalously low-energy around 2 meV. This characteristic frequency drops by approximately 5 percent as magnetic correlations become established with decreasing temperature, and we connect this behavior with the location and structure of the cyclopentylammonium rings. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Spectroscopic Determination of Site-Selective Ligand Binding on Single Anisotropic Nanocrystals

Authors: Dong Le, Wade Shipley, Alexandria Do, Liya Bi, Yufei Wang, Krista P. Balto, Rourav Basak, Hans A. Bechtel, Stephanie N. Gilbert Corder, Ilya Mazalov, Tesa Manto, Reno Sammons, Yutong She, Fiona Liang, Ganesh Raghavendran, Joshua S. Figueroa, Shaowei Li, Tod A. Pascal, Andrea R. Tao, Alex Frano

Abstract: Organic surface ligands are integral components of nanocrystals and nanoparticles that have a strong influence on their physicochemical properties, their interaction with the environment, and their ability to self-assemble and order into higher-order structures. These hybrid nanomaterials are tunable with applications in catalysis, directed self-assembly, next-generation optoelectronics, and chemi… ▽ More Organic surface ligands are integral components of nanocrystals and nanoparticles that have a strong influence on their physicochemical properties, their interaction with the environment, and their ability to self-assemble and order into higher-order structures. These hybrid nanomaterials are tunable with applications in catalysis, directed self-assembly, next-generation optoelectronics, and chemical and quantum sensing. Critically, future advances depend on our ability to rationally engineer their surface chemistry. However, fundamental knowledge of ligand-nanoparticle behavior is limited by uncertainty in where and how these ligands bind to surfaces. For nanoparticles, in particular, few characterization techniques offer both the high spatial resolution and the precise chemical mapping needed to identify specific ligand binding sites. In this study, we utilized synchrotron infrared nanospectroscopy (SINS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM) together with first-principles computer simulations to validate the site-selective adsorption of organic ligands on a shaped nanocrystal surface. Specifically, we demonstrate that the sterically encumbered isocyanide ligands (CNAr^{Mes2}) preferentially bind to the high curvature features of Ag nanocubes (NCs), where low-coordinate Ag atoms are present. In contrast, isocyanide ligands that do not exhibit these steric properties show no surface selectivity. SINS serves as an effective tool to validate these surface binding interactions at the near-single molecule level. These results indicate that steric effects can be successfully harnessed to design bespoke organic ligands for fine-tuning nanocrystal surface chemistry and the properties of the nanocrystal ligand shell. △ Less

Submitted 14 October, 2025; originally announced October 2025.

Comments: 12 pages, 4 figures

The superconducting grid-states qubit

Authors: Long B. Nguyen, Hyunseong Kim, Dat T. Le, Thomas Ersevim, Sai P. Chitta, Trevor Chistolini, Christian Jünger, W. Clarke Smith, T. M. Stace, Jens Koch, David I. Santiago, Irfan Siddiqi

Abstract: Decoherence errors arising from noisy environments remain a central obstacle to progress in quantum computation and information processing. Quantum error correction (QEC) based on the Gottesman-Kitaev-Preskill (GKP) protocol offers a powerful strategy to overcome this challenge, with successful demonstrations in trapped ions, superconducting circuits, and photonics. Beyond active QEC, a compelling… ▽ More Decoherence errors arising from noisy environments remain a central obstacle to progress in quantum computation and information processing. Quantum error correction (QEC) based on the Gottesman-Kitaev-Preskill (GKP) protocol offers a powerful strategy to overcome this challenge, with successful demonstrations in trapped ions, superconducting circuits, and photonics. Beyond active QEC, a compelling alternative is to engineer Hamiltonians that intrinsically enforce stabilizers, offering passive protection akin to topological models. Inspired by the GKP encoding scheme, we implement a superconducting qubit whose eigenstates form protected grid states - long envisioned but not previously realized - by integrating an effective Cooper-quartet junction with a quantum phase-slip element embedded in a high-impedance circuit. Spectroscopic measurements reveal pairs of degenerate states separated by large energy gaps, in excellent agreement with theoretical predictions. Remarkably, our observations indicate that the circuit tolerates small disorders and gains robustness against environmental noise as its parameters approach the ideal regime, establishing a new framework for exploring superconducting hardware. These findings also showcase the versatility of the superconducting circuit toolbox, setting the stage for future exploration of advanced solid-state devices with emergent properties. △ Less

Submitted 18 September, 2025; originally announced September 2025.

Comments: 30 pages, 22 figures

TbPt6Al3: A rare-earth-based g-wave altermagnet with a honeycomb structure

Authors: R. Oishi, T. Taniguchi, D. T. Adroja, M. D. Le, M. Aouane, T. Onimaru, K. Umeo, I. Ishii, T. Takabatake

Abstract: The magnetic properties of the Tb-honeycomb lattice compound TbPt6Al3, which crystallizes in the NdPt6Al3-type trigonal structure, have been studied by the measurements of electrical resistivity, magnetization M(T, B), and specific heat on single-crystalline samples. The magnetic susceptibility, M(T)/B, for B || c = 0.1 T shows a cusp at TN = 3.5 K, which temperature decreases with increasing the… ▽ More The magnetic properties of the Tb-honeycomb lattice compound TbPt6Al3, which crystallizes in the NdPt6Al3-type trigonal structure, have been studied by the measurements of electrical resistivity, magnetization M(T, B), and specific heat on single-crystalline samples. The magnetic susceptibility, M(T)/B, for B || c = 0.1 T shows a cusp at TN = 3.5 K, which temperature decreases with increasing the magnitude of B || c, while M(T)/B for B || a = 0.1 T remains constant with decreasing temperature below TN. This anisotropic behavior suggests a collinear antiferromagnetic (AFM) order of the Tb3+ moments pointing along the c axis. The data of M(T)/B for T > 10 K on the single crystal and that of inelastic neutron scattering from powdered samples have been simultaneously analyzed using the crystal field model. The analysis reveals the non-Kramers doublet ground state for the Tb3+ ion under the trigonal crystal field. The neutron powder diffraction measurement shows that the collinear AFM structure with a magnetic propagation vector k = [0, 0, 0] is associated with moments of 5.1 μB/Tb pointing along the c axis. Comparison of the magnetic point group with the nontrivial spin Laue group indicates that TbPt6Al3 is classified into bulk g-wave altermagnets. △ Less

Submitted 11 September, 2025; originally announced September 2025.

Comments: 17 pages, 10 figures

Single-ion anisotropy driven chiral magnetic order in a spin-1 antiferromagnetic chain

Authors: S. Vaidya, S. P. M. Curley, P. Manuel, J. Ross Stewart, M. Duc Le, A. Hernández-Melián, T. J. Hicken, C. Wang, H. Luetkens, J. Krieger, S. J. Blundell, T. Lancaster, K. A. Wheeler, D. Y. Villa, Z. E. Manson, J. A. Villa, J. L. Manson, J. Singleton, R. D. Johnson, P. A. Goddard

Abstract: Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in $S=1$ chains remains largely unexplored. Here, we present a comprehensive experimental study of a chiral antiferromagnetic (AFM) $S=1$ chain, [Ni(pym)(H$_{2}$O)$_{4}$]SO$_{4} \cdot$ H$_{2}$O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a four-fold chiral periodicity. Muon spin… ▽ More Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in $S=1$ chains remains largely unexplored. Here, we present a comprehensive experimental study of a chiral antiferromagnetic (AFM) $S=1$ chain, [Ni(pym)(H$_{2}$O)$_{4}$]SO$_{4} \cdot$ H$_{2}$O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a four-fold chiral periodicity. Muon spin rotation measurements indicate the onset of long-range magnetic order below $T_{\rm N} = 1.82(2)\,\mathrm{K}$. Neutron diffraction measurements reveal a chiral AFM order driven by a chiral modulation of the easy-axis anisotropy direction, rather than the typical scenario of Dzyaloshinskii-Moriya interactions, geometrical frustration or higher-order interactions. Inelastic neutron scattering (INS) measurements reveal dispersive spin-wave excitations well described by linear spin-wave theory, with Hamiltonian parameters $J_{0} = 6.81(1)\,\mathrm{K}$ (intrachain exchange), $J'_{1\rm a} = -0.091(1)\,\mathrm{K}$ (interchain exchange), and $D = -3.02(1)\,\mathrm{K}$ (easy-axis single-ion anisotropy). These parameters are further validated by Monte Carlo simulations of the magnetisation. Additionally, the INS data reveal multiple dispersionless bands, suggesting the presence of further excitations beyond the scope of our linear spin-wave theory. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Comments: 9 pages, 8 figures

Crystalline electric field excitations in Weyl semimetal \textit{R}AlSi (\textit{R} = Ce, Pr and Nd)

Authors: Lin Yang, Yili Sun, Xiutong Deng, Weizheng Cao, Xiaoyan Ma, Yinguo Xiao, Zhentao Wang, Ze Hu, Xiaowen Hao, Yuan Yuan, Zecong Qin, Wei Luo, Qingyong Ren, Xin Tong, Mohamed Aouane, Manh Duc Le, Youguo Shi, Yanpeng Qi, Devashibhai Adroja, Huiqian Luo

Abstract: The rare earth intermetallic system \textit{R}Al\textit{X} (\textit{R} = rare earth elements, \textit{X} = Si and Ge) is known to be a promising candidate of magnetic Weyl semimetal. Due to the complex interactions between the rare earth elements and surrounding atoms, as well as hybridization with itinerant electrons, this family likely possesses highly intriguing and novel magnetic structures an… ▽ More The rare earth intermetallic system \textit{R}Al\textit{X} (\textit{R} = rare earth elements, \textit{X} = Si and Ge) is known to be a promising candidate of magnetic Weyl semimetal. Due to the complex interactions between the rare earth elements and surrounding atoms, as well as hybridization with itinerant electrons, this family likely possesses highly intriguing and novel magnetic structures and thus exhibits dynamic behaviors. We systematically probe polycrystalline samples of \textit{R}AlSi (\textit{R} = La, Ce, Pr and Nd) combining inelastic neutron scattering (INS), heat capacity and magnetic susceptibility measurements. The INS measurements identify well-resolved crystalline electric field (CEF) excitations at 19.2 and 24.9 meV in CeAlSi, at 5.4 meV in PrAlSi, and at 2.5 and 4.2 meV in NdAlSi. We analyzed the INS data using the corresponding CEF models and determined the CEF parameters and ground state wave functions of \textit{R}AlSi (\textit{R} = Ce, Pr and Nd). Our results suggest strong single-ion anisotropy in their ground states: $|\pm3/2\rangle$ (94.5\%) in CeAlSi, $|\pm3\rangle$ (99.2\%) in PrAlSi, and $|\pm9/2\rangle$ (76.2\%) in NdAlSi. Notably, the weaker anisotropy and strong exchange interactions in NdAlSi promote competing magnetic orders and CEF splitting at low temperature, contrasting with the robust CEF levels in magnetic states of CeAlSi and PrAlSi. △ Less

Submitted 14 August, 2025; originally announced August 2025.

