Showing 1–50 of 374 results for author: Kumar, R

Synergistic Interplay between Surface Polarons and Adsorbates for Photocatalytic Nitrogen Reduction on TiO$_2$(110)

Authors: Manoj Dey, Ritesh Kumar, Abhishek Kumar Singh

Abstract: Photocatalytic nitrogen reduction under ambient conditions represents a promising pathway toward sustainable ammonia production. However, the fundamental mechanisms, particularly the role of photogenerated charge carriers and their interactions with surface defects and adsorbates, remain elusive. Here, we employ density functional theory with Hubbard U corrections and hybrid functionals to demonst… ▽ More Photocatalytic nitrogen reduction under ambient conditions represents a promising pathway toward sustainable ammonia production. However, the fundamental mechanisms, particularly the role of photogenerated charge carriers and their interactions with surface defects and adsorbates, remain elusive. Here, we employ density functional theory with Hubbard U corrections and hybrid functionals to demonstrate that the synergistic interactions between photogenerated electron polarons and point defects are essential for enabling nitrogen reduction on TiO$_2$(110). We reveal that water adsorption promotes polaron migration from subsurface to surface sites, while subsequent water dissociation stabilizes polarons near oxygen vacancies through proton coupled electron polaron transfer (PCEpT). This surface localization of polarons is critical for effective N$_2$ adsorption and activation. Our findings are consistent with previous experimental reports utilizing EPR that confirm the presence of reduced Ti species and STM, which shows the presence of water dimers on the surface. Moreover, the simultaneous interaction between polarons and reaction intermediates facilitates polaron transfer, thereby driving the completion of the nitrogen reduction reaction. Our findings elucidate the pivotal role of surface polarons in photocatalytic nitrogen fixation and provide mechanistic insights applicable to a broad range of oxide surfaces and interfaces capable of hosting small polarons, offering new design principles for efficient photocatalysts operating under ambient conditions. △ Less

Submitted 10 April, 2026; originally announced April 2026.

Immiscible to miscible quenching instabilities in two-dimensional binary Bose-Einstein condensates

Authors: Lauro Tomio, S. Sabari, Arnaldo Gammal, R. K. Kumar

Abstract: Immiscible to miscible quenching transitions (IMQT) in homogeneous Bose-Einstein condensate are investigated, considering rubidium isotopes $^{85}$Rb and $^{87}$Rb confined in a two-dimensional (2D) circular box, under two different initial configurations. These IMQT instabilities, triggered by sudden reductions in the two-body interspecies scattering length $a_{12}$, are explored under two distin… ▽ More Immiscible to miscible quenching transitions (IMQT) in homogeneous Bose-Einstein condensate are investigated, considering rubidium isotopes $^{85}$Rb and $^{87}$Rb confined in a two-dimensional (2D) circular box, under two different initial configurations. These IMQT instabilities, triggered by sudden reductions in the two-body interspecies scattering length $a_{12}$, are explored under two distinct initialconditions, highlighting the critical role of nonlinear dynamics in their evolution. The numerical simulations indicate that the instability dynamics are primarily driven by the production of large vortices and the propagation of sound waves (phonons), with sound wave excitations prevailing in the long-term evolution. The compressible and incompressible parts of the kinetic energy spectra, in terms of the wave number $k$, are confronted with the classical Kolmogorov scaling, $k^{-5/3}$ for turbulence, which is observed in the onset of instabilities. Before reaching the ultraviolet dissipation region at small scales, the IMQT spectra exhibit a bottleneck effect, indicating a clear departure from classical scaling behavior. In the time asymptotic miscible regime, it is observed that the vorticity and sound-wave production remain practically stable. In this regime, for both cases investigated, a linear relation is also recognized between the miscibility parameter and the initial IMQT configuration. △ Less

Submitted 9 April, 2026; originally announced April 2026.

Comments: 14 pages, 8 figures. Contribution to the 9th Asia-Pacific conference on Few-body problems inPhysics

Microscopic contributions to the deviation from Amontons friction law

Authors: Suresh Ravisankar, Ravikant Kumar, Antonio Cammarata, Thilo Glatzel, Tomas Polcar

Abstract: We investigate the nanoscale friction behaviour of MX2 monolayers (M = Mo, W; X = S, Se) on Au(111) and Ag(111) substrates with a silicon tip using classical molecular dynamics simulations with machine-learning-based force fields. This approach enables an accurate description of tip-surface interactions and friction mechanisms at the atomic scale. We observe a pronounced non-monotonic dependence o… ▽ More We investigate the nanoscale friction behaviour of MX2 monolayers (M = Mo, W; X = S, Se) on Au(111) and Ag(111) substrates with a silicon tip using classical molecular dynamics simulations with machine-learning-based force fields. This approach enables an accurate description of tip-surface interactions and friction mechanisms at the atomic scale. We observe a pronounced non-monotonic dependence of the friction force on the applied normal load, indicating a breakdown of Amontons's law at the nanoscale. Analysis of lateral force' signals and their spatial Fourier transforms reveals the coexistence of multiple sliding modes, including longitudinal sliding, lateral slip, and zig-zag motions. We show that the overall friction response is governed by the relative contributions of these motions. While the qualitative features of friction are largely substrate-independent, both the magnitude of friction and the balance between sliding modes depend sensitively on the substrate-monolayer combination. In particular, Au/MoSe2/Si exhibits significantly reduced friction due to suppression of lateral slip motion. Our results indicate that the method is broadly applicable for probing nanoscale friction in related heterostructures. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Heat Capacity-A Powerful Tool for Studying Exotic States of Matter

Authors: K. Ramesh Kumar, Xudong Huai, Michał J. Winiarski, Allen O. Scheie, Thao T. Tran

Abstract: Heat capacity measurements are a powerful tool that researchers rely on when studying the relationship between microscopic degrees of freedom and macroscopic behavior in condensed matter. This uniqueness stems from heat capacity capturing contributions from lattice, electronic, and magnetic components, as well as energy-level populations, enabling an effective approach to studying phase transition… ▽ More Heat capacity measurements are a powerful tool that researchers rely on when studying the relationship between microscopic degrees of freedom and macroscopic behavior in condensed matter. This uniqueness stems from heat capacity capturing contributions from lattice, electronic, and magnetic components, as well as energy-level populations, enabling an effective approach to studying phase transitions and excitations across different classes of materials. However, analyzing heat capacity data presents a common, appreciable challenge for new researchers. Although comprehensive theoretical aspects of heat capacity are presented in several elegant textbooks, practical application remains a daunting task. To overcome this challenge, this tutorial guides researchers in collecting, analyzing, and interpreting heat capacity data in contemporary quantum materials. We outline the connections between thermodynamics, heat capacity, and entropy, as well as measurement methodology and data analysis for representative examples, including phonon dynamics, spin waves, superconductors, magnetic skyrmions, proximate quantum spin liquids, and heavy-fermion materials. Our goal is to provide a concise, accessible guide that enables new researchers to utilize heat capacity as a quantitative lens for understanding exotic states of matter. △ Less

Submitted 14 April, 2026; v1 submitted 13 March, 2026; originally announced March 2026.

Selective braiding of different anyons in the even-denominator fractional quantum Hall effect

Authors: Jehyun Kim, Amit Shaer, Ravi Kumar, Alexey Ilin, Kenji Watanabe, Takashi Taniguchi, Ady Stern, David F. Mross, Yuval Ronen

Abstract: Even-denominator quantum Hall states can host several types of anyons with distinct exchange statistics. Depending on the anyon type, exchanging two quasiparticles can impart a phase to the many-body wave function or even transform it into a different state. Here, we realize a gate-tunable Fabry-Pérot interferometer with an embedded antidot that provides local control over the number of anyons wit… ▽ More Even-denominator quantum Hall states can host several types of anyons with distinct exchange statistics. Depending on the anyon type, exchanging two quasiparticles can impart a phase to the many-body wave function or even transform it into a different state. Here, we realize a gate-tunable Fabry-Pérot interferometer with an embedded antidot that provides local control over the number of anyons within the interference loop. By independently tuning the magnetic field, carrier densities across the device, and the antidot potential, we access regimes in which localized anyons form reproducibly and measure the associated statistical phases $e^{i θ_\mathrm{braid}}$. We resolve braiding phases of $θ_{\mathrm{braid}}=π$ and $θ_{\mathrm{braid}}=\fracπ{2}$, which we attribute to $e/2$ quasiparticles encircling either $e/2$ or $e/4$ quasiparticles, respectively. We further observe switching between different anyon occupancies of the antidot over time, directly resolving individual anyon tunnelling events into the interference loop. Similar behavior occurs at filling factor one third. Our work addresses one of the two key challenges in observing non-Abelian braiding, which requires control of both localized and interfering anyon types. △ Less

Submitted 11 March, 2026; originally announced March 2026.