Comments: 10 pages, 7 figures, Accepeted by Physical Review B

Journal ref: Physical Review B 112, 054439 (2025)

Radiative Heat Transfer and 2D Transition Metal Dichalcogenide Materials

Authors: Long Ma, Dai-Nam Le, Lilia M. Woods

Abstract: Radiative heat transfer is of great interest from a fundamental point of view and for energy harvesting applications. This is a material dependent phenomenon where confined plasmonic excitations, hyperbolicity and other properties can be effective channels for enhancement, especially at the near field regime. Materials with reduced dimensions may offer further benefits of enhancement compared to t… ▽ More Radiative heat transfer is of great interest from a fundamental point of view and for energy harvesting applications. This is a material dependent phenomenon where confined plasmonic excitations, hyperbolicity and other properties can be effective channels for enhancement, especially at the near field regime. Materials with reduced dimensions may offer further benefits of enhancement compared to the bulk systems. Here we study the radiative thermal power in the family of transition metal dichalcogenide monolayers in their H- and T-symmetries. For this purpose, the computed from first principles electronic and optical properties are then used in effective models to understand the emerging scaling laws for metals and semiconductors as well as specific materials signatures as control knobs for radiative heat transfer. Our combined approach of analytical modeling with properties from ab initio simulations can be used for other materials families to build a materials database for radiative heat transfer. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Comments: 28 pages, 4 figures

Journal ref: J. Phys. Chem. Lett. 16, 11348-11355 (2025)

Berry Monopole Scattering in the Synthetic Momentum Space of a Bilayer Photonic Crystal Slab

Authors: Ngoc Duc Le, D. -H. -Minh Nguyen, Dung Xuan Nguyen, Hai Son Nguyen, Dario Bercioux

Abstract: Berry monopoles-quantized sources of Berry curvature-are fundamental to topological phases, yet their scattering remains unexplored. Here, we report for the first time the adiabatic scattering of Berry monopoles in a bilayer photonic crystal slab combining one genuine and one synthetic momentum. Two monopoles approach, collide, and scatter within this hybrid parameter space. The process is describ… ▽ More Berry monopoles-quantized sources of Berry curvature-are fundamental to topological phases, yet their scattering remains unexplored. Here, we report for the first time the adiabatic scattering of Berry monopoles in a bilayer photonic crystal slab combining one genuine and one synthetic momentum. Two monopoles approach, collide, and scatter within this hybrid parameter space. The process is described by an effective coupled-mode model and confirmed by full-wave simulations. We further propose an experimental scheme using chiral edge states, opening a route to probe monopole interactions in synthetic photonic systems. △ Less

Submitted 24 July, 2025; v1 submitted 16 July, 2025; originally announced July 2025.

Comments: 46 pages, 21 figures

Magnetic excitations and exchange parameters of a nickel chain compound PbMn$_2$Ni$_6$Te$_3$O$_{18}$: Neutron scattering and density functional theory studies

Authors: S. Uthayakumar, D. T. Adroja, Amit Pokhriyal, A. K. Bera, Haranath Ghosh, Tatiana Gudi, Manh Duc Le, Christian Balz, R. A. Ewings, Minal Gupta, P. R. Sagdeo, D. Prabhakaran, J. P. Goff

Abstract: We have investigated the quasi-one dimensional Ni-chain compound PbMn$_2$Ni$_6$Te$_2$O$_{18}$ using theoretical DFT calculations, inelastic neutron scattering and optical spectroscopy in order to understand the nature of magnetic exchange interactions. Our inelastic neutron scattering study at 5 K on a powder sample reveals two bands of magnetic excitations, the first near 8 meV and the second nea… ▽ More We have investigated the quasi-one dimensional Ni-chain compound PbMn$_2$Ni$_6$Te$_2$O$_{18}$ using theoretical DFT calculations, inelastic neutron scattering and optical spectroscopy in order to understand the nature of magnetic exchange interactions. Our inelastic neutron scattering study at 5 K on a powder sample reveals two bands of magnetic excitations, the first near 8 meV and the second near 18 meV originating from the antiferromagnetic zone center near $Q$ = 1~Å. On the other hand at 100 K (which is above T$_N$ = 86 K) a broad diffuse scattering signal is observed indicating the presence of short range magnetic correlations. We have analyzed the magnetic excitations based on the Linear Spin Wave Theory (LSWT) and compared the experimentally estimated exchange parameters with the DFT calculations. Our analysis reveals that the value of the exchange parameter at the larger distance (d=3.654 $Å$) $J_3$=4.21(8) meV between Ni-Ni (from inter-chain) is the strongest amongst the allowed six exchange parameters, which suggests that this system is not really a quasi-one-dimensional and confirmed by the absence of a Haldane gap. We have also presented the electronic structure calculations. The spin-polarized partial density of states (DOS) projected onto the Mn-d and Ni-d orbitals reveals that the Ni-d$_{x^2-y^2}$ contribution is dominant below the Fermi level in the spin-up and spin-down channel, while a minimal contribution from spin-up Mn states in the occupied region, suggesting a nearly high-spin state. The estimated Néel temperature, based on experimental exchange parameters is found to be in close agreement with the experimental value. △ Less

Submitted 11 June, 2025; originally announced June 2025.

Comments: 16 pages, 13 figures

Journal ref: Physical Review B 111, 184432 (2025)

Coherent phonon motions and ordered vacancy compound mediated quantum path interference in Cu-poor CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) with attosecond transient absorption

Authors: Hugo Laurell, Jonah R. Adelman, Elizaveta Yakovleva, Carl Hägglund, Kostiantyn Sopiha, Axel Stenquist, Han K. D. Le, Peidong Yang, Marika Edoff, Stephen R. Leone

Abstract: In this study, coherent phonon motion is observed in bandgap excited CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) utilizing extreme ultraviolet (XUV) attosecond transient absorption spectroscopy across the Se M$_{4,5}$ absorption edge. Two frequencies of coherent phonon motion are resolved, a low frequency mode attributed through Raman measurements to the $A_{1g}$ phonon motion of a Cu-deficient ordered va… ▽ More In this study, coherent phonon motion is observed in bandgap excited CuIn$_{x}$Ga$_{(1-x)}$Se$_2$ (CIGS) utilizing extreme ultraviolet (XUV) attosecond transient absorption spectroscopy across the Se M$_{4,5}$ absorption edge. Two frequencies of coherent phonon motion are resolved, a low frequency mode attributed through Raman measurements to the $A_{1g}$ phonon motion of a Cu-deficient ordered vacancy compound (OVC), while the high frequency mode originates from the $A_{1g}$ phonon motion in the chalcopyrite phase. The two oscillations lead to modulations in the XUV differential absorption $ΔA(ε,τ)$ due to energy shifts of the Se M$_{4,5}$ edge, with a minima occuring approximately 1 ps after the band gap excitation. The hot carrier cooling time of holes and electrons are disentangled and the observed slower cooling of holes is attributed to the higher density of hole states in the valence band. We also observe fast oscillations (18.6(3) fs period) across the Se absorption edge, which are interpreted to originate from quantum path interference between the electronic conduction bands of the chalcopyrite CIGS and OVC phases, opening the possibility towards quantum coherent metrology in photovoltaics on the femtosecond timescale. The complex interplay between the chalcopyrite and OVC phases are revealed in this investigation through both coherent vibrational and electronic motions. △ Less

Submitted 18 August, 2025; v1 submitted 5 June, 2025; originally announced June 2025.

Atypical Ferrimagnetism in Ni$_4$Nb$_2$O$_9$

Authors: Jhuma Sannigrahi, Roumita Roy, Richard Waite, Anupam Banerjee, Mohamad Numan, Manh Duc Le, D. T. Adroja, Sudipta Kanungo, Subham Majumdar

Abstract: Ferrimagnetism typically emerges from chemically distinct magnetic ions or the same element at two inequivalent crystallographic sites, rendering unequal moments. In contrast, Ni4Nb2O9 has been recently discovered to show a different mechanism, where identical Ni2 ions with the same ligand coordination develop unequal magnetic moments purely due to differences in local environments. Here, we inves… ▽ More Ferrimagnetism typically emerges from chemically distinct magnetic ions or the same element at two inequivalent crystallographic sites, rendering unequal moments. In contrast, Ni4Nb2O9 has been recently discovered to show a different mechanism, where identical Ni2 ions with the same ligand coordination develop unequal magnetic moments purely due to differences in local environments. Here, we investigate the microscopic origin of this emergent mechanism through a synergy of powder neutron diffraction, inelastic neutron scattering, and first principle based calculations. We demonstrate that the NiA and NiB sublattices, while sharing the same nominal valence, differ in their magnetic dimensionality NiA forms quasi one dimensional chains with enhanced p d hybridization and a reduced magnetic moment, whereas NiB retains a nearly two-dimensional geometry and a full S 1 moment. Our results underscore the pivotal role of spin dimensionality and local structural distortions in stabilizing ferrimagnetism in systems with electronically equivalent magnetic ions. △ Less

Submitted 6 August, 2025; v1 submitted 9 May, 2025; originally announced May 2025.