NMR Determination of the Low-Field Magnetic Structure of the Cu-Based Mineral Rouaite Cu$_2$(OH)$_3$NO$_3$

Authors: Issei Niwata, R. Kumar, Aswathi Mannathanath Chakkingal, Anton A. Kulbakov, Maxim Avdeev, Dmytro S. Inosov, Darren C. Peets, Yoshihiko Ihara

Abstract: Frustrated interactions in the Cu-based mineral rouaite with alternating antiferromagnetic and ferromagnetic spin chains, Cu$_2$(OH)$_3$NO$_3$, introduce non-trivial magnetic ground states and exotic excitations arising from them. We investigated the magnetic structure of Cu$_2$(OH)$_3$NO$_3$ by $^1$H- and $^2$H-NMR measurements on single crystals. The internal fields in the ordered state were mic… ▽ More Frustrated interactions in the Cu-based mineral rouaite with alternating antiferromagnetic and ferromagnetic spin chains, Cu$_2$(OH)$_3$NO$_3$, introduce non-trivial magnetic ground states and exotic excitations arising from them. We investigated the magnetic structure of Cu$_2$(OH)$_3$NO$_3$ by $^1$H- and $^2$H-NMR measurements on single crystals. The internal fields in the ordered state were microscopically measured using the H nuclear moments as a local probe. The directions of the ordered moments were determined by comparing the experimental results to model calculations. The obtained magnetic structure suggests the importance of Dzyaloshinskii-Moriya interactions in stabilizing the low-field magnetic structure. The present result advances the theoretical understanding of the low-field magnetic states and will enable exploration of the exotic magnetic states emerging in high magnetic fields. △ Less

Submitted 2 March, 2026; originally announced March 2026.

Comments: 8 pages, 9 figures

Journal ref: J. Phys. Soc. Japan 95, 044710 (2026)

Van der Waals Antiferromagnets: From Early Discoveries to Future Directions in the 2D Limit

Authors: Rahul Kumar, Je-Geun Park

Abstract: The emergence of a long-range magnetic order in the atomically thin, two-dimensional (2D) limit has long remained a fundamental question in condensed matter physics. The advent of exfoliable van der Waals (vdW) materials, particularly transition-metal phosphorus trisulfides (T MPS3; T M = Fe, Ni, and Mn), provided the first experimental access to this regime and established a foundational platform… ▽ More The emergence of a long-range magnetic order in the atomically thin, two-dimensional (2D) limit has long remained a fundamental question in condensed matter physics. The advent of exfoliable van der Waals (vdW) materials, particularly transition-metal phosphorus trisulfides (T MPS3; T M = Fe, Ni, and Mn), provided the first experimental access to this regime and established a foundational platform for investigating 2D magnetism. The 2016 experimental demonstrations of intrinsic magnetism in monolayer FePS3 provided a platform to test key aspects of 2D Ising criticality in the true 2D limit. It was followed by a rapid growth resulting in a wealth of emergent phenomena arising from the interplay of low-dimensional magnetism and quantum materials. We begin this review with the historical development of vdW antiferromagnets and highlight the key physical insights gained over the past decade. We finish with emerging opportunities in which vdW antiferromagnets can serve as versatile platforms for exploring low-dimensional magnetism and its interplay with other quantum degrees of freedom. △ Less

Submitted 28 February, 2026; originally announced March 2026.

Comments: Journal of Magnetism and Magnetic Materials for its 50th Anniversary Commemorative Special Issue

Exact dimer ground state and quantum phase transitions in a coupled spin ladder

Authors: Manas Ranjan Mahapatra, Rakesh Kumar

Abstract: Spin ladders are key models that act as intermediaries between one-dimensional and two-dimensional spin systems. In this study, we examine a coupled spin-$1/2$ ladder, where frustrated ladders with leg, rung, and diagonal interactions are linked through a horizontal coupling. By introducing a spatially anisotropic third-nearest-neighbor interaction along the horizontal direction, the model was fou… ▽ More Spin ladders are key models that act as intermediaries between one-dimensional and two-dimensional spin systems. In this study, we examine a coupled spin-$1/2$ ladder, where frustrated ladders with leg, rung, and diagonal interactions are linked through a horizontal coupling. By introducing a spatially anisotropic third-nearest-neighbor interaction along the horizontal direction, the model was found to possess an exact dimer ground state, characterized by a product of singlets forming a columnar dimer phase. The model is analyzed using bond-operator mean-field theory (BOMFT) and the density matrix renormalization group (DMRG). BOMFT reveals three distinct phases: a double-stripe ordered phase, a Néel ordered phase, and a quantum disordered dimerized phase. The critical points for the transitions are $J_1 = -0.81$ (double-stripe to dimerized) and $J_1 = 2.81$ (dimerized to Néel phase). DMRG results corroborate the exact ground state and refine the critical points to $J_1 = -0.79$ and $J_1 = 2.29$ for the respective transitions. Additionally, another transition is identified as the Néel order vanishes for $J_1 > 4.5$. The static spin structure factor further corroborates the nature of the ordered phases. △ Less

Submitted 13 February, 2026; originally announced February 2026.

Low magnetic moment and unconventional magneto-transport in half-Heusler alloy CoVGe

Authors: Ravinder Kumar, Jyotiraditya Pandey, Shoaib Akhtar, Sachin Majee, Dibyendu Majee, Samik DuttaGupta, Sachin Gupta

Abstract: In the present work, we experimentally realize CoVGe for the first time and investigate its structural, magnetic, and transport properties, supported by theoretical calculations. The material crystallizes in a cubic structure and exhibits a very low magnetic moment of 0.13 μB per formula unit at 5 K. The temperature dependence of electrical resistivity suggests half-metallic behaviour. Magnetoresi… ▽ More In the present work, we experimentally realize CoVGe for the first time and investigate its structural, magnetic, and transport properties, supported by theoretical calculations. The material crystallizes in a cubic structure and exhibits a very low magnetic moment of 0.13 μB per formula unit at 5 K. The temperature dependence of electrical resistivity suggests half-metallic behaviour. Magnetoresistance shows a positive, non-saturating linear field dependence at low temperature that gradually weakens with increasing temperature. The combination of low magnetic moment and unusual magnetotransport behaviour positions CoVGe as a promising platform for exploring spin-dependent transport in Heusler-based materials. △ Less

Submitted 11 February, 2026; originally announced February 2026.

Comments: 15 pages, 6 figures

Chiral Polar Eu2(SeO3)2(SO4)(H2O)2: A Pathway Toward Narrow Optical Linewidths and Microsecond Lifetimes for Quantum Memory Candidates

Authors: Uchenna Chinaegbomkpa, Ebube Oyeka, Xudong Huai, Ramesh Kumar, Mingli Liang, Jakoah Brgoch, Hugo Sanabria, Thao T. Tran

Abstract: Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate J = 0 transition, which enables precise mapping of optical quantum states into their hyperfine structure for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achiev… ▽ More Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate J = 0 transition, which enables precise mapping of optical quantum states into their hyperfine structure for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achieving narrow spectral linewidths and long optical lifetimes, is a daunting task. Here, we discover Eu2(SeO3)2(SO4)(H2O)2, a rare Eu(III) material that exhibits chiral polar symmetries encompassing both local and global structures. This unique structure is shaped by an appropriate combination of asymmetric ligands. The chirality fosters dipole-dipole interactions and J-mixing, as characterized by second-harmonic generation, photoluminescence, and magnetic susceptibility. The broken inversion symmetry is supported by the phase-matching behavior of second-harmonic generation. The J = 0 transition is observed at 578 nm with a narrow linewidth at 78 K and a microsecond-scale optical lifetime. The analysis of magnetic susceptibility data using Van Vleck theory results in an effective magnetic moment of 3.33 μB/Eu3+ and J-mixing. Heat capacity data reveal underlying phonon dynamics in the material. This study demonstrates a pathway toward realizing new stoichiometric Eu3+ compounds with potential for optically addressable quantum memory applications. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Enhanced Terahertz Photoresponse via Acoustic Plasmon Cavity Resonances in Scalable Graphene

Authors: Domenico De Fazio, Sebastián Castilla, Karuppasamy P. Soundarapandian, Tetiana Slipchenko, Ioannis Vangelidis, Simone Marconi, Riccardo Bertini, Vlad Petrica, Yang Hao, Alessandro Principi, Elefterios Lidorikis, Roshan K. Kumar, Luis Martín-Moreno, Frank H. L. Koppens

Abstract: Precise control and nanoscale confinement of terahertz (THz) fields are essential requirements for emerging applications in photonics, quantum technologies, wireless communications, and sensing. Here, we demonstrate a polaritonic cavity enhanced THz photoresponse in an antenna coupled device based on chemical vapor deposited (CVD) monolayer graphene. The dipole antenna lobes simultaneously serve a… ▽ More Precise control and nanoscale confinement of terahertz (THz) fields are essential requirements for emerging applications in photonics, quantum technologies, wireless communications, and sensing. Here, we demonstrate a polaritonic cavity enhanced THz photoresponse in an antenna coupled device based on chemical vapor deposited (CVD) monolayer graphene. The dipole antenna lobes simultaneously serve as two gate electrodes, concentrate the impinging THz field, and efficiently launch acoustic graphene plasmons (AGPs), which drive a strong photo-thermoelectric (PTE) signal. Between 6 and 90 K, the photovoltage exhibits pronounced peaks, modulating the PTE response by up to 40\%, that we attribute to AGPs forming a Fabry Pérot THz cavity in the full or half graphene channel. Combined full wave and transport thermal simulations accurately reproduce the gate controlled plasmon wavelength, spatial absorption profile, and the resulting nonuniform electron heating responsible for the PTE response. The lateral and vertical maximum confinement factors of the AGP wavelength relative to the incident wavelength are 165 and 4000, respectively, for frequencies from 1.83 to 2.52 THz. These results demonstrate that wafer scalable CVD graphene, without hBN encapsulation, can host coherent AGP resonances and exhibit an efficient polaritonic enhanced photoresponse under appropriate gating, antenna coupling, and AGP cavity design, opening a route to scalable, polarization and frequency selective, liquid nitrogen cooled, and low power consumption THz detection platforms based on plasmon thermoelectric transduction. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Selective Amplification of the Topological Hall Signal in Cr$_2$Te$_3$: The Role of Molecular Exchange Coupling

Authors: Suman Mundlia, Ritesh Kumar, Anshika Mishra, Malavika Chandrasekhar, Narayan Mohanta, Karthik V. Raman

Abstract: Layered magnetic transition-metal chalcogenides (TMCs) are a focal point of research, revealing a variety of intriguing magnetic and topological ground states. Within this family of TMCs, chromium telluride has garnered significant attention because of its excellent tunability in magnetic response, owing to the presence of competing magnetic exchange interactions. We here demonstrate the manipulat… ▽ More Layered magnetic transition-metal chalcogenides (TMCs) are a focal point of research, revealing a variety of intriguing magnetic and topological ground states. Within this family of TMCs, chromium telluride has garnered significant attention because of its excellent tunability in magnetic response, owing to the presence of competing magnetic exchange interactions. We here demonstrate the manipulation of magnetic anisotropy in ultra-thin Cr$_2$Te$_3$ films through growth engineering leading to a controlled transition from in-plane to out-of-plane orientation with an intermediate non-coplanar magnetic ground phase characterized by a topological Hall effect. Moreover, interfacing these films with Vanadyl phthalocyanine (VOPc) molecules prominently enhances the non-coplanar magnetic phase, attributing its presence to the competing interfacial magnetic exchange interactions over the spin-orbit-driven interfacial effects. These findings pave the way for the realization of novel topological spintronic devices through interface-modulated exchange coupling. △ Less

Submitted 30 December, 2025; originally announced December 2025.

Orbitally tuned composite-fermion metal-to-superfluid transitions

Authors: Ravi Kumar, Tomer Firon, André Haug, Misha Yutushui, Alon Ner Gaon, Kenji Watanabe, Takashi Taniguchi, David F. Mross, Yuval Ronen

Abstract: The effective interaction between composite fermions, set entirely by the Coulomb potential and the underlying electronic Landau level orbitals, can stabilize exotic fractional quantum Hall states. In particular, half-filled Landau levels with different orbital character can host either metallic or paired phases of composite fermions. Here, we leverage experimental control over the orbital composi… ▽ More The effective interaction between composite fermions, set entirely by the Coulomb potential and the underlying electronic Landau level orbitals, can stabilize exotic fractional quantum Hall states. In particular, half-filled Landau levels with different orbital character can host either metallic or paired phases of composite fermions. Here, we leverage experimental control over the orbital composition to realize a composite-fermion pairing transition in the first excited Landau level of bilayer graphene. Transport measurements at filling factors v = 9/2 and 11/2 reveal conductive states giving way to well-developed plateaus with increasing displacement fields. These states are insensitive to an in-plane magnetic field, indicating single-component ground states and thus pointing at non-Abelian orders. Our numerical study, based on displacement-field-dependent Landau-level wavefunctions, supports the orbital origin of the pairing transition and suggests Moore-Read or anti-Pfaffian ground states. △ Less

Submitted 24 December, 2025; originally announced December 2025.

Comments: 14 Pages, 12 Figures

Energy-Dependent Magnetic Modifications in HOPG via Microbeam Scanning

Authors: Ram Kumar, Aditya H. Kelkar, Neeraj Shukla, Paras Poswal, Sheshmani Singh

Abstract: Medium-energy ion irradiation is a promising technique for inducing magnetism in materials with partially filled d or f electron bands. This approach enables precise control over the density and spatial distribution of irradiation-induced defects, which play a crucial role in modifying the electronic and magnetic properties of the system. The primary objective of this experiment was to investigate… ▽ More Medium-energy ion irradiation is a promising technique for inducing magnetism in materials with partially filled d or f electron bands. This approach enables precise control over the density and spatial distribution of irradiation-induced defects, which play a crucial role in modifying the electronic and magnetic properties of the system. The primary objective of this experiment was to investigate the influence of ion energy variation on the magnetic properties of highly oriented pyrolytic graphite (HOPG). To achieve this, HOPG samples were irradiated with protons 1-3 MeV and carbon ions 600 keV - 2 MeV. A significant change in the magnetic moment was observed with respect to the irradiation energy for both ion species. The effect of energy variation was analyzed using a vibrating sample magnetometer (VSM) and SRIM simulations. The results demonstrate that ion-beam-induced magnetic ordering strongly depends on both the ion species and the beam energy. Magnetic measurements were performed with varying irradiation energies, showing that carbon ion irradiation produces a higher degree of magnetic ordering compared to proton irradiation at the same dose. The maximum magnetization was obtained at 1.2 MeV carbon ion irradiation. SRIM simulations confirm that carbon ions create a greater number of lattice defects than proton ions, which correlates with the enhanced magnetic response. △ Less

Submitted 20 November, 2025; originally announced November 2025.

Comments: 5 pages, three figures, 2 tables

Nanobubble size controls gas hydrate nucleation in supercooled water

Authors: Ramkhelavan Kanaujiya, Atanu K. Metya, Rajnish Kumar, Tarak K Patra

Abstract: Gas hydrates are crystalline compounds formed when water molecules encapsulate guest gas molecules under high pressure and low temperatures. They have gained significant interest due to their potential as alternative energy resources and their applications in gas storage, transportation, and carbon sequestration. However, the fundamental mechanisms governing their formation, especially the influen… ▽ More Gas hydrates are crystalline compounds formed when water molecules encapsulate guest gas molecules under high pressure and low temperatures. They have gained significant interest due to their potential as alternative energy resources and their applications in gas storage, transportation, and carbon sequestration. However, the fundamental mechanisms governing their formation, especially the influence of gas bubbles, remain poorly understood. In this study, we use molecular dynamics (MD) simulations to examine how methane nanobubble size modulates hydrate formation in supercooled water. Nanobubbles of different sizes are generated by modulating the methane concentration in a methane-water mixture during equilibration under high-temperature and low-pressure conditions, followed by quenching to low temperature and high pressure to induce gas hydrate nucleation and subsequent growth. The simulations reveal a strong correlation between nanobubble size and the extent of hydrate formation. Specifically, the extent of hydrate formation increases with bubble size in the small-to-intermediate regime. However, beyond a critical bubble size threshold, the hydrate formation efficiency declines. The work provides new molecular-level insight into how nanobubble size modulates gas hydrate nucleation and growth dynamics. △ Less

Submitted 10 November, 2025; originally announced November 2025.

Low Temperature Two Fluid State in SmB6

Authors: Sayantan Ghosh, Sugata Paul, Tamoghna Chattoraj, Ritesh Kumar, Zachary Fisk, S. S. Banerjee

Abstract: Comprehensive study using DC transport, specific heat, magnetization, and two-coil mutual inductance measurements unveils an understanding of three temperature regimes in SmB$_6$: (i) $T \geq T^{*}$ ($\sim66$K), (ii) $T_g$ ($\sim40$ K) $\leq T < T^{*}$, and (iii) $T < T_g$. Onset of Kondo breakdown below $T^{*}$ releases disorder-driven magnetic fluctuations, which splits the bulk ($\sim116$K) and… ▽ More Comprehensive study using DC transport, specific heat, magnetization, and two-coil mutual inductance measurements unveils an understanding of three temperature regimes in SmB$_6$: (i) $T \geq T^{*}$ ($\sim66$K), (ii) $T_g$ ($\sim40$ K) $\leq T < T^{*}$, and (iii) $T < T_g$. Onset of Kondo breakdown below $T^{*}$ releases disorder-driven magnetic fluctuations, which splits the bulk ($\sim116$K) and surface Kondo temperature ($T_k^{s} \approx 7$ K). Below $T_g$, as magnetic fluctuations subside, surface Kondo screening revives, stabilizing the topological surface state and generating an in-gap feature ($\sim2.2$ meV) across which Dirac-like carriers are excited. Nyquist impedance analysis reveals a crossover from purely capacitive to capacitive-inductive behavior, signalling a disorder-driven two-fluid phase of heavy quasiparticles and light, high-mobility carriers below $T_g$. We identify a characteristic length scale, $L_{ν_0}(T)$, associated with the high-mobility phase, exhibiting an almost divergent trend below $T_k^{s}$. These findings underscore the complex nature of the surface conducting state in SmB$_6$. △ Less

Submitted 10 November, 2025; originally announced November 2025.

Elemental Frequency-Based Supervised Classification Approach for the Search of Novel Topological Materials

Authors: Zodinpuia Ralte, Ramesh Kumar, Mukhtiyar Singh

Abstract: The machine learning based approaches efficiently solve the goal of searching the best materials candidate for the targeted properties. The search for topological materials using traditional first-principles and symmetry-based methods often requires lots of computing power or is limited by the crystalline symmetries. In this study, we present frequency-based statistical descriptors for machine lea… ▽ More The machine learning based approaches efficiently solve the goal of searching the best materials candidate for the targeted properties. The search for topological materials using traditional first-principles and symmetry-based methods often requires lots of computing power or is limited by the crystalline symmetries. In this study, we present frequency-based statistical descriptors for machine learning-driven topological material's classification that is independent of crystallographic symmetry of wave functions. This approach predicts the topological nature of a material based on its chemical formula. With a balanced dataset of 3910 materials, we have achieved classification accuracies of 82\% with the Support Vector Machine (SVM) model and 83\% with the Random Forest (RF) model, where both models have trained on common frequency based features. We have verified the performances of the models using $5-fold$ cross-validation approach. Further, we have validated the models on a dataset of unseen binary compounds and have efficiently identified 22 common materials using both the models. Next, we implemented the $first-principles$ approach to confirm the topological nature of these predicted materials and found the topological signatures of Dirac, Weyl, and nodal-line semimetallic phases. Therefore, we have demonstrated that the implications of frequency-based descriptors is a practical and less complex way to find novel topological materials with certain physical post-processing filters. This approach lays the groundwork for scalable, data-driven topological property screening of complex materials. △ Less

Submitted 20 September, 2025; v1 submitted 12 September, 2025; originally announced September 2025.