Comments: 9 pages, 6 figures, regular article

Slow ferromagnetic fluctuations in the kagome metal Sc$_3$Mn$_3$Al$_7$Si$_5$ revealed by $^{27}$Al NMR

Authors: Qing-Ping Ding, Charles Taylor, Yongbin Lee, Charuni Dissanayake, Vireshwar Mishra, Dang Khoa Le, Manh-Huong Phan, Yasuyuki Nakajima, Yuji Furukawa

Abstract: Static and dynamical magnetic and electronic properties of the kagome metal Sc$_3$Mn$_3$Al$_7$Si$_5$ have been investigated by $^{27}$Al nuclear magnetic resonance (NMR) measurements. The temperature dependence of Knight shift ($K$) shows a similar temperature dependence of the DC magnetic susceptibility $χ$ except for the low-temperature region below $\sim$ 50 K where $K$ is almost constant while… ▽ More Static and dynamical magnetic and electronic properties of the kagome metal Sc$_3$Mn$_3$Al$_7$Si$_5$ have been investigated by $^{27}$Al nuclear magnetic resonance (NMR) measurements. The temperature dependence of Knight shift ($K$) shows a similar temperature dependence of the DC magnetic susceptibility $χ$ except for the low-temperature region below $\sim$ 50 K where $K$ is almost constant while $χ$ keeps increasing, which suggests that the increase in $χ$ at low temperatures is not intrinsic. $^{27}$Al spin-lattice relaxation rate divided by temperature ($1/T_1T$) is found to be constant, confirming the metallic state of Sc$_3$Mn$_3$Al$_7$Si$_5$ from a microscopic point of view. Based on a Korringa ratio analysis using the $T_1$ and $K$ data, ferromagnetic spin fluctuations are found to dominate in Sc$_3$Mn$_3$Al$_7$Si$_5$. These fluctuations are suggested to be very slow with frequencies on the order of kilohertz or lower. △ Less

Submitted 26 April, 2025; originally announced April 2025.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 112, L121105 (2025)

Interface Magnetism in Vanadium-doped MoS$_2$/Graphene Heterostructures

Authors: Diem Thi-Xuan Dang, Yen Thi-Hai Pham, Da Zhou, Dai-Nam Le, Mauricio Terrones, Manh-Huong Phan, Lilia M. Woods

Abstract: Magnetism in two-dimensional materials is of great importance in discovering new physical phenomena and developing new devices at the nanoscale. In this paper, first-principles simulations are used to calculate the electronic and magnetic properties of heterostructures composed of Graphene and MoS$_2$ considering the influence of point defects and Vanadium doping. It is found that the concentratio… ▽ More Magnetism in two-dimensional materials is of great importance in discovering new physical phenomena and developing new devices at the nanoscale. In this paper, first-principles simulations are used to calculate the electronic and magnetic properties of heterostructures composed of Graphene and MoS$_2$ considering the influence of point defects and Vanadium doping. It is found that the concentration of the dopants and the types of defects can result in induced magnetic moments leading to ferromagnetically polarized systems with sharp interfaces. This provides a framework for interpreting the experimental observations of enhanced ferromagnetism in both MoS$_2$/Graphene and V-doped MoS$_2$/Graphene heterostructures. The computed electronic and spin polarizations give a microscopic understanding of the origin of ferromagnetism in these systems and illustrate how doping and defect engineering can lead to targeted property tunability. Our work has demonstrated that through defects engineering, ferromagnetism can be achieved in V-doped MoS$_2$/Graphene heterostructures, providing a potential way to induce magnetization in other TMDC/Graphene materials and opening new opportunities for their applications in nano-spintronics. △ Less

Submitted 25 April, 2025; originally announced April 2025.

Comments: 27 pages, 8 pages, 1 table. Submitted

Journal ref: RSC Appl. Interfaces, 2025

Retrieval of fundamental material parameters of monolayer transition metal dichalcogenides from experimental exciton energies: An analytical approach

Authors: Duy-Nhat Ly, Dai-Nam Le, Dang-Khoa D. Le, Van-Hoang Le

Abstract: We propose a straightforward and highly accurate method for extracting material parameters such as screening length, bandgap energy, exciton reduced mass, and the dielectric constant of the surrounding medium from experimental magnetoexciton energies available for monolayer transition metal dichalcogenides (TMDCs). Our approach relies on analytical formulations that allow us to calculate the scree… ▽ More We propose a straightforward and highly accurate method for extracting material parameters such as screening length, bandgap energy, exciton reduced mass, and the dielectric constant of the surrounding medium from experimental magnetoexciton energies available for monolayer transition metal dichalcogenides (TMDCs). Our approach relies on analytical formulations that allow us to calculate the screening length $r_0$ and bandgap energy $E_g$ directly from the experimental $s$-state exciton energies $E_{1s}$, $E_{2s}$, and $E_{3s}$. We also establish a relationship between the surrounding dielectric constant $κ$ and the exciton reduced mass $μ$. This relationship simplifies the Schr{ö}dinger equation for a magnetoexciton in a TMDC monolayer, transforming it into a one-parameter equation that depends solely on the single material parameter $μ$. Furthermore, we develop an analytical formula with high accuracy for magnetoexciton energies as a function of the exciton reduced mass: $E(B,μ)$. Then, the inverse of this formula allows us to calculate the exciton reduced mass from experimental data on magnetoexciton energies. By applying this method, we extract key material parameters, $E_g$, $r_0$, $μ$, and $κ$, from the magnetoexciton energies of monolayer TMDCs, including WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$, encapsulated by hexagonal boron nitride (hBN) slabs in various current experiments. The material properties we retrieve complement and correct existing experimental and theoretical data. Additionally, we develop an analytical method for calculating diamagnetic coefficients and exciton radii with high accuracy compared to numerical calculations. Based on this method, we provide diamagnetic coefficients and exciton radii computed using the extracted material parameters. △ Less

Submitted 1 October, 2025; v1 submitted 19 April, 2025; originally announced April 2025.

Comments: 11 pages, 5 figures, 4 tables. To be published on Physical Review B

Solvation enhances folding cooperativity and the topology dependence of folding rates in a lattice protein model

Authors: Nhung T. T. Nguyen, Pham Nam Phong, Duy Manh Le, Minh-Tien Tran, Trinh Xuan Hoang

Abstract: The aqueous solvent profoundly influences protein folding, yet its effects are relatively poorly understood. In this study, we investigate the impact of solvation on the folding of lattice proteins by using Monte Carlo simulations. The proteins are modelled as self-avoiding 27-mer chains on a cubic lattice, with compact native states and structure-based Gō potentials. Each residue that makes no co… ▽ More The aqueous solvent profoundly influences protein folding, yet its effects are relatively poorly understood. In this study, we investigate the impact of solvation on the folding of lattice proteins by using Monte Carlo simulations. The proteins are modelled as self-avoiding 27-mer chains on a cubic lattice, with compact native states and structure-based Gō potentials. Each residue that makes no contacts with other residues in a given protein conformation is assigned a solvation energy ε_s , representing its full exposure to the solvent. We find that a negative ε_s , indicating a favorable solvation, increases the cooperativity of the folding transition by lowering the free energy of the unfolded state, increasing the folding free energy barrier, and narrowing the folding routes. This favorable solvation also significantly improves the correlation between folding rates and the native topology, measured by the relative contact order. Our results suggest that Gō model may overestimate the importance of native interactions and a solvation potential countering the native bias can play a significant role. The solvation energy in our model can be related to the polar interaction between water and peptide groups in the protein backbone. It is therefore suggested that the solvation of peptide groups may significantly contribute to the exceptional folding cooperativity and the pronounced topology-dependence of folding rates observed in two-state proteins. △ Less

Submitted 29 March, 2025; originally announced March 2025.

Comments: 11 pages, 8 figures, accepted for publication in J. Chem. Phys

Unveiling Coverage Dependent Interactions of N-Methylaniline with the Pt(111) Surface

Authors: Bushra Ashraf, Nils Brinkmann, Dave Austin, Duy Le, Katharina Al Shamery, Talat S. Rahman

Abstract: This study aims to elucidate the adsorption and surface chemistry of N-methylaniline (NMA) on Pt(111), using it as a model molecule to probe the activation mechanisms of aromatic amines on catalytic surfaces. Through a combination of density functional theory (DFT) calculations and experimental techniques such as temperature programmed X-ray photoelectron spectroscopy (TP-XPS), temperature program… ▽ More This study aims to elucidate the adsorption and surface chemistry of N-methylaniline (NMA) on Pt(111), using it as a model molecule to probe the activation mechanisms of aromatic amines on catalytic surfaces. Through a combination of density functional theory (DFT) calculations and experimental techniques such as temperature programmed X-ray photoelectron spectroscopy (TP-XPS), temperature programmed desorption (TPD), and Fourier transform infrared reflection absorption spectroscopy(FT-IRRAS),we explored the coverage-dependent behaviour of NMA on Pt(111) to identify key steps in the activation process. The population of certain reaction paths is driven by a coverage dependent balance between molecule surface charge transfer and intermolecular interactions, dictating the selective activation of specific bonds. Our findings reveal how coverage influences the orientation and bonding of NMA on the Pt(111)surface. At lower coverages, the molecule binds to the surface through the phenyl ring and activation, facilitating C-N bond cleavage to the ring under HCN formation. In comparison, at higher coverages, the molecule binds only through the nitrogen atom and desorbs intact. These insights into variable bond activation lay the ground work for understanding the fundamental processes involved in potential heterogeneously catalyzed reactions of aromatic amines, contributing to the development of new catalytic strategies. △ Less

Submitted 24 March, 2025; originally announced March 2025.

Twisting in h-BN bilayers and their angle-dependent properties

Authors: Diem Thi-Xuan Dang, Dai-Nam Le, Lilia M. Woods

Abstract: In this paper, we systematically investigate the structural and electronic properties of twisted h-BN bilayers to understand the role of the twisting angle. Using first-principles methods with relaxation taken into account, we simulate h-BN bilayers with commensurate supercells with the smallest angle being $2.88^{\circ}$ until $60^{\circ}$. We find that the interlayer separation is not constant t… ▽ More In this paper, we systematically investigate the structural and electronic properties of twisted h-BN bilayers to understand the role of the twisting angle. Using first-principles methods with relaxation taken into account, we simulate h-BN bilayers with commensurate supercells with the smallest angle being $2.88^{\circ}$ until $60^{\circ}$. We find that the interlayer separation is not constant throughout each bilayer because of the various stacking patterns of AA, AA', AB, AB', and A'B throughout the layers, which also play a significant role in their unique charge redistribution. The calculations for the 110 generated structures show the existence of flat bands in several twisted h-BN bilayers, as well as the emergence of different trends in their properties as a function of the twist angle. These results are useful for establishing a systematic base line of registry-dependent relations for the development of more advanced computational methods to access incommensurate h-BN bilayers. △ Less

Submitted 4 March, 2025; originally announced March 2025.