Comments: 10 pages, 9 figures, 6 tables

Evolution from Topological Dirac Metal to Flat-band-Induced Antiferromagnet in Layered KxNi4S2 (0<=x<=1)

Authors: Hengdi Zhao, Xiuquan Zhou, Hyowon Park, Tianqi Deng, Brandon Wilfong, Alann P. Au II, Samuel E. Pate, Craig M. Brown, Hui Wu, Tushar Bhowmick, Tessa McNamee, Ravhi Kumar, Yu-Sheng Chen, Zhi-Li Xiao, Russell Hemley, Weizhao Cai, Shanti Deemyad, Duck-Young Chung, Stephan Rosenkranz, Mercouri G. Kanatzidis

Abstract: Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the… ▽ More Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the Fermi surface can be fine-tuned continuously over a wide range of energies. Consequently, a non-magnetic Dirac-metal state with a topological nontrivial Z2 index of 1;(000), supported by first-principles calculations and high mobility up to 1471 cm2V-1s-1, is observed on the K-rich x = 1 side, whereas a flat-band induced antiferromagnetic state with TN up to 10.1 K emerges as K-content approaches 0. The KxNi4S2 system offers a versatile platform for exploring emerging phenomena and underscores a viable pathway for in-situ control of quantum materials dominated by Dirac cones, flat bands, and their interplay. △ Less

Submitted 11 September, 2025; originally announced September 2025.

CoRuTiGe: A Possible Spin Gapless Semiconductor

Authors: Ravinder Kumar, Tufan Roy, Baisali Ghadai, Rakesh Kumar, Sucheta Mondal, Anil Kumar, Archana Lakhani, Devendra Kumar, Masafumi Shirai, Sachin Gupta

Abstract: We report experimental and theoretical investigations on the quaternary Heusler alloy CoRuTiGe, synthesized using the arc melting technique. Crystal structure analysis reveals a tetragonal structure at room temperature. Magnetization measurements as a function of temperature and magnetic field indicate ferromagnetic nature with a saturation magnetization of 0.681 mB/f.u. at 5 K. The temperature de… ▽ More We report experimental and theoretical investigations on the quaternary Heusler alloy CoRuTiGe, synthesized using the arc melting technique. Crystal structure analysis reveals a tetragonal structure at room temperature. Magnetization measurements as a function of temperature and magnetic field indicate ferromagnetic nature with a saturation magnetization of 0.681 mB/f.u. at 5 K. The temperature dependence of electrical resistivity shows a nearly linear decrease in the high-temperature range, indicating the spin gapless semiconductor like behavior of the material. This SGS nature is further supported by the temperature-independent carrier concentration and mobility. Hall effect analysis reveals that the anomalous Hall effect in CoRuTiGe arises from both intrinsic and extrinsic mechanisms. Additionally, a well-defined symmetric negative magnetoresistance is observed at low temperatures. These findings suggest that CoRuTiGe holds significant promise for spintronic applications. △ Less

Submitted 17 August, 2025; originally announced August 2025.

Comments: 22 pages, 12 figures

Journal ref: Phys. Rev. Materials 10, 034413 (2026)

Light-Addressable Smart Nanostructures via Resonant Nanoheating

Authors: Victor Tabouillot, Douglas Murad, Rahul Kumar, Paula L. Lalaguna, Maryam Hajji, Affar Karimullah, Nikolaj Gadegaard, Aurélie Malfait, Patrice Woisel, Graeme Cooke, Malcolm Kadodwala

Abstract: Selective spatial control of chemical reactions at the level of individual nanostructures remains a significant challenge. We introduce a light-activated system that combines plasmonic gold nanorods with a poly(N-isopropylacrylamide) monolayer to gate surface reactivity based on each rod's geometry under optical illumination. Laser excitation tuned to a rod's plasmon resonance and polarization col… ▽ More Selective spatial control of chemical reactions at the level of individual nanostructures remains a significant challenge. We introduce a light-activated system that combines plasmonic gold nanorods with a poly(N-isopropylacrylamide) monolayer to gate surface reactivity based on each rod's geometry under optical illumination. Laser excitation tuned to a rod's plasmon resonance and polarization collapses the polymer into a compact shell on that rod, blocking reactive head groups and creating a long-lived, kinetically trapped inert state stable for days. During this interval, orthogonal chemical transformations can be performed on adjacent, unilluminated rods without interference. Subsequent diffusion-limited rehydration restores the swollen brush conformation and renews surface activity, effectively erasing the chemical memory. Numerical simulations based on real nanorod geometries confirm that switching selectivity follows the rods' absorption profiles. This mask-free, fully reversible strategy turns passive polymer films into dynamic chemical interfaces, offering a route to high-resolution patterning and on-demand control of nanoscale reactions for electronic and sensing applications. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Superconductivity emerging from the N${é}$el state in ${\it infinite}$-${\it stage}$ single-layer cuprate La$_2$CuO$_{4+δ}$

Authors: Yoshihiko Ihara, Ramender Kumar, Kota Miyakoshi, Migaku Oda, Kenji Ishida

Abstract: In copper oxides (cuprates) with single CuO$_2$ layer such as La$_{2-x}$Ba(Sr)$_x$CuO$_4$, antiferromagnetism coexists with superconductivity at small doping levels $x$, where chemical disorders are significant. Here, we report that superconductivity occurs in a uniform and fully ordered N${é}$el state in a single-layer cuprate La$_2$CuO$_{4+δ}$ with a small amount of excess oxygen $(δ= 0.015)$ as… ▽ More In copper oxides (cuprates) with single CuO$_2$ layer such as La$_{2-x}$Ba(Sr)$_x$CuO$_4$, antiferromagnetism coexists with superconductivity at small doping levels $x$, where chemical disorders are significant. Here, we report that superconductivity occurs in a uniform and fully ordered N${é}$el state in a single-layer cuprate La$_2$CuO$_{4+δ}$ with a small amount of excess oxygen $(δ= 0.015)$ as demonstrated by the $^{139}$La nuclear quadrupole resonance measurement. A uniform oxygen distribution in the crystal is crucial for achieving microscopic phase coexistence and overcoming the miscibility gap associated with the staging instability; self-organized periodic oxygen arrangement driven by mobile oxygen atoms. This finding prompts the reconsideration of superconductivity in cuprates, highlighting that it can emerge in a robust N${é}$el state that retains sizable magnetic moments and hosts only a small carrier density. △ Less

Submitted 27 July, 2025; originally announced July 2025.

Comments: 32 pages, 8 figures

Antibonding and Electronic Instabilities in GdRu2X2 (X = Si, Ge, Sn): A New Pathway Toward Developing Centrosymmetric Skyrmion Materials

Authors: Dasuni N. Rathnaweera, Xudong Huai, K. Ramesh Kumar, Sumanta Tewari, Michał J. Winiarski, Richard Dronskowski, Thao T. Tran

Abstract: Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknow… ▽ More Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknown. Here, we study GdRu2X2 (X = Si, Ge, Sn) as a model system to study the connection among chemical bonding, electronic instability, and the critical temperature and magnetic field at which skyrmions evolve. The nature of the electronic structure of GdRu2X2 is characterized by chemical bonding, Fermi surface analysis, and density of energy function. As X-p orbitals become more extended from Si-3p to Ge-4p and Sn-5p, improved interactions between the Gd spins and the [Ru2X2] conduction layer and increased destabilizing energy contributions are obtained. GdRu2Si2 possesses a Fermi surface nesting (FSN) vector [Q = (q, 0, 0)], whereas GdRu2Ge2 displays two inequivalent FSN vectors [Q = (q, 0, 0); QA = (q, q, 0)] and GdRu2Sn2 features multiple Q vectors. In addition, competing ferromagnetic and antiferromagnetic exchange interactions in the Gd plane become more pronounced as a function of X. These results reveal some correlation among the electronic instability, the competing interaction strength, and the temperature and magnetic field conditions at which the skyrmions emerge. This work demonstrates how chemical bonding and electronic structure enable a new framework for understanding and developing skyrmions under desired conditions that would otherwise be impossible. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Topologically nontrivial multicritical points

Authors: Ranjith R Kumar, Pasquale Marra

Abstract: Recently, the intriguing interplay between topology and quantum criticality has been unveiled in one-dimensional topological chains with extended nearest-neighbor couplings. In these systems, topologically distinct critical phases emerge with localized edge modes despite the vanishing bulk gap. In this work, we study the topological multicritical points at which distinct gapped and critical phases… ▽ More Recently, the intriguing interplay between topology and quantum criticality has been unveiled in one-dimensional topological chains with extended nearest-neighbor couplings. In these systems, topologically distinct critical phases emerge with localized edge modes despite the vanishing bulk gap. In this work, we study the topological multicritical points at which distinct gapped and critical phases intersect. Specifically, we consider a topological chain with coupling up to the third nearest neighbors, which shows stable localized edge modes at the multicritical points. These points possess only nontrivial gapped and critical phases around them and are also characterized by the quadratic dispersion around the gap-closing points. We characterize the topological multicritical points in terms of the topological invariant obtained from the zeros of the complex function associated with the Hamiltonian. Further, we analyze the nature of zeros in the vicinity of the multicritical points by calculating the discriminants of the associated polynomial. The discriminant uniquely identifies the topological multicritical points and distinguishes them from the trivial ones. We finally study the robustness of the zero-energy modes at the multicritical points at weak disorder strengths, and reveal the presence of a topologically nontrivial gapless Anderson-localized phase at strong disorder strengths. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 12 pages, 13 figures

Structural optimization of lattice-matched Sc0.14Al0.86N/GaN superlattices for photonic applications

Authors: Rajendra Kumar, Govardan Gopakumar, Zain Ul Abdin, Michael J. Manfra, Oana Malis

Abstract: ScxAl1-xN is an emerging III-nitride material known for its high piezoelectric coefficient and ferroelectric properties. Integration of wide-bandgap ScxAl1-xN with GaN is particularly attractive for quantum photonic devices. Achieving low defect complex multilayers incorporating ScxAl1-xN, though, requires precise lattice-matching and carefully optimized growth parameters. This study systematicall… ▽ More ScxAl1-xN is an emerging III-nitride material known for its high piezoelectric coefficient and ferroelectric properties. Integration of wide-bandgap ScxAl1-xN with GaN is particularly attractive for quantum photonic devices. Achieving low defect complex multilayers incorporating ScxAl1-xN, though, requires precise lattice-matching and carefully optimized growth parameters. This study systematically investigates the molecular-beam epitaxy of short-period ScxAl1-xN/GaN superlattices with total thicknesses of up to 600 nm on GaN templates. X-ray diffraction reciprocal space mapping confirmed lattice-matching at x = 0.14 Sc composition regardless of the thickness of GaN interlayers, as evidenced by symmetric superlattice satellites aligned in-plane with the underlying substrate peak. Superlattices with Sc compositions deviating from this lattice-matching condition exhibited strain-induced defects ranging from crack formation to partial relaxation. Scanning transmission electron microscopy (STEM) investigation of the ScxAl1-xN/GaN interfaces identified temperature-dependent intermixing as a major factor in setting the nitride composition variation and implicitly band structure profile along the growth direction. Energy-dispersive X-ray spectroscopy also revealed that Sc incorporation exhibits delays relative to Al at both onset and termination. Optimal growth conditions were observed at approximately 600°C and 550°C for superlattices with thick GaN layers (6 nm), and ultra-thin GaN layers (< 2 nm), respectively. △ Less

Submitted 11 July, 2025; originally announced July 2025.

Complex single-site magnetism and magnetotransport in single-crystalline Gd$_{2}$AlSi$_{3}$

Authors: Ram Kumar, Shanta R. Saha, Jarryd Horn, A. Ikeda, Danila Sokratov, Yash Anand, Prathum Saraf, Ryan Dorman, E. Hemley, K. K. Iyer, Johnpierre Paglione

Abstract: We present a detailed investigation of single-crystal samples of the magnetic compound Gd$_{2}$AlSi$_{3}$, which crystallizes in the $α$-ThSi$_2$ type tetragonal structure. We report the temperature and magnetic field dependence of the magnetic susceptibility, magnetization, heat capacity, electrical resistivity, and magnetoresistance for magnetic fields applied along both the tetragonal $c$-axis… ▽ More We present a detailed investigation of single-crystal samples of the magnetic compound Gd$_{2}$AlSi$_{3}$, which crystallizes in the $α$-ThSi$_2$ type tetragonal structure. We report the temperature and magnetic field dependence of the magnetic susceptibility, magnetization, heat capacity, electrical resistivity, and magnetoresistance for magnetic fields applied along both the tetragonal $c$-axis and in the basal $ab$-plane. X-ray diffraction measurements confirm a centrosymmetric, $I4_{1}/amd$ space group of the crystal structure. Despite single-site occupancy of the Gd position in this tetragonal structure, we identify two successive antiferromagnetic phase transitions at Neél temperatures 32~K and 23~K via magnetic susceptibility, heat capacity and transport measurements, as well as a complex magnetic interaction with a magnetic anisotropy that plays an important role in the direction-dependent transport response. Our identification of multiple magnetic phases in Gd$_{2}$AlSi$_{3}$, where Gd is the only magnetic species, helps to elucidate the field-induced skyrmionic behavior in the Gd-based intermetallic compounds. △ Less

Submitted 17 June, 2025; originally announced June 2025.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 111, 214426 (2025)

Erbium-implanted WS2 flakes with room-temperature photon emission at telecom wavelengths

Authors: Guadalupe García-Arellano, Gabriel I. López Morales, Zav Shotan, Raman Kumar, Ben Murdin, Cyrus E. Dreyer, Carlos A. Meriles

Abstract: Optically addressable spin impurities in crystals along with device engineering provide an attractive route to realizing quantum technologies in the solid state, but reconciling disparate emitter and host material constraints for a given target application is often challenging. Rare-earth ions in two-dimensional (2D) materials could mitigate this problem given the atomic-like transitions of the em… ▽ More Optically addressable spin impurities in crystals along with device engineering provide an attractive route to realizing quantum technologies in the solid state, but reconciling disparate emitter and host material constraints for a given target application is often challenging. Rare-earth ions in two-dimensional (2D) materials could mitigate this problem given the atomic-like transitions of the emitters and the versatile nature of van der Waals systems. Here we combine ion implantation, confocal microscopy, and ab-initio calculations to examine the photon emission of Er-doped WS2 flakes. Optical spectroscopy reveals narrow, long-lived photo-luminescence lines in the telecom band, which we activate after low-temperature thermal annealing. Spectroscopic and polarization-selective measurements show a uniform response across the ensemble, while the fluorescence brightness remains mostly unchanged with temperature, suggesting non-radiative relaxation channels are inefficient. Our results create opportunities for novel solid state devices coupling 2D-hosted, telecom-band emitters to photonic heterostructures separately optimized for photon manipulation. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Journal ref: Nano Lett. 25, 9070 (2025)

Magnetic excitations in the 1/3 plateau state in InCu$_3$(OH)$_6$Cl$_3$

Authors: Moyu Kato, Hiroyuki K. Yoshida, R. Kumar, Yoshihiko Ihara

Abstract: Magnetic dynamics in InCu$_3$(OH)$_6$Cl$_3$ was investigated from the NMR relaxation rate measurement. In InCu$_3$(OH)$_6$Cl$_3$, the magnetization isotherm shows a plateau at the 1/3 of full-saturation magnetization, characterizing the 1/3 plateau state. As the 1/3 plateau state appears above 7 T upto 14 T, the microscopic magnetic properties were investigated with the NMR measurement in steady f… ▽ More Magnetic dynamics in InCu$_3$(OH)$_6$Cl$_3$ was investigated from the NMR relaxation rate measurement. In InCu$_3$(OH)$_6$Cl$_3$, the magnetization isotherm shows a plateau at the 1/3 of full-saturation magnetization, characterizing the 1/3 plateau state. As the 1/3 plateau state appears above 7 T upto 14 T, the microscopic magnetic properties were investigated with the NMR measurement in steady fields. The temperature and field dependence of $1/T_1$ measurement reveals a gap in the magnetic excitation spectrum and its evolution with field in the 1/3 plateau state. The field dependence of spin gap provides an important information to understand the microscopic origin of 1/3 plateau state in the kagome antiferromagnets. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Comments: 5 pages, 5 figures

Report number: Published from Journal of the Physical Society of Japan, Vol. 94, 083704

Dilute Paramagnetism and Non-Trivial Topology in Quasicrystal Approximant Fe$_4$Al$_{13}$

Authors: Keenan E. Avers, Jarryd A. Horn, Ram Kumar, Shanta R. Saha, Yuanfeng Xu, B. Andrei Bernevig, Peter Zavalij, Johnpierre Paglione

Abstract: A very fundamental property of both weakly and strongly interacting materials is the nature of its magnetic response. In this work we detail the growth of crystals of the quasicrystal approximant Fe$_4$Al$_{13}$ with an Al flux solvent method. We characterize our samples using electrical transport and heat capacity, yielding results consistent with a simple non-magnetic metal. However, magnetizati… ▽ More A very fundamental property of both weakly and strongly interacting materials is the nature of its magnetic response. In this work we detail the growth of crystals of the quasicrystal approximant Fe$_4$Al$_{13}$ with an Al flux solvent method. We characterize our samples using electrical transport and heat capacity, yielding results consistent with a simple non-magnetic metal. However, magnetization measurements portray an extremely unusual response for a dilute paramagnet and do not exhibit the characteristic Curie-Weiss behavior expected for a weakly interacting material at high temperature. Electronic structure calculations confirm metallic behavior, but also indicate that each isolated band near the Fermi energy hosts non-trivial topologies including strong, weak and nodal components, with resultant topological surface states distinguishable from bulk states on the (001) surface. With half-filled flat bands apparent in the calculation but absence of long-range magnetic order, the unusual paramagnetic response suggests the dilute paramagnetic behavior in this quasicrystal approximant is surprising and may serve as a test of the fundamental assumptions that are taken for granted for the magnetic response of weakly interacting systems. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Comments: 7 pages, 5 figures

Journal ref: Crystals 2025, 15, 485

Single-photon detection enabled by negative differential conductivity in moiré superlattices

Authors: Krystian Nowakowski, Hitesh Agarwal, Sergey Slizovskiy, Robin Smeyers, Xueqiao Wang, Zhiren Zheng, Julien Barrier, David Barcons Ruiz, Geng Li, Riccardo Bertini, Matteo Ceccanti, Iacopo Torre, Bert Jorissen, Antoine Reserbat-Plantey, Kenji Watanabe, Takashi Taniguchi, Lucian Covaci, Milorad V. Milošević, Vladimir Fal'ko, Pablo Jarillo-Herrero, Roshan Krishna Kumar, Frank H. L. Koppens