Comments: 9 pages, 6 figures, submitted

Journal ref: Physical Review B 112, 085102 (2025)

U(1) Dirac quantum spin liquid candidate in triangular-lattice antiferromagnet CeMgAl$_{11}$O$_{19}$

Authors: Yantao Cao, Akihiro Koda, M. D. Le, V. Pomjakushin, Benqiong Liu, Zhendong Fu, Zhiwei Li, Jinkui Zhao, Zhaoming Tian, Hanjie Guo

Abstract: Quantum spin liquid represents an intriguing state where electron spins are highly entangled yet spin fluctuation persists even at 0 K. Recently, the hexaaluminates \textit{R}MgAl$_{11}$O$_{19}$ (\textit{R} = rare earth) have been proposed to be a platform for realizing the quantum spin liquid state with dominant Ising anisotropic correlations. Here, we report detailed low-temperature magnetic sus… ▽ More Quantum spin liquid represents an intriguing state where electron spins are highly entangled yet spin fluctuation persists even at 0 K. Recently, the hexaaluminates \textit{R}MgAl$_{11}$O$_{19}$ (\textit{R} = rare earth) have been proposed to be a platform for realizing the quantum spin liquid state with dominant Ising anisotropic correlations. Here, we report detailed low-temperature magnetic susceptibility, muon spin relaxation, and thermodynamic studies on the CeMgAl$_{11}$O$_{19}$ single crystal. Ising anisotropy is revealed by magnetic susceptibility measurements. Muon spin relaxation and ac susceptibility measurements rule out any long-range magnetic ordering or spin freezing down to 50 mK despite the onset of spin correlations below $\sim$0.8 K. Instead, the spins keep fluctuating at a rate of 1.0(2) MHz at 50 mK. Specific heat results indicate a gapless excitation with a power-law dependence on temperature, $C_m(T) \propto T^α$. The quasi-quadratic temperature dependence with $α$ = 2.28(4) in zero field and linear temperature dependence in 0.25 T support the possible realization of the U(1) Dirac quantum spin liquid state. △ Less

Submitted 26 February, 2025; originally announced February 2025.

Comments: Accepted by Sci. China - Phys. Mech. Astron. 7 pages main text + 8 pages supplementary materials

Journal ref: Sci. China-Phys. Mech. Astron. 68, 267011 (2025)

Quantum stick-slip motion in nanoscaled friction

Authors: Dai-Nam Le, Pablo Rodriguez-Lopez, Lilia M. Woods

Abstract: Friction in atomistic systems is usually described by the classical Prandtl-Tomlinson model suitable for capturing the dragging force of a nanoparticle in a periodic potential. Here we consider the quantum mechanical version of this model in which the dissipation is facilitated by releasing heat to an external bath reservoir. The time evolution of the system is captured with the Liouville-von Neum… ▽ More Friction in atomistic systems is usually described by the classical Prandtl-Tomlinson model suitable for capturing the dragging force of a nanoparticle in a periodic potential. Here we consider the quantum mechanical version of this model in which the dissipation is facilitated by releasing heat to an external bath reservoir. The time evolution of the system is captured with the Liouville-von Neumann equation through the density matrix of the system in the Markov approximation. We examine several kinetic and dissipative properties of the nanoparticle by delineating classical vs quantum mechanical effects. We find that that the Landau-Zener tunneling is a key factor in the overall reduction of the frictional dissipation when compared to the classical motion in which such tunneling is absent. This in-depth study analyzes the interplay between velocity, strength of interaction, and temperature to control the frictional process and provide useful guidelines for experimental data interpretation. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Comments: Main text: 12 pages, 4 figures, 1 table; Supplementary Information: 20 pages, 4 figures (1 animation GIF figure)

Low-temperature magnetic behaviour on the triangular lattice in hexagonal Ba$_3$Tb(BO$_3$)$_3$

Authors: Nicola Kelly, Manh Duc Le, Denis Sheptyakov, Camilla Tacconis, Cheng Liu, Gavin Stenning, Peter Baker, Siân Dutton

Abstract: The hexagonal polymorph of Ba$_3$Tb(BO$_3$)$_3$ contains Tb$^{3+}$ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba$_3$Tb(BO$_3$)$_3$ have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS) and muon-spin relaxation spectroscopy ($μ$SR). No long-range magnetic ordering is observed down t… ▽ More The hexagonal polymorph of Ba$_3$Tb(BO$_3$)$_3$ contains Tb$^{3+}$ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba$_3$Tb(BO$_3$)$_3$ have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS) and muon-spin relaxation spectroscopy ($μ$SR). No long-range magnetic ordering is observed down to the lowest measured temperatures of 75 mK in PND and specific heat data and 1.5 K in the $μ$SR data. Modelling the INS spectrum using a point charge model suggests that the ground state is a singlet with a low-lying doublet on each of the two crystallographically independent Tb$^{3+}$ sites and that both the Tb ions display weak XY single-ion anisotropy. △ Less

Submitted 28 January, 2025; originally announced January 2025.

Comments: 22 pages, 7 figures

Journal ref: Physical Review B 111, 054427 (2025)

Magnetic properties of the zigzag ladder compound SrTb2O4

Authors: F. Orlandi, M. Ciomaga Hatnean, D. A. Mayoh, J. P. Tidey, S. X. M. Riberolles, G. Balakrishnan, P. Manuel, D. D. Khalyavin, H. C. Walker, M. D. Le, B. Ouladdiaf, A. R. Wildes, N. Qureshi, O. A. Petrenko

Abstract: We report on the properties of SrTb2O4, a frustrated zigzag ladder antiferromagnet, studied by single crystal neutron diffraction (with polarised neutrons in zero field and unpolarised neutrons in an applied magnetic field), as well as by neutron spectroscopy on a polycrystalline sample. The neutron scattering results are supported by single crystal magnetisation and heat capacity measurements. In… ▽ More We report on the properties of SrTb2O4, a frustrated zigzag ladder antiferromagnet, studied by single crystal neutron diffraction (with polarised neutrons in zero field and unpolarised neutrons in an applied magnetic field), as well as by neutron spectroscopy on a polycrystalline sample. The neutron scattering results are supported by single crystal magnetisation and heat capacity measurements. In zero field, neutron diffraction data show no transition to a magnetically ordered state down to the lowest experimentally available temperature of 35 mK, and the material remains magnetically disordered down to this temperature. Polarised neutron diffraction measurements reveal the presence of a diffuse scattering signal suggesting only very weak spin-spin correlations in the ground state. For H // c (the easy magnetisation direction), we followed the magnetisation process using neutron diffraction measurements and observed the appearance of field-induced magnetic Bragg peaks with integer h and k indices in the (hk0) scattering plane. No magnetic peaks with a non-zero propagation vector were detected. The observed in-field data fit well to a simple two-sublattice model with magnetic moments aligned along the field direction but being significantly different in magnitude for the two inequivalent Tb3+ sites in the unit cell. Overall, the collected data point to a nonmagnetic ground state in SrTb2O4 despite the presence of strong interactions. △ Less

Submitted 17 January, 2025; originally announced January 2025.

Comments: 13 pages, 11 figures

Journal ref: Phys. Rev. B 111, 054415 (2025)

Orbital fluctuations and spin-orbital-lattice coupling in Bi2Fe4O9

Authors: Aditya Prasad Roy, M. K. Chattopadhyay, Ranjan Mittal, Srungarpu N. Achary, Avesh K. Tyagi, Manh Duc Le, Dipanshu Bansal

Abstract: Magnetic frustrations and degeneracies profoundly affect ground-state magnetic properties emerging from competing exchange interactions. Controlling such frustrations using orbital and phonon engineering via the Kugel-Khomskii-type (KK-type) interactions has recently enabled the orbital enhancement of magnetoelectric (ME) coupling. Using combined spectroscopic techniques and first-principle simula… ▽ More Magnetic frustrations and degeneracies profoundly affect ground-state magnetic properties emerging from competing exchange interactions. Controlling such frustrations using orbital and phonon engineering via the Kugel-Khomskii-type (KK-type) interactions has recently enabled the orbital enhancement of magnetoelectric (ME) coupling. Using combined spectroscopic techniques and first-principle simulations, here we demonstrate that the magnetically frustrated Cairo lattice, Bi2Fe4O9, exhibits a strong KK-type interaction resulting in a coupled spin-orbital phase below 1.8 times the Neel temperature (TN = 245 K). We observe an order of magnitude change in phonon linewidths that is not explainable considering spin-phonon coupling channels alone. Instead, the observed change is reminiscent of orbitally active materials, which we explicitly confirm by measuring the T-dependence of low-energy orbital excitations. We further find that Bi2Fe4O9 harbors an unstable polar mode, driving the lattice to a symmetry-lowered ferroelectric (FE) phase below TN, in line with the previously reported hysteresis in polarization. Nonetheless, the FE phase leads to extremely small calculated superlattice Bragg peak intensities that are yet to be experimentally confirmed. Moreover, thermal conductivity measurements do not show any measurable effect of KK-type interactions on thermal transport across TN. But, we observe a repeatable anomaly near 57 K appearing only in the heating cycle, which co-occurs with the 400 meV broad continuum observed in Raman measurements. The observed KK-type interaction in Bi2Fe4O9 provides an opportunity for orbital enhancement of ME coupling by phonon control of superexchange interactions. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Comments: 7 figures

Enhancement of the superconducting transition temperature due to multiband effect in the topological nodal-line semimetal Pb$_{1-x}$Sn$_{x}$TaSe$_{2}$

Authors: K. Kumarasinghe, A. Rahman, M. Tomlinson, D. Le, F. Joshua, L. Zhai, Y. Nakajima

Abstract: We report a systematic study of the normal-state and superconducting properties of single crystal Pb$_{1-x}$Sn$_{x}$TaSe$_{2}$ $(0\leq x \leq 0.23)$. Sn doping enhances the superconducting temperature $T_{c}$ up to 5.1 K while also significantly increasing impurity scattering in the crystals. For $x=0$ and 0.018, the specific heat jump at $T_{c}$ exceeds the Bardeen-Cooper-Schrieffer (BCS) weak-co… ▽ More We report a systematic study of the normal-state and superconducting properties of single crystal Pb$_{1-x}$Sn$_{x}$TaSe$_{2}$ $(0\leq x \leq 0.23)$. Sn doping enhances the superconducting temperature $T_{c}$ up to 5.1 K while also significantly increasing impurity scattering in the crystals. For $x=0$ and 0.018, the specific heat jump at $T_{c}$ exceeds the Bardeen-Cooper-Schrieffer (BCS) weak-coupling value of 1.43, indicating the realization of strong-coupling superconductivity in undoped and slightly Sn-doped PbTaSe$_{2}$. Substituting Pb with more Sn lowers the specific heat jump at $T_{c}$ below the BCS value of 1.43, which cannot be explained by a single-gap model. Rather, the observed specific heat data of moderately Sn-doped PbTaSe$_{2}$ ($x= 0.08$ and 0.15) are reproduced by a two-gap model. Our density functional theory calculations suggest that three-dimensional Fermi pockets appear due to a reduction of the spin-orbit gap with Sn doping, and the multiband effect arising from these emergent Fermi pockets enhances the effective electron-phonon coupling strength, leading to the increase in $T_{c}$ of Pb$_{1-x}$Sn$_{x}$TaSe$_{2}$. △ Less

Submitted 6 August, 2025; v1 submitted 29 November, 2024; originally announced November 2024.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. B 112, 054504 (2025)

Room temperature quantum metric effect in TbMn6Sn6

Authors: Weiyao Zhao, Kaijian Xing, Yufei Zhao, Lei Chen, Min Hong, Yuefeng Yin, Yang Liu, Khoa Dang Le, Jacob Gayles, Fang Tang, Yong Fang, Binghai Yan, Julie Karel

Abstract: Quantum geometry, including Berry curvature and the quantum metric, of the electronic Bloch bands has been studied via nonlinear responses in topological materials. Naturally, these material systems with intrinsic strong nonlinear responses also form the key component in nonlinear electronic devices. However, the previous reported quantum geometry effects are mainly observed at cryogenic temperatu… ▽ More Quantum geometry, including Berry curvature and the quantum metric, of the electronic Bloch bands has been studied via nonlinear responses in topological materials. Naturally, these material systems with intrinsic strong nonlinear responses also form the key component in nonlinear electronic devices. However, the previous reported quantum geometry effects are mainly observed at cryogenic temperatures, hindering their application in practical devices. Here we report the tuneable strong room-temperature second-harmonic transport response in a quantum magnet, TbMn6Sn6, which is governed by the quantum metric and can be tuned with applied magnetic fields. We show that around room temperature, which is close to the spontaneous spin-reorientation transition, the magnetic configurations, and therefore the related symmetry breaking phases, are easily controlled via magnetic fields. Our results also show that manipulation of the symmetries of the magnetic structure presents an effective route to tuneable quantum-geometry-based devices. △ Less

Submitted 12 June, 2025; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: 12 pages, 3 figures

Coherently Coupled Carrier and Phonon Dynamics in Elemental Tellurium Probed by XUV Transient Absorption

Authors: Jonah R. Adelman, Hugo Laurell, Lauren B. Drescher, Han K. D. Le, Peidong Yang, Stephen R. Leone

Abstract: The narrow bandgap semiconductor elemental tellurium (Te) has a unique electronic structure due to strong spin-orbit splitting and a lack of inversion symmetry of it's helical lattice. Using broadband extreme ultraviolet core-level transient absorption, we measure simultaneously the coherently coupled photo-induced carrier and lattice dynamics at the Te N$_{4,5}$ edge initiated by a few-cycle NIR… ▽ More The narrow bandgap semiconductor elemental tellurium (Te) has a unique electronic structure due to strong spin-orbit splitting and a lack of inversion symmetry of it's helical lattice. Using broadband extreme ultraviolet core-level transient absorption, we measure simultaneously the coherently coupled photo-induced carrier and lattice dynamics at the Te N$_{4,5}$ edge initiated by a few-cycle NIR pulse. Ultrafast excitation of carriers leads to a coherently excited A$_{\rm{1}}$ phonon oscillation and the generation of a hot carrier population distribution that oscillates in temperature, and the phonon excursion and hot carrier temperature are $π$ out of phase with respect to each other. The depths of modulation suggest a significant coupling between the electronic and lattice degrees of freedom in Te. A long-lived shift of the absorption edge suggests an excited state of Te in a new equilibrium potential energy surface that lives on the order of the carrier recombination timescale. The observed phonon-induced oscillations of the hot carriers are supportive of a change in the metallicity, whereby Te becomes more metallic with increasing phonon-induced displacement. Additionally, near the Fermi level we observe an energy-dependent phase of the displacive excitation of the A$_{\rm{1}}$ phonon mode. The discovery of coherent coupling between the lattice and hot carriers in Te provides the basis to investigate coherent interactions between spin and orbital degrees of freedom. The results spectrally and temporally resolve the correlation between photo-excited hot carriers and coherent lattice excitations, providing insight on the optical manipulation of the Te electronic structure at high carrier densities exceeding $10^{21}\,\mathrm{cm}^{-3}$. △ Less

Submitted 6 May, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Comments: 12 Figures

Magnetic and crystal electric field studies in the rare-earth-based square lattice antiferromagnet NdKNaNbO$_5$

Authors: S. Guchhait, A. Painganoor, S. S. Islam, J. Sichelschmidt, M. D. Le, N. B. Christensen, R. Nath

Abstract: The interplay of magnetic correlations, crystal electric field interactions, and spin-orbit coupling in low-dimensional frustrated magnets fosters novel ground states with unusual excitations. Here, we report the magnetic properties and crystal electric field (CEF) scheme of a rare-earth-based square-lattice antiferromagnet NdKNaNbO$_5$ investigated via magnetization, specific heat, electron spin… ▽ More The interplay of magnetic correlations, crystal electric field interactions, and spin-orbit coupling in low-dimensional frustrated magnets fosters novel ground states with unusual excitations. Here, we report the magnetic properties and crystal electric field (CEF) scheme of a rare-earth-based square-lattice antiferromagnet NdKNaNbO$_5$ investigated via magnetization, specific heat, electron spin resonance (ESR), and inelastic neutron scattering (INS) experiments. The low-temperature Curie-Weiss temperature $θ_{\rm CW} \simeq -0.6$ K implies net antiferromagnetic interactions between the Nd$^{3+}$ ions. Two broad maxima are observed in the low temperature specific heat data in magnetic fields, indicating multilevel Schottky anomalies due to the effect of CEF. No magnetic long-range-order is detected down to 0.4 K. The CEF excitations of Kramers' ion Nd$^{3+}$ ($J=9/2$) probed via INS experiments evince dispersionless excitations characterizing the transitions among the CEF energy levels. The fit of the INS spectra enabled the mapping of the CEF Hamiltonian and the energy eigenvalues of the Kramers' doublets. The simulation using the obtained CEF parameters reproduces the broad maxima in specific heat in zero-field as well as in different applied fields. The significant contribution from $J_z = \pm 1/2$ state to the wave function of the ground state doublet indicates the role of strong quantum fluctuations at low temperatures. The magnetic ground state is found to be a Kramers' doublet with effective spin $J_{\rm eff} = 1/2$ at low temperatures. △ Less

Submitted 30 August, 2024; originally announced August 2024.

Comments: 14 pages, 10 figures

Phonon-assisted Casimir interactions between piezoelectric materials

Authors: Dai-Nam Le, Pablo Rodriguez-Lopez, Lilia M. Woods

Abstract: The strong coupling between electromagnetic field and lattice oscillation in piezoelectric materials gives rise to phonon polariton excitations. Such quasiparticles open up new directions in modulating the ubiquitous Casimir force. Here by utilizing the generalized Born-Huang hydrodynamics model, three types of phonons in piezoelectrics are studied: longitudinal optical phonon, transverse optical… ▽ More The strong coupling between electromagnetic field and lattice oscillation in piezoelectric materials gives rise to phonon polariton excitations. Such quasiparticles open up new directions in modulating the ubiquitous Casimir force. Here by utilizing the generalized Born-Huang hydrodynamics model, three types of phonons in piezoelectrics are studied: longitudinal optical phonon, transverse optical phonon and phonon polariton. The phonon-electromagnetic coupling results in a complex set of Fresnel reflection matrices which prevents the utilization of the standard Lifshitz approach for calculating Casimir forces in the imaginary frequency domain. Our calculations are based on an approach within real frequency and finite temperatures, through which various regimes of the Casimir interaction are examined. Our study shows that piezoelectrics emerge as a set of materials where this ubiquitous force can be controlled via phonon properties for the first time. The Casimir interaction appears as a suitable means to distinguish between different types of surface phonon polaritons associated with different structural piezoelectric polytypes. △ Less

Submitted 23 August, 2024; originally announced August 2024.

Comments: 8 pages, 4 figures, supplementary information with 10 pages, 4 figures; submitted

Journal ref: Commun. Mater. 5, 260 (2024)

Magnetic properties of a staggered $S=1$ chain Ni(pym)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$ with an alternating single-ion anisotropy direction

Authors: S. Vaidya, S. P. M. Curley, P. Manuel, J. Ross Stewart, M. Duc Le, C. Balz, T. Shiroka, S. J. Blundell, K. A. Wheeler, I. Calderon-Lin, Z. E. Manson, J. L. Manson, J. Singleton, T. Lancaster, R. D. Johnson, P. A. Goddard

Abstract: Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states which are sensitive to the single-ion-anisotropy (SIA) energy ($D$), and intrachain ($J_{0}$) and interchain ($J'_{i}$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is… ▽ More Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states which are sensitive to the single-ion-anisotropy (SIA) energy ($D$), and intrachain ($J_{0}$) and interchain ($J'_{i}$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of Ni(pyrimidine)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$, a chain compound where the tilting of Ni octahedra leads to a 2-fold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3$\,K and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_{0} = 5.107(7)$\,K, $D = 2.79(1)$\,K, $J'_{2}=0.18(3)$\,K and a rhombic anisotropy energy $E=0.19(9)$\,K. Mean-field modelling reveals that the ground state structure hosts spin canting of $φ\approx6.5^{\circ}$, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte-Carlo simulation of the powder-averaged magnetization, $M(H)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M(H)$ simulations provide insight into features observed in the data. △ Less

Submitted 28 November, 2024; v1 submitted 25 July, 2024; originally announced July 2024.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 111, 014421 (2025)

Magnetic properties and field-induced phenomena in the Jeff = 1/2 distorted kagome antiferromagnet

Authors: A. Yadav, A. Elghandour, T. Arh, D. T. Adroja, M. D. Le, G. B. G. Stenning, M. Aouane, S. Luther, F. Hotz, T. J. Hicken, H. Luetkens, A. Zorko, R. Klingeler, P. Khuntia

Abstract: The intertwining between competing degrees of freedom, anisotropy, and frustration-induced quantum fluctuations offers an ideal ground to realize exotic quantum phenomena in the rare-earth-based kagome lattice. The magnetic susceptibility reveals the presence of two energy scales in agreement with the INS results. The higher energy state is dominated by CEF excitations, where the lowest Kramers gr… ▽ More The intertwining between competing degrees of freedom, anisotropy, and frustration-induced quantum fluctuations offers an ideal ground to realize exotic quantum phenomena in the rare-earth-based kagome lattice. The magnetic susceptibility reveals the presence of two energy scales in agreement with the INS results. The higher energy state is dominated by CEF excitations, where the lowest Kramers ground-state doublet is well separated from the excited state suggesting that the compound realizes a low-energy state at low temperatures. The second energy scale is witnessed via thermodynamic results that reveal an anomaly at 0.3 K typical of a phase transition, which is attributed to the presence of complex magnetic ordering phenomena. The broad maximum in the specific heat well above 0.3 K indicates the presence of short-range spin correlations that is corroborated by muon spin relaxation rate results. The isothermal magnetization reveals a field-induced 1/3 magnetization plateau at low temperatures. muSR relaxation rate experiments, on the other hand, neither show the signature of a phase transition nor spin-freezing down to 34 mK. The ZF muSR relaxation is governed by the Orbach process and reveals the presence of a fluctuating state owing to the depopulation of crystal field levels reflected as a constant value of relaxation rate in the temperature range 0.4-10 K. NMR results indicate the presence of fluctuating Nd3+ moments down to 1.8 K consistent with muSR experiments. Our comprehensive results reveal that a field-induced quantum critical phenomenon is at play in this frustrated kagome magnet and enable us to construct a phase diagram exemplifying the proximity effect of competing magnetic states. This sets the stage to investigate the broad RE3BWO9 family of rare-earth kagome magnets promising to host exotic quantum states driven by spin-orbit coupling and geometrical frustration. △ Less