Abstract: Detecting individual light quanta is essential for quantum information, space exploration, advanced machine vision, and fundamental science. Here, we introduce a novel single photon detection mechanism using highly photosensitive non-equilibrium electron phases in moiré materials. Using tunable bands in bilayer graphene/hexagonal-boron nitride superlattices, we engineer negative differential condu… ▽ More Detecting individual light quanta is essential for quantum information, space exploration, advanced machine vision, and fundamental science. Here, we introduce a novel single photon detection mechanism using highly photosensitive non-equilibrium electron phases in moiré materials. Using tunable bands in bilayer graphene/hexagonal-boron nitride superlattices, we engineer negative differential conductance and a sensitive bistable state capable of detecting single photons. Operating in this regime, we demonstrate single-photon counting at mid-infrared (11.3 microns) and visible wavelengths (675 nanometres) and temperatures up to 25 K. This detector offers new prospects for broadband, high-temperature quantum technologies with CMOS compatibility and seamless integration into photonic integrated circuits (PICs). Our analysis suggests the mechanism underlying our device operation originates from negative differential velocity, and represents an important milestone in the field of high-bias transport in two-dimensional moiré quantum materials. △ Less

Submitted 19 May, 2025; originally announced May 2025.

Nitrogen-Vacancy Magnetometry of Edge Magnetism in WS2 Flakes

Authors: Ilja Fescenko, Raman Kumar, Thitinun Gas-Osoth, Yifei Wang, Suvechhya Lamichhane, Tianlin Li, Adam Erickson, Nina Raghavan, Tom Delord, Cory D. Cress, Nicholas Proscia, Samuel W. LaGasse, Sy-Hwang Liou, Xia Hong, Jose J. Fonseca, Toshu An, Carlos A. Meriles, Abdelghani Laraoui

Abstract: Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature… ▽ More Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature wide-field quantum diamond magnetometry to image pristine and Fe-implanted WS2 flakes of varying thicknesses (45-160 nm), exfoliated from bulk crystals and transferred to NV-doped diamond substrates. We observe direct evidence of edge-localized stray magnetic fields, which scale linearly with applied external magnetic field (4.4-220 mT), reaching up to 4.7 uT. The edge signal shows a limited dependence on the flake thickness, consistent with dipolar field decay and sensing geometry. Magnetic simulations using five alternative models favor the presence of edge magnetization aligned along an axis slightly tilted from the normal to the WS2 flake plane, consistent with spin canting in antiferromagnetically coupled edge states. Our findings establish WS2 as a promising platform for edge-controlled 2D spintronics. △ Less

Submitted 27 July, 2025; v1 submitted 16 May, 2025; originally announced May 2025.

Journal ref: Advanced Functional Materials e12391 (2025)

Al$_2$MnCu: A magnetically ordered member of the Heusler alloy family despite having a valence electron count of 24

Authors: Soumya Bhowmik, Santanu Pakhira, Renu Choudhary, Ravi Kumar, Rajashri Urkude, Biplab Ghosh, D. Bhattacharyya, Maxim Avdeev, Chandan Mazumdar

Abstract: The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,μ_B/f.u.$, where $N_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected as well as experimentally reported to have any magne… ▽ More The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,μ_B/f.u.$, where $N_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected as well as experimentally reported to have any magnetic ordering. Recently, a special class of Heusler alloys with 50\% concentration of $p$-block elements (anti-Heusler) have been identified, although none of such reported compounds belong to the VEC 24 category. Here, we report a new anti-Heusler alloy, Al$_2$MnCu, that undergoes long-range ferromagnetic (FM) ordering with $T_{\rm C}\sim$315 K and a large magnetic moment of $\sim$1.8 $μ_B$/f.u. despite having VEC 24. A phenomenological model based on molecular orbital hybridization is also proposed to understand the magnetism and unusual deviation from the standard S-P rule. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Comments: 11 pages, 14 figures

Journal ref: Phys. Rev. B 111, (2025) 174417

Mitigating parasitic contributions in measured piezoresponse for accurate determination of piezoelectric coefficients in Sc-alloyed-AlN thin films using piezo-response force microscopy

Authors: Ch Kishan Singh, K. Rajalakshmi, N. Balamurugan, Rakesh kumar, Mukul Gupta, R. Ramaseshan, Kiran Baraik

Abstract: We present a methodology to mitigate the effect of the parasitic electrostatic contribution usually present in piezoresponse force microscopy (PFM) measurement for quantitative characterization of polycrystalline piezoelectric thin films using a case study on a set of Al1-xScxN thin films. It involves minimizing the voltage sensitivity of the measured piezoresponse by optimizing the optical lever… ▽ More We present a methodology to mitigate the effect of the parasitic electrostatic contribution usually present in piezoresponse force microscopy (PFM) measurement for quantitative characterization of polycrystalline piezoelectric thin films using a case study on a set of Al1-xScxN thin films. It involves minimizing the voltage sensitivity of the measured piezoresponse by optimizing the optical lever sensitivity using the laser positioning of the beam-bounce system. Additionally, applying a dc-voltage offset (determined through Kelvin probe force microscopy) during PFM scans and positioning the probe over the interior or edge portion of the specimen are explored to minimize the local and non-local electrostatic tip-sample interaction. The results shows that the effective piezoelectric coefficient (d33-eff) of our c-axis oriented wurtzite (wz)-Al1.0Sc0.0N thin film is 4.9 pm per Volt. The highest enhancement in the d33-eff value occurred in the wz-Al0.58Sc0.42N thin film. Above x = 0.42, the d33-eff reduces due to phase-mixing of the wz-Al1-xScxN phase with cubic-Sc3AlN phase till the piezoelectricity finally disappear at x = 0.51 △ Less

Submitted 6 May, 2025; originally announced May 2025.

Comments: 20 pages, 10 figures (some containing multiple figures)

Interaction-driven quantum phase transitions between topological and crystalline orders of electrons

Authors: André Haug, Ravi Kumar, Tomer Firon, Misha Yutushui, Kenji Watanabe, Takashi Taniguchi, David F. Mross, Yuval Ronen

Abstract: Topological and crystalline orders of electrons both benefit from enhanced Coulomb interactions in partially filled Landau levels. In bilayer graphene (BLG), the competition between fractional quantum Hall liquids and electronic crystals can be tuned electrostatically. Applying a displacement field leads to Landau-level crossings, where the interaction potential is strongly modified due to changes… ▽ More Topological and crystalline orders of electrons both benefit from enhanced Coulomb interactions in partially filled Landau levels. In bilayer graphene (BLG), the competition between fractional quantum Hall liquids and electronic crystals can be tuned electrostatically. Applying a displacement field leads to Landau-level crossings, where the interaction potential is strongly modified due to changes in the orbital wave functions. Here, we leverage this control to investigate phase transitions between topological and crystalline orders at constant filling factors in the lowest Landau level of BLG. Using transport measurements in high-quality hBN-encapsulated devices, we study transitions as a function of displacement field near crossings of $N=0$ and $N=1$ orbitals. The enhanced Landau-level mixing near the crossing stabilizes electronic crystals at all fractional fillings, including a resistive state at $ν= \frac{1}{3}$ and a reentrant integer quantum Hall state at $ν= \frac{7}{3}$. On the $N=0$ side, the activation energies of the crystal and fractional quantum Hall liquid vanish smoothly and symmetrically at the transition, while the $N=1$ transitions out of the crystal appear discontinuous. Additionally, we observe quantized plateaus forming near the crystal transition at half filling of the $N=0$ levels, suggesting a paired composite fermion state stabilized by Landau level mixing. △ Less

Submitted 9 March, 2026; v1 submitted 25 April, 2025; originally announced April 2025.

Comments: 15 pages, 7 figures

Journal ref: Phys. Rev. X 16, 011039 (2026)

Novel Heusler Materials for Spintronic Applications: Growth, Characterizations and Applications

Authors: Ravinder Kumar, Sachin Gupta

Abstract: Spintronics is a rapidly evolving technology that utilizes the spin of electrons along with their charge to enable high speed, low power and non volatile electronic devices. The development of novel materials with tailored magnetic and electronic properties is critical to exploit the full potential of spintronic applications. Among these, Heusler alloys stand out due to their tunable multifunction… ▽ More Spintronics is a rapidly evolving technology that utilizes the spin of electrons along with their charge to enable high speed, low power and non volatile electronic devices. The development of novel materials with tailored magnetic and electronic properties is critical to exploit the full potential of spintronic applications. Among these, Heusler alloys stand out due to their tunable multifunctional properties. This review presents a comprehensive overview of various Heusler based materials including half metallic ferromagnets, spin gapless semiconductors, magnetic semiconductors, spin semimetals, and nearly zero moment materials focusing on their synthesis, structural and magnetic characterizations, and transport behavior. The role of crystal structure, and structural disorder in governing their magnetic and electronic properties is discussed in detail. Emphasis is placed on experimental results and their implications for spintronic devices. By bringing together recent advancements, the review highlights the critical role of Heusler alloys in advancing the next-generation spintronic technologies and outlines future directions for their integration in practical applications. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Comments: 44 pages, 17 figures

Spin dynamics and 1/3 magnetization plateau in a coupled distorted diamond chain compound K2Cu3(MoO4)4