Submitted 18 December, 2024; v1 submitted 12 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. B 111, 094408 (2025)

Classical spin liquid state in the emergent honeycomb-lattice material TbBO3

Authors: J. Khatua, D. Tay, T. Shiroka, M. Pregelj, U. Jena, M. Barik, K. Kargeti, S. K. Panda, G. B. G. Stenning, P. Manuel, M. D. Le, D. T. Adroja, P. Khuntia

Abstract: The classical spin liquid state, owing to spin frustration between classical Ising spins on triangular motifs and interplay between competing degrees of freedom, is characterized by a macroscopically degenerate ground state and topological excitations linked to emergent gauge theories. Here, by using thermodynamic and local-probe measurements down to 16 mK, we demonstrate the exotic magnetism and… ▽ More The classical spin liquid state, owing to spin frustration between classical Ising spins on triangular motifs and interplay between competing degrees of freedom, is characterized by a macroscopically degenerate ground state and topological excitations linked to emergent gauge theories. Here, by using thermodynamic and local-probe measurements down to 16 mK, we demonstrate the exotic magnetism and spin dynamics in the nearly perfect honeycomb lattice material TbBO3. The latter embodies a frustrated lattice with a superimposed triangular lattice, constituted by additional Tb3+ ions at the center of each hexagon. Thermodynamic experiments reveal the presence of dominant antiferromagnetic exchange and significant dipolar interactions. Despite sizable antiferromagnetic exchange interactions between the Tb3+ moments, muon-spin relaxation does not detect any signatures of long-range magnetic order or spin freezing down to 16 mK, corroborating the specific heat and ac susceptibility down to 45 mK. This suggests that the spin-orbit driven anisotropic exchange interaction engenders a strong frustration, crucial to induce a persistent spin dynamics. The scaling of muon relaxation rate as a function of the characteristic energy scale for several spin-liquid candidates, including TbBO3, demonstrates that a common underlying mechanism is at play. This is consistent with the NMR results on TbBO3 and reminiscent of a universal spin-liquid behavior, here attributed to dominant antiferromagnetic short-range spin correlations, confirmed by the presence of a broad magnetic diffuse scattering in the elastic and low-energy inelastic neutron scattering at Q=1.03 Ang^-1 at low temperatures. Our results demonstrate that TbBO3 hosts a classical spin liquid induced by spin-orbit driven anisotropy on a frustrated honeycomb lattice antiferromagnet. △ Less

Submitted 16 May, 2025; v1 submitted 8 July, 2024; originally announced July 2024.

Analytical exciton energies in monolayer transition-metal dichalcogenides

Authors: Hanh T. Dinh, Ngoc-Hung Phan, Duy-Nhat Ly, Dai-Nam Le, Ngoc-Tram D. Hoang, Nhat-Quang Nguyen, Phuoc-Thien Doan, Van-Hoang Le

Abstract: We derive an analytical expression for $s$-state exciton energies in monolayer transition-metal dichalcogenides (TMDCs): $E_{\text{ns}}=-{\text{Ry}}^*\times P_n/{(n-1/2+0.479\, r^*_0/κ)^2}$, $n=1,2,...$, where $r^*_0$ and $κ$ are the dimensionless screening length and dielectric constant of the surrounding medium; $\text{Ry}^*$ is an effective Rydberg energy scaled by the dielectric constant and e… ▽ More We derive an analytical expression for $s$-state exciton energies in monolayer transition-metal dichalcogenides (TMDCs): $E_{\text{ns}}=-{\text{Ry}}^*\times P_n/{(n-1/2+0.479\, r^*_0/κ)^2}$, $n=1,2,...$, where $r^*_0$ and $κ$ are the dimensionless screening length and dielectric constant of the surrounding medium; $\text{Ry}^*$ is an effective Rydberg energy scaled by the dielectric constant and exciton reduced mass; $P_n(r^*_0/κ)$ is a function of variables $n$ and $r^*_0/κ$. Its values are around 1.0 so we consider it a term that corrects the Rydberg energy. Despite the simple form, the suggested formula gives exciton energies with high precision compared to the exact numerical solutions that accurately describe recent experimental data for a large class of TMDC materials, including WSe$_2$, WS$_2$, MoSe$_2$, MoS$_2$, and MoTe$_2$. To achieve these results, we have developed a so-called regulated perturbation theory by combining the conventional perturbation method with several elements of the Feranchuk-Komarov operator method, including the Levi-Civita transformation, the algebraic calculation technique via the annihilation and creation operators, and the introduction of a free parameter to optimize the convergence rate of the perturbation series. This universal form of exciton energies could be helpful in various physical analyses, including retrieval of the material parameters such as reduced exciton mass and screening length from the available measured exciton energies. △ Less

Submitted 6 July, 2024; v1 submitted 1 July, 2024; originally announced July 2024.

Comments: 5 pages, 1 figure, 3 tables, 1 supplementary

Journal ref: Phys. Rev. B 111, 035443 (2025)

Fano-enhanced low-loss on-chip superconducting microwave circulator

Authors: N. Pradeep Kumar, Dat Thanh Le, Prasanna Pakkiam, Thomas M. Stace, Arkady Fedorov

Abstract: Ferrite-free circulators that are passive and readily integratable on a chip are highly sought-after in quantum technologies based on superconducting circuits. In our previous work, we implemented such a circulator using a three-Josephson-junction loop that exhibited unambiguous nonreciprocity and signal circulation, but required junction energies to be within $1\%$ of design values. This toleranc… ▽ More Ferrite-free circulators that are passive and readily integratable on a chip are highly sought-after in quantum technologies based on superconducting circuits. In our previous work, we implemented such a circulator using a three-Josephson-junction loop that exhibited unambiguous nonreciprocity and signal circulation, but required junction energies to be within $1\%$ of design values. This tolerance is tighter than standard junction fabrication methods provide, so we propose and demonstrate a design improvement that relaxes the required junction fabrication precision, allowing for higher device performance and fabrication yield. Specifically, we introduce large direct capacitive couplings between the waveguides to create strong Fano scattering interference. We measure enhanced `circulation fidelity' above $97\%$, with optimised on-resonance insertion loss of $0.2$~dB, isolation of $18$~dB, and power reflectance of $-15$~dB, in good agreement with model calculations. △ Less

Submitted 20 June, 2024; originally announced June 2024.

Comments: 12 pages, 9 figures

Dissecting van der Waals interactions with Density Functional Theory -- Wannier-basis approach

Authors: Diem Thi-Xuan Dang, Dai-Nam Le, Lilia M. Woods

Abstract: A new scheme for the computation of dispersive interactions from first principles is presented. This cost-effective approach relies on a Wannier function representation compatible with density function theory descriptions. This is an electronic-based many-body method that captures the full electronic and optical response properties of the materials. It provides the foundation to discern van der Wa… ▽ More A new scheme for the computation of dispersive interactions from first principles is presented. This cost-effective approach relies on a Wannier function representation compatible with density function theory descriptions. This is an electronic-based many-body method that captures the full electronic and optical response properties of the materials. It provides the foundation to discern van der Waals and induction energies as well as the role of anisotropy and different stacking patterns when computing dispersive interactions in systems. Calculated results for binding energies in benchmarked materials and layered materials, such as graphite, hBN, and MoS$_2$ give encouraging comparisons with available experimental data. Strategies for broadened computational descriptions of dispersive interactions are also discussed. Our investigation aims at stimulating new experimental studies to measure van der Waals energies in a wider range of materials, especially in layered systems. △ Less

Submitted 31 January, 2025; v1 submitted 17 June, 2024; originally announced June 2024.

Comments: 29 pages, 1 abstract figure, 5 figures, source codes: 10.6084/m9.figshare.26023024.v2 or https://github.com/Diem-TX-Dang/vdW-MBDWan-code/tree/main, published on Computer Physics Communications

Journal ref: Computer Physics Communications 310, 109525 (2025)

Kitaev Interactions Through an Extended Superexchange Pathway in the jeff = 1/2 Ru3+ Honeycomb Magnet, RuP3SiO11

Authors: Aly H. Abdeldaim, Hlynur Gretarsson, Sarah J. Day, M. Duc Le, Gavin B. G. Stenning, Pascal Manuel, Robin S. Perry, Alexander A. Tsirlin, Gøran J. Nilsen, Lucy Clark

Abstract: Magnetic materials are composed of the simple building blocks of magnetic moments on a crystal lattice that interact via magnetic exchange. Yet from this simplicity emerges a remarkable diversity of magnetic states. Some reveal the deep quantum mechanical origins of magnetism, for example, quantum spin liquid (QSL) states in which magnetic moments remain disordered at low temperatures despite bein… ▽ More Magnetic materials are composed of the simple building blocks of magnetic moments on a crystal lattice that interact via magnetic exchange. Yet from this simplicity emerges a remarkable diversity of magnetic states. Some reveal the deep quantum mechanical origins of magnetism, for example, quantum spin liquid (QSL) states in which magnetic moments remain disordered at low temperatures despite being strongly correlated through quantum entanglement. A promising theoretical model of a QSL is the Kitaev model, composed of unusual bond-dependent exchange interactions, but experimentally, this model is challenging to realise. Here we show that the material requirements for the Kitaev QSL survive an extended pseudo-edge-sharing superexchange pathway of Ru3+ octahedra within the honeycomb layers of the inorganic framework solid, RuP3SiO11. We confirm the requisite jeff = 1/2 state of Ru3+ in RuP3SiO11 and resolve the hierarchy of exchange interactions that provide experimental access to an unexplored region of the Kitaev model. △ Less

Submitted 14 October, 2024; v1 submitted 28 March, 2024; originally announced March 2024.

Comments: 28 Pages, 5 figures, 1 table

Vanadium-Doped Molybdenum Disulfide Monolayers with Tunable Electronic and Magnetic Properties: Do Vanadium-Vacancy Pairs Matter?