Authors: G. Senthil Murugan, J. Khatua, Suyoung Kim, Eundeok Mun, K. Ramesh Babu, Heung-Sik Kim, C. -L. Huang, R. Kalaivanan, U. Rajesh Kumar, I. Panneer Muthuselvam, W. T. Chen, Sritharan Krishnamoorthi, K. -Y. Choi, R. Sankar

Abstract: We investigate magnetic properties of the $s$ = 1/2 compound K$_{2}$Cu$_{3}$(MoO$_{4}$)$_{4}$ by combining magnetic susceptibility, magnetization, specific heat, and electron spin resonance (ESR) with density functional calculations. Its monoclinic structure features alternating Cu$^{2+}$ ($s$ = 1/2) monomers and edge-shared dimers linked by MoO$_{4}$ units, forming a distorted diamond chain along… ▽ More We investigate magnetic properties of the $s$ = 1/2 compound K$_{2}$Cu$_{3}$(MoO$_{4}$)$_{4}$ by combining magnetic susceptibility, magnetization, specific heat, and electron spin resonance (ESR) with density functional calculations. Its monoclinic structure features alternating Cu$^{2+}$ ($s$ = 1/2) monomers and edge-shared dimers linked by MoO$_{4}$ units, forming a distorted diamond chain along the $a$-axis. Antiferromagnetic order occurs at $T_{\rm N}$ = 2.3 K, as evident from a $λ$-type anomaly in specific heat and magnetic susceptibility derivatives. Inverse magnetic susceptibility reveals coexisting ferro- and antiferromagnetic interactions. Specific heat and ESR data show two characteristic temperatures: one at 20 K, associated with spin-singlet formation in Cu$_{2}$O$_{9}$ dimers, and another at 3.68 K, indicating short-range correlations between dimers and monomers. Magnetization measurements reveal a metamagnetic transition at 2.6 T and a critical magnetic field $μ_{0}H_{c}$ = 3.4 T, where a 1/3 magnetization plateau emerges with saturation near 0.35 $μ_{\rm B}$. Low-temperature specific heat and magnetization data reveal the suppression of long-range order at $μ_{0}H_{c}$, enabling the construction of a temperature-magnetic field phase diagram showing multiple magnetic phases near the $μ_{0}H_{c}$. Density functional theory confirms a distorted diamond chain with $J_{1}$ dimers and competing $J_2$, $J_4$, $J_3$, and $J_5$ interactions with monomer spins as an effective low-temperature spin model. △ Less

Submitted 21 April, 2025; originally announced April 2025.

Journal ref: Phys.Rev.B111,144420(2025)

Topological transition between gapless phases in quantum walks

Authors: Ranjith R Kumar, Hideaki Obuse

Abstract: Topological gapless phases of matter have been a recent interest among theoretical and experimental condensed matter physicists. Fermionic chains with extended nearest neighbor couplings have been observed to show unique topological transition at the multicritical points between distinct gapless phases. In this work, we show that such topological gapless phases and the transition between them can… ▽ More Topological gapless phases of matter have been a recent interest among theoretical and experimental condensed matter physicists. Fermionic chains with extended nearest neighbor couplings have been observed to show unique topological transition at the multicritical points between distinct gapless phases. In this work, we show that such topological gapless phases and the transition between them can be simulated in a quantum walk. We consider a three-step discrete-time quantum walk and identify various critical or gapless phases and multicriticalities from the topological phase diagram along with their distinguished energy dispersions. We reconstruct the scaling theory based on the curvature function to study transition between gapless phases in the quantum walk. We show the interesting features observed in fermionic chains, such as diverging, sign flipping and swapping properties of curvature function, can be simulated in the quantum walk. Moreover, the renormalization group flow and Wannier state correlation functions also identify transition at the multicritical points between gapless phases. We observe the scaling law and overlapping of critical and fixed point properties at the multicritical points of the fermionic chains can also be observed in the quantum walk. Furthermore, we categorize the topological transitions at various multicritical points using the group velocity of the energy eigenstates. Finally, the topological characters of various gapless phases are captured using winding number which allows one to distinguish various gapless phases and also show the transitions at the multicritical points. △ Less

Submitted 7 April, 2025; originally announced April 2025.

Comments: 12 pages, 11 figures

Roadmap for Photonics with 2D Materials

Authors: F. Javier García de Abajo, D. N. Basov, Frank H. L. Koppens, Lorenzo Orsini, Matteo Ceccanti, Sebastián Castilla, Lorenzo Cavicchi, Marco Polini, P. A. D. Gonçalves, A. T. Costa, N. M. R. Peres, N. Asger Mortensen, Sathwik Bharadwaj, Zubin Jacob, P. J. Schuck, A. N. Pasupathy, Milan Delor, M. K. Liu, Aitor Mugarza, Pablo Merino, Marc G. Cuxart, Emigdio Chávez-Angel, Martin Svec, Luiz H. G. Tizei, Florian Dirnberger , et al. (123 additional authors not shown)

Abstract: Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combinat… ▽ More Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners. △ Less

Submitted 14 April, 2025; v1 submitted 6 April, 2025; originally announced April 2025.

Comments: 199 pages, 42 figures, 1154 references

Journal ref: ACS Photonics 12, 3961 (2025)

The 2D Materials Roadmap

Authors: Wencai Ren, Peter Bøggild, Joan Redwing, Kostya Novoselov, Luzhao Sun, Yue Qi, Kaicheng Jia, Zhongfan Liu, Oliver Burton, Jack Alexander-Webber, Stephan Hofmann, Yang Cao, Yu Long, Quan-Hong Yang, Dan Li, Soo Ho Choi, Ki Kang Kim, Young Hee Lee, Mian Li, Qing Huang, Yury Gogotsi, Nicholas Clark, Amy Carl, Roman Gorbachev, Thomas Olsen , et al. (48 additional authors not shown)

Abstract: Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and developme… ▽ More Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and development, spanning synthesis, properties and commercial applications. We specifically present roadmaps for high impact 2D materials, including graphene and its derivatives, transition metal dichalcogenides, MXenes as well as their heterostructures and moiré systems. The discussions are organized into thematic sections covering emerging research areas (e.g., twisted electronics, moiré nano-optoelectronics, polaritronics, quantum photonics, and neuromorphic computing), breakthrough applications in key technologies (e.g., 2D transistors, energy storage, electrocatalysis, filtration and separation, thermal management, flexible electronics, sensing, electromagnetic interference shielding, and composites) and other important topics (computational discovery of novel materials, commercialization and standardization). This roadmap focuses on the current research landscape, future challenges and scientific and technological advances required to address, with the intent to provide useful references for promoting the development of 2D materials. △ Less

Submitted 28 April, 2025; v1 submitted 28 March, 2025; originally announced March 2025.

Comments: 97 pages, to be published in 2D Materials

Pressure tuning of Kitaev spin liquid candidate Na$_3$Co$_2$SbO$_6$

Authors: E. H. T. Poldi, R. Tartaglia, G. Fabbris, N. Nguyen, H. Park, Z. Liu, M. van Veenendaal, R. Kumar, G. Jose, S. Samanta, W. Bi, Y. Xiao, D. Popov, Y. Wu, J. -W. Kim, H. Zheng, J. Yan, J. F. Mitchell, R. J. Hemley, D. Haskel

Abstract: The search for Kitaev's quantum spin liquid (KQSL) state in real materials has recently expanded with the prediction that honeycomb lattices of divalent, high-spin cobalt ions could host the dominant bond-dependent exchange interactions required to stabilize the elusive entangled quantum state. The layered honeycomb Na$_3$Co$_2$SbO$_6$ has been singled out as a leading candidate provided that the… ▽ More The search for Kitaev's quantum spin liquid (KQSL) state in real materials has recently expanded with the prediction that honeycomb lattices of divalent, high-spin cobalt ions could host the dominant bond-dependent exchange interactions required to stabilize the elusive entangled quantum state. The layered honeycomb Na$_3$Co$_2$SbO$_6$ has been singled out as a leading candidate provided that the trigonal crystal field acting on Co $3d$ orbitals, which enhances non-Kitaev exchange interactions between $J_{\rm eff}=\frac{1}{2}$ spin-orbital pseudospins, is reduced. We find that applied pressure leads to anisotropic compression of the layered structure, significantly reducing the trigonal distortion of CoO$_6$ octahedra. A strong enhancement of ferromagnetic correlations between pseudospins is observed in the spin-polarized (3 Tesla) phase up to about 60 GPa. Higher pressures drive a spin transition into a low-spin state destroying the $J_{\rm eff}=\frac{1}{2}$ local moments required to map the spin Hamiltonian into Kitaev's model. The spin transition strongly suppresses the low-temperature magnetic susceptibility and appears to stabilize a paramagnetic phase driven by frustration. Although applied pressure fails to realize a KQSL state, the possible emergence of frustrated magnetism of localized, low-spin $S=\frac{1}{2}$ moments opens the door for exploration of novel magnetic quantum states in compressed honeycomb lattices of divalent cobaltates. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Low-Moment Semiconducting Properties of Quaternary Heusler Alloy CoRuTiSn