Authors: Da Zhou, Yen Thi Hai Pham, Diem Thi-Xuan Dang, David Sanchez, Aaryan Oberoi, Ke Wang, Andres Fest, Alexander Sredenschek, Mingzu Liu, Humberto Terrones, Saptarshi Das, Dai-Nam Le, Lilia M. Woods, Manh-Huong Phan, Mauricio Terrones

Abstract: Monolayers of molybdenum disulfide (MoS2) are the most studied two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its exceptional optical, electronic, and opto-electronic properties. Recent studies have shown the possibility of incorporating a small amount of magnetic transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic semiconductor (2D-DMS). However, t… ▽ More Monolayers of molybdenum disulfide (MoS2) are the most studied two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its exceptional optical, electronic, and opto-electronic properties. Recent studies have shown the possibility of incorporating a small amount of magnetic transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic semiconductor (2D-DMS). However, the origin of the observed ferromagnetism has remained elusive, due to the presence of randomly generated sulfur vacancies during synthesis that can pair with magnetic dopants to form complex dopant-vacancy configurations altering the magnetic order induced by the dopants. By combining high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with first-principles density functional theory (DFT) calculations and magnetometry data, we demonstrate the critical effects of sulfur vacancies and their pairings with vanadium atoms on the magnetic ordering in V-doped MoS2 (V-MoS2) monolayers. Additionally, we fabricated a series of field effect transistors on these V-MoS2 monolayers and observed the emergence of p-type behavior as the vanadium concentration increased. Our study sheds light on the origin of ferromagnetism in V-MoS2 monolayers and provides a foundation for future research on defect engineering to tune the electronic and magnetic properties of atomically thin TMD-based DMSs. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Giant anisotropy and Casimir phenomena: the case of carbon nanotube metasurfaces

Authors: Pablo Rodriguez-Lopez, Dai-Nam Le, Igor V. Bondarev, Mauro Antezza, Lilia M. Woods

Abstract: The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between any types of objects, the materials or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single wall carbon nanotub… ▽ More The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between any types of objects, the materials or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single wall carbon nanotubes in the dilute limit with their chirality and collective electronic and optical response properties taken into account. It is found that the Casimir interaction is dominated by thermal fluctuations at sub-micron separations, while the torque is primarily determined by quantum mechanical effects. This peculiar quantum vs. thermal separation is attributed to the strong influence of reduced dimensionality and inherent anisotropy of the materials. Our study suggests that nanostructured anisotropic materials can serve as novel platforms to uncover new functionalities in ubiquitous Casimir phenomena. △ Less

Submitted 8 November, 2023; originally announced November 2023.

Comments: 9 pages, 4 figures, submitted to Phys. Rev. B

Journal ref: Phys. Rev. B 109, 035422 (2024)

Role of crystal field ground state in the classical spin-liquid behavior of a quasi-one dimensional spin-chain system Sr3NiPtO6

Authors: V. K. Anand, D. T. Adroja, S. Rayaprol, A. D. Hillier, J. Sannigrahi, M. Rotter, M. D. Le, E. V. Sampathkumaran

Abstract: The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground state. We have investigated the nature of ground state of Sr3NiPtO6 using magnetic susceptibility $χ(T)$, heat capacity $C_{\rm p}(T)$, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS) measurements. The $χ(T)$ and $C_{\rm p}(T)$ do not exhibit any pronounced anomaly that can be associated with a phase transi… ▽ More The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground state. We have investigated the nature of ground state of Sr3NiPtO6 using magnetic susceptibility $χ(T)$, heat capacity $C_{\rm p}(T)$, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS) measurements. The $χ(T)$ and $C_{\rm p}(T)$ do not exhibit any pronounced anomaly that can be associated with a phase transition to a magnetically ordered state. Our $μ$SR data confirm the absence of long-range magnetic ordering down to 0.04 K. Furthermore, the muon spin relaxation rate increases below 20 K and exhibits temperature independent behavior at low temperature, very similar to that observed in a quantum spin-liquid system. The INS data show a large excitation near 8~meV, and the analysis of the INS data reveals a singlet CEF ground state with a first excited CEF doublet state at $Δ_{\rm CEF}$ = 7.7 meV. The estimated CEF parameters reveal a strong planar anisotropy in the calculated $χ(T)$, consistent with the reported behavior of the $χ(T)$ of single crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a large $Δ_{\rm CEF}$ (much larger than the exchange interaction $\mathcal{J}_{\rm ex}$) are responsible for the absence of long-range magnetic ordering and can mimic a classical spin-liquid behavior in this quasi-1D spin chain system Sr3NiPtO6. The classical spin-liquid ground state observed in Sr3NiPtO6 is due to the single-ion property, which is different from the quantum spin-liquid ground state observed in geometrically frustrated systems, where two-ion exchanges play an important role. △ Less

Submitted 24 October, 2023; originally announced October 2023.

Comments: 11 pages, 10 figures, 1 table

Journal ref: Phys. Rev. B 108 (2023) 144426

Nonlinear effects in many-body van der Waals interactions

Authors: Dai-Nam Le, Pablo Rodriguez-Lopez, Lilia M. Woods

Abstract: Van der Waals interactions are ubiquitous and they play an important role for the stability of materials. Current understanding of this type of coupling is based on linear response theory, while optical nonlinearities are rarely considered in this context. Many materials, however, exhibit strong optical nonlinear response, which prompts further evaluation of dispersive forces beyond linear respons… ▽ More Van der Waals interactions are ubiquitous and they play an important role for the stability of materials. Current understanding of this type of coupling is based on linear response theory, while optical nonlinearities are rarely considered in this context. Many materials, however, exhibit strong optical nonlinear response, which prompts further evaluation of dispersive forces beyond linear response. Here we present a $\textit{Discrete Coupled Nonlinear Dipole}$ approach that takes into account linear and nonlinear properties of all dipolar nanoparticles in a given system. This method is based on a Hamiltonian for nonlinear dipoles, which we apply in different systems uncovering a complex interplay of distance, anisotropy, polarizibilities, and hyperpolarizabilities in the vdW energy. This investigation broadens our basic understanding of dispersive interactions, especially in the context of nonlinear materials. △ Less

Submitted 23 June, 2023; originally announced July 2023.

Comments: 7 pages, 3 figures; submitted; a part of this work has been presented at APS March Meeting 2023 (N17.00010)

MSC Class: 81V45; 81V55; 81V70; 81T55

Inelastic neutron scattering investigation of the crystal field excitations of NdCo$_5$

Authors: F. de Almeida Passos, G. J. Nilsen, C. E. Patrick, M. D. Le, G. Balakrishnan, Santosh Kumar, A. Thamizhavel, D. R. Cornejo, J. Larrea Jiménez

Abstract: We present an inelastic neutron scattering study of the crystal electric field levels in the intermetallic ferrimagnets RECo$_{5}$ (RE = Nd and Y). In NdCo$_{5}$, measurements at $5~$K reveal two levels at approximately 28.9 and 52.9 meV. Crystal field calculations including the exchange field $B_{\textrm{exc}}$ from the Co sites account for both of these, as well as the spectrum at temperatures a… ▽ More We present an inelastic neutron scattering study of the crystal electric field levels in the intermetallic ferrimagnets RECo$_{5}$ (RE = Nd and Y). In NdCo$_{5}$, measurements at $5~$K reveal two levels at approximately 28.9 and 52.9 meV. Crystal field calculations including the exchange field $B_{\textrm{exc}}$ from the Co sites account for both of these, as well as the spectrum at temperatures above the spin-reorientation transition at $\sim 280$~K. In particular, it is found that both a large hexagonal crystal field parameter $A_{6}^6\langle r^6 \rangle$ and $B_{\textrm{exc}}$ are required to reproduce the data, with the latter having a much larger value than that deduced from previous computational and experimental studies. Our study sheds light on the delicate interplay of terms in the rare-earth Hamiltonian of RECo$_5$ systems, and is therefore expected to stimulate further experimental and computational work on the broader family of rare-earth permanent magnets. △ Less

Submitted 1 June, 2023; originally announced June 2023.

Weyl metallic state induced by helical magnetic order

Authors: Jian-Rui Soh, Irián Sánchez-Ramírez, Xupeng Yang, Jinzhao Sun, Ivica Zivkovic, J. Alberto Rodríguez-Velamazán, Oscar Fabelo, Anne Stunault, Alessandro Bombardi, Christian Balz, Manh Duc Le, Helen C. Walker, J. Hugo Dil, Dharmalingam Prabhakaran, Henrik M. Rønnow, Fernando de Juan, Maia G. Vergniory, Andrew T. Boothroyd

Abstract: In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetal… ▽ More In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that EuCuAs develops a planar helical structure below $T_\textrm{N}$ = 14.5 K which induces Weyl nodes along the $Γ$--A high symmetry line in the Brillouin zone. △ Less

Submitted 29 April, 2023; originally announced May 2023.

Giant phonon softening and avoided crossing in aliovalence-doped heavy-band thermoelectrics

Authors: Shen Han, Shengnan Dai, Jie Ma, Qingyong Ren, Chaoliang Hu, Ziheng Gao, Manh Duc Le, Denis Sheptyakov, Ping Miao, Shuki Torii, Takashi Kamiyama, Claudia Felser, Jiong Yang, Chenguang Fu, Tiejun Zhu

Abstract: Aliovalent doping has been adopted to optimize the electrical properties of semiconductors, while its impact on the phonon structure and propagation is seldom paid proper attention to. This work reveals that aliovalent doping can be much more effective in reducing the lattice thermal conductivity of thermoelectric semiconductors than the commonly employed isoelectronic alloying strategy. As demons… ▽ More Aliovalent doping has been adopted to optimize the electrical properties of semiconductors, while its impact on the phonon structure and propagation is seldom paid proper attention to. This work reveals that aliovalent doping can be much more effective in reducing the lattice thermal conductivity of thermoelectric semiconductors than the commonly employed isoelectronic alloying strategy. As demonstrated in the heavy-band NbFeSb system, a large reduction of 65% in the lattice thermal conductivity is achieved through only 10% aliovalent Hf-doping, compared to the 4 times higher isoelectronic Ta-alloying. It is elucidated that aliovalent doping introduces free charge carriers and enhances the screening, leading to the giant softening and deceleration of optical phonons. Moreover, the heavy dopant can induce the avoided-crossing of acoustic and optical phonon branches, further decelerating the acoustic phonons. These results highlight the significant role of aliovalent dopants in regulating the phonon structure and suppressing the phonon propagation of semiconductors. △ Less

Submitted 20 March, 2023; originally announced March 2023.

Diffusive Excitonic Bands from Frustrated Triangular Sublattice in a Singlet-Ground-State System

Authors: Bin Gao, Tong Chen, Xiao-Chuan Wu, Michael Flynn, Chunruo Duan, Lebing Chen, Chien-Lung Huang, Jesse Liebman, Shuyi Li, Feng Ye, Matthew B. Stone, Andrey Podlesnyak, Douglas L. Abernathy, Devashibhai T. Adroja, Manh Duc Le, Qingzhen Huang, Andriy H. Nevidomskyy, Emilia Morosan, Leon Balents, Pengcheng Dai

Abstract: Magnetic order in most materials occurs when magnetic ions with finite moments in a crystalline lattice arrange in a particular pattern below the ordering temperature determined by exchange interactions between the ions. However, when the crystal electric field (CEF) effect results in a spin-singlet ground state on individual magnetic sites, the collective ground state of the system can either rem… ▽ More Magnetic order in most materials occurs when magnetic ions with finite moments in a crystalline lattice arrange in a particular pattern below the ordering temperature determined by exchange interactions between the ions. However, when the crystal electric field (CEF) effect results in a spin-singlet ground state on individual magnetic sites, the collective ground state of the system can either remain non-magnetic, or more intriguingly, the exchange interactions between neighboring ions, provided they are sufficiently strong, can admix the excited CEF levels, resulting in a magnetically ordered ground state. The collective magnetic excitations in such a state are so-called spin excitons that describe the CEF transitions propagating through the lattice. In most cases, spin excitons originating from CEF levels of a localized single ion are dispersion-less in momentum (reciprocal) space and well-defined in both the magnetically ordered and paramagnetic states. Here we use thermodynamic and neutron scattering experiments to study stoichiometric Ni2Mo3O8 without site disorder, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has long-range magnetic order. Furthermore, CEF spin excitons from the triangular-lattice arrangement of tetrahedral sites form, in both the antiferromagnetic and paramagnetic states, a dispersive diffusive pattern around the Brillouin zone boundary in reciprocal space. The present work thus demonstrates that spin excitons in an ideal triangular lattice magnet can have dispersive excitations, irrespective of the existence of static magnetic order, and this phenomenon is most likely due to spin entanglement and geometric frustrations. △ Less

Submitted 17 March, 2023; originally announced March 2023.

Retrieval of material properties of monolayer transition-metal dichalcogenides from magnetoexciton energy spectra

Authors: Duy-Nhat Ly, Dai-Nam Le, Duy-Anh P. Nguyen, Ngoc-Tram D. Hoang, Ngoc-Hung Phan, Hoang-Minh L. Nguyen, Van-Hoang Le

Abstract: Reduced exciton mass, polarizability, and dielectric constant of the surrounding medium are essential properties for semiconducting materials, and they have been extracted recently from the magnetoexciton energies. However, the acceptable accuracy of the suggested method requires very high magnetic intensity. Therefore, in the present paper, we propose an alternative method of extracting these mat… ▽ More Reduced exciton mass, polarizability, and dielectric constant of the surrounding medium are essential properties for semiconducting materials, and they have been extracted recently from the magnetoexciton energies. However, the acceptable accuracy of the suggested method requires very high magnetic intensity. Therefore, in the present paper, we propose an alternative method of extracting these material properties from recently available experimental magnetoexciton s-state energies in monolayer transition-metal dichalcogenides (TMDCs). The method is based on the high sensitivity of exciton energies to the material parameters in the Rytova-Keldysh model. It allows us to vary the considered material parameters to get the best fit of the theoretical calculation to the experimental exciton energies for the $1s$, $2s$, and $3s$ states. This procedure gives values of the exciton reduced mass and $2D$ polarizability. Then, the experimental magnetoexciton spectra compared to the theoretical calculation also determine the average dielectric constant. Concrete applications are presented only for monolayers WSe$_2$ and WS$_2$ from the recently available experimental data; however, the presented approach is universal and can be applied to other monolayer TMDCs. The mentioned fitting procedure requires a fast and effective method of solving the Schrödinger equation of an exciton in monolayer TMDCs with a magnetic field. Therefore, we also develop such a method in this paper for highly accurate magnetoexciton energies. △ Less

Submitted 24 April, 2023; v1 submitted 14 March, 2023; originally announced March 2023.

Comments: 10 pages, 5 figures, 4 tables; submitted to Phys. Rev. B

Thickness dependence of superconductivity in FeSe films

Authors: Jia Shi, Duy Le, Volodymyr Turkowski, Naseem Ud Din, Tao Jiang, Qiang Gu, Talat S. Rahman

Abstract: The films of FeSe on substrates have attracted attention because of their unusually high-temperature (Tc) superconducting properties whose origins continue to be debated. To disentangle the competing effects of the substrate and interlayer and intralayer processes, we present here results of density functional theory (DFT)-based analysis of the electronic structure of unsupported FeSe films consis… ▽ More The films of FeSe on substrates have attracted attention because of their unusually high-temperature (Tc) superconducting properties whose origins continue to be debated. To disentangle the competing effects of the substrate and interlayer and intralayer processes, we present here results of density functional theory (DFT)-based analysis of the electronic structure of unsupported FeSe films consisting of 1 to 5 layers (1L-5L). Furthermore, by solving the Bardeen-Schrieffer-Cooper (BCS) equation with spin-wave exchange attraction derived from the Hubbard model, we find the superconducting critical temperature Tc for 1L-5L and bulk FeSe systems in reasonable agreement with experimental data. Our results point to the importance of correlation effects in superconducting properties of single- and multi-layer FeSe films, independently of the role of substrate. △ Less

Submitted 19 February, 2023; originally announced February 2023.

Dark exciton energy splitting in monolayer WSe2: insights from time-dependent density-functional theory

Authors: Jia Shi, Volodymyr Turkowski, Duy Le, Talat S. Rahman

Abstract: We present here a formalism based on time-dependent density-functional theory (TDDFT) to describe characteristics of both intra- and inter-valley excitons in semiconductors, the latter of which had remained a challenge. Through the usage of an appropriate exchange-correlation kernel (nanoquanta), we trace the energy difference between the intra- and inter-valley dark excitons in monolayer (1L) WSe… ▽ More We present here a formalism based on time-dependent density-functional theory (TDDFT) to describe characteristics of both intra- and inter-valley excitons in semiconductors, the latter of which had remained a challenge. Through the usage of an appropriate exchange-correlation kernel (nanoquanta), we trace the energy difference between the intra- and inter-valley dark excitons in monolayer (1L) WSe2 to the domination of the exchange part in the exchange-correlation energies of these states. Furthermore, our calculated transition contribution maps establish the momentum resolved weights of the electron-hole excitations in both bright and dark excitons thereby providing a comprehensive understanding of excitonic properties of 1L WSe2. We find that the states consist of hybridized excitations around the corresponding valleys which leads to brightening of the dark excitons, i.e., significantly decreasing their lifetime which is reflected in the PL spectrum. Using many-body perturbation theory, we calculate the phonon contribution to the energy bandgap and the linewidths of the excited electrons, holes and (bright) exciton to find that as the temperature increases the bandgap significantly decreases, while the linewidths increase. Our work paves for describing the ultrafast charge dynamics of transition metal dichalcogenide within an ab initio framework. △ Less

Submitted 16 February, 2023; originally announced February 2023.

Inelastic neutron scattering study of crystalline electric field excitations in the caged compounds NdT2Zn20 (T = Co, Rh, and Ir)

Authors: Rikako Yamamoto, Manh Duc Le, Devashibhai T. Adroja, Yasuyuki Shimura, Toshiro Takabatake, Takahiro Onimaru

Abstract: We have measured crystalline electric field (CEF) excitations of Nd3+ ions in the two-channel Kondo lattice candidates NdT2Zn20 (T = Co, Rh, and Ir) by means of inelastic neutron scattering (INS). In the INS measurements at 5 K, dispersionless excitations were observed at 3.8 and 7.2 meV for T = Co, 3.1 and 5.8 meV for T = Rh, and 3.0 and 5.3 meV for T = Ir. Analyses of the temperature dependence… ▽ More We have measured crystalline electric field (CEF) excitations of Nd3+ ions in the two-channel Kondo lattice candidates NdT2Zn20 (T = Co, Rh, and Ir) by means of inelastic neutron scattering (INS). In the INS measurements at 5 K, dispersionless excitations were observed at 3.8 and 7.2 meV for T = Co, 3.1 and 5.8 meV for T = Rh, and 3.0 and 5.3 meV for T = Ir. Analyses of the temperature dependence of the INS spectra confirm that the CEF ground states are the Gamma 6 doublet, that is a requisite for manifestation of the magnetic two-channel Kondo effect. For T = Co, a shoulder was observed at 7.7 meV close to the CEF excitation peak centered at 7.2 meV. The shoulder is attributed to a bound state of the CEF and low-lying optical phonon excitations. △ Less

Submitted 20 January, 2023; originally announced January 2023.

Comments: 6 pages, 3 figures

Inelastic Neutron Scattering Study of the Spin Dynamics in the Breathing Pyrochlore System LiGa0.95In0.05Cr4O8

Authors: Yu Tanaka, Rafał Wawrzyńczak, Manh Duc Le, Tatiana Guidi, Yoshihiko Okamoto, Takeshi Yajima, Zenji Hiroi, Masashi Takigawa, Gøran J. Nilsen

Abstract: The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present her… ▽ More The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian. △ Less

Submitted 12 January, 2023; originally announced January 2023.

Journal ref: J. Phys. Soc. Jpn. 87, 073710 (2018)

Soft Anharmonic Coupled Vibrations of Li and SiO4 Enable Li-ion Diffusion in Amorphous Li2Si2O5

Authors: Sajan Kumar, Mayanak K. Gupta, Prabhatasree Goel, Ranjan Mittal, Sanghamitra Mukhopadhyay, Manh Duc Le, Rakesh Shukla, Srungarpu N. Achary, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening c… ▽ More We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening corresponding to the fast Li+ diffusion and relaxation of SiO4 units to the crystalline phase. Our INS measurements clearly show the presence of large phonon density of states (PDOS) at low energy (low-E) in the superionic amorphous phase, which disappear in the non-superionic crystalline phase, corroborating the role of low-E modes in Li+ diffusion. The frustrated energy landscape and host flexibility (due to random orientation and vibrational motion of SiO4 polyhedral units) play an essential role in diffusing the Li+. We used AIMD simulations to identify that these low-E modes involve a large amplitude of Li vibrations coupled with SiO4 vibrations in the amorphous phase. At elevated temperatures, these vibrational dynamics accelerate the Li+ diffusion via a paddle-wheel like coupling mechanism. Above 775 K, these SiO4 vibrational dynamics drive the system into the crystalline phase by locking SiO4 and Li+ into deeper minima of the free energy landscape and disappear in the crystalline phase. Both experiments and simulations provide valuable information about the atomic level stochastic and vibrational dynamics in Li2Si2O5 and their role in Li+ diffusion and vitrification. △ Less

Submitted 17 October, 2022; originally announced October 2022.

Comments: 31 pages, 7 Figures, Supplementary Materials