Authors: Ravinder Kumar, Tufan Roy, Masafumi Shirai, Sachin Gupta

Abstract: We investigate structural, magnetic and transport properties of CoRuTiSn equiatomic quaternary Heusler alloy. CoRuTiSn was synthesized by arc-melt technique. The room temperature powder XRD pattern was analyzed, and it was found that CoRuTiSn has a tetragonal crystal structure. Magnetic measurements show non-zero but small hysteresis indicating CoRuTiSn as a soft ferromagnetic with a Curie tempera… ▽ More We investigate structural, magnetic and transport properties of CoRuTiSn equiatomic quaternary Heusler alloy. CoRuTiSn was synthesized by arc-melt technique. The room temperature powder XRD pattern was analyzed, and it was found that CoRuTiSn has a tetragonal crystal structure. Magnetic measurements show non-zero but small hysteresis indicating CoRuTiSn as a soft ferromagnetic with a Curie temperature of ~200 K. The magnetic moment determined from magnetization data is found to be 0.84 μB/f.u. at 4 K, which is close to the value, calculated using first principles calculations. Electrical resistivity decreases with temperatures, indicating semiconducting nature of CoRuTiSn. Hall effect measurements show anomalous behavior, consistent with the ferromagnetic nature of the sample. The low moment ferromagnetic semiconducting nature of CoRuTiSn could make this material promising for semiconducting spintronics. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Comments: 18 pages, 9 figures

Journal ref: Journal of Alloys and Compounds 1026, 180530 (2025)

In situ engineering hexagonal boron nitride in van der Waals heterostructures with selective SF6 etching

Authors: Hitesh Agarwal, Antoine Reserbat-Plantey, David Barcons Ruiz, Karuppasammy Soundarapandian, Geng Li, Vahagn Mkhitaryan, Johann Osmond, Helena Lozano, Kenji Watanabe, Takashi Taniguchi, Petr Stepanov, Frank. H. L. Koppens, Roshan Krishna Kumar

Abstract: Van der Waals heterostructures are at the forefront in materials heterostructure engineering, offering the ultimate control in layer selectivity and capability to combine virtually any material. Hexagonal boron nitride (hBN), the most commonly used dielectric material, has proven indispensable in this field, allowing the encapsulation of active 2D materials preserving their exceptional electronic… ▽ More Van der Waals heterostructures are at the forefront in materials heterostructure engineering, offering the ultimate control in layer selectivity and capability to combine virtually any material. Hexagonal boron nitride (hBN), the most commonly used dielectric material, has proven indispensable in this field, allowing the encapsulation of active 2D materials preserving their exceptional electronic quality. However, not all device applications require full encapsulation but rather require open surfaces, or even selective patterning of hBN layers. Here, we report on a procedure to engineer top hBN layers within van der Waals heterostructures while preserving the underlying active 2D layers. Using a soft selective SF6 etching combined with a series of pre and post-etching treatments, we demonstrate that pristine surfaces can be exposed with atomic flatness while preserving the active layers electronic quality. We benchmark our technique using graphene encapsulated with hBN Hall bar devices. Using Raman spectroscopy combined with quantum transport, we show high quality can be preserved in etched regions by demonstrating low temperature carrier mobilities of 200,000 cm2Vs-1, ballistic transport probed through magnetic focusing, and intrinsic room temperature phonon-limited mobilities. Atomic force microscopy brooming and O2 plasma cleaning are identified as key pre-etching steps to obtaining pristine open surfaces while preserving electronic quality. The technique provides a clean method for opening windows into mesoscopic van der Waals devices that can be used for local probe experiments, patterning top hBN in situ, and exposing 2D layers to their environment for sensing applications. △ Less

Submitted 18 March, 2025; originally announced March 2025.

Rayleigh-Taylor, Kelvin-Helmholtz and immiscible to miscible quenching instabilities in binary Bose-Einstein condensates

Authors: R. Kishor Kumar, S. Sabari, Arnaldo Gammal, Lauro Tomio

Abstract: We investigate three kinds of instabilities in binary immiscible homogeneous Bose-Einstein condensate, considering rubidium isotopes $^{85}$Rb and $^{87}$Rb confined in two-dimensional circular box. Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) instability types are studied under strong perturbations. Without external perturbation, instabilities are also probed by immiscible to miscible quenching… ▽ More We investigate three kinds of instabilities in binary immiscible homogeneous Bose-Einstein condensate, considering rubidium isotopes $^{85}$Rb and $^{87}$Rb confined in two-dimensional circular box. Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) instability types are studied under strong perturbations. Without external perturbation, instabilities are also probed by immiscible to miscible quenching transition (IMQT), under two different initial configurations. Our numerical simulations show that all such instability dynamics are dominated by large vortex productions and sound-wave (phonon) propagations. For long-term propagation, vortex dynamics become dominant over sound waves in the KH instability, while sound wave excitations predominate in the other cases. For all the dynamical simulations, the emergence of possible scaling laws are investigated for the compressible and incompressible parts of the kinetic energy spectra, in terms of the wave number $k$. The corresponding results are compared with the classical Kolmogorov scalings, $k^{-5/3}$ and $k^{-3}$, for turbulence, which are observed in the kinetic energy spectra at some specific time intervals. Deviating from the classical scaling, a kind of ``Bottleneck effect" is noticed in the IMQT spectra. △ Less

Submitted 7 August, 2025; v1 submitted 17 March, 2025; originally announced March 2025.

Comments: 26 pages, 14 figures, supplemented by 4 mp4 files

Journal ref: Phys. Rev. A 112, 033312 (2025)

Observation of a gapped phase in the one-dimensional $S = {\frac{1}{2}}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr

Authors: Saikat Nandi, Monika Jawale, Sanjay Bachhar, Rahul Kumar, Marlis Schuller, Rabindranath Bag, J. Wilkinson, Jörg Sichelschmidt, A. Sundaresan, Sara Haravifard, N. Büttgen, A. V. Mahajan

Abstract: Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C$_6$H… ▽ More Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C$_6$H$_8$N$_2$ = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^1$H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation ($μ$SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu$^{2+}$ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu$^{2+}$ spins as reflected by a Curie-Weiss temperature of about $-9$ K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the $^1$H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field $μ$SR evidences lack of any static magnetism. △ Less

Submitted 17 October, 2025; v1 submitted 13 March, 2025; originally announced March 2025.

Comments: 14 pages, 12 figures, submitted to Physical Review B

Journal ref: Phys. Rev. B 112 (2025) 134437

Unravelling magnetic vortex-like excitations through rapid thermal quenching in low-carbon steel

Authors: P. C. Mahato, Suprotim Saha, Ritesh Kumar, D. Banik, K. Mondal, Shyam Kumar Choudhary, Ashish Garg, S. S. Banerjee

Abstract: Steel, traditionally valued for its structural strength, emerges in this study as a remarkable material for exploring novel magnetic phenomena. We investigate how common processing techniques-thermal treatments and mechanical strain-significantly affect the magnetic properties of low-carbon steels (0.05 percent by weight). Our findings show that slow annealing enlarges the grain size, enhancing ma… ▽ More Steel, traditionally valued for its structural strength, emerges in this study as a remarkable material for exploring novel magnetic phenomena. We investigate how common processing techniques-thermal treatments and mechanical strain-significantly affect the magnetic properties of low-carbon steels (0.05 percent by weight). Our findings show that slow annealing enlarges the grain size, enhancing magnetic susceptibility, while rapid quenching reduces grain size, resulting in a decreased magnetic response. Quenching low-carbon steel produces significant increase in the fraction of high-angle grain boundaries and a rapid spatial variation of local magnetic anisotropy between grains, a feature which is unachievable with mechanical straining even up to the material's ultimate tensile strength. Tensile-straining of low-carbon steel enhances magnetic susceptibility through altered magnetic anisotropy, contrary to the observed decrease of susceptibility in quenched low-carbon steel. Magnetic force microscopy and micromagnetic modelling of our data reveal that, the reduced magnetic susceptibility in quenched steel is a result of the presence of intriguing magnetic excitations akin to magnetic vortices. These localized structures act as strong magnetic domain wall pinning centres, causing the observed decrease in magnetic susceptibility in these quenched low-carbon steels. Beyond its established structural utility, low-carbon steel combines mechanical stability with favourable magnetic properties, positioning it as a strong platform for magnetic device applications. △ Less

Submitted 9 December, 2025; v1 submitted 11 March, 2025; originally announced March 2025.

Atomically Modulating Competing Exchange Interactions in Centrosymmetric Skyrmion Hosts GdRu2X2 (X = Si, Ge)

Authors: Dasuni N. Rathnaweera, Xudong Huai, K. Ramesh Kumar, Michal J. Winiarski, Tomasz Klimczuk, Allana G. Iwanicki, Satya Kushwaha, Martin Mourigal, Tyrel M. McQueen, Thao T. Tran

Abstract: Magnetic skyrmions are topologically protected spin states enabling high-density, low-power spin electronics. Despite growing efforts to find new skyrmion host systems, the microscopic mechanisms leading to skyrmion phase transitions at specific temperatures and magnetic fields remain elusive. Here, we systematically study the isostructural centrosymmetric magnets- GdRu2X2 (X = Si and Ge), and the… ▽ More Magnetic skyrmions are topologically protected spin states enabling high-density, low-power spin electronics. Despite growing efforts to find new skyrmion host systems, the microscopic mechanisms leading to skyrmion phase transitions at specific temperatures and magnetic fields remain elusive. Here, we systematically study the isostructural centrosymmetric magnets- GdRu2X2 (X = Si and Ge), and the role of X-p orbitals in modifying magnetic exchange interactions. GdRu2Ge2 single crystals, synthesized by arc melting, exhibit two high-entropy pockets associated with skyrmion phases at 0.9 T < H < 1.2 T and 1.3 T < H < 1.7 T, 2 K < T < 30 K-more accessible condition at lower fields and higher temperatures than that in the Si counterpart. Entropy estimations from heat capacity measurements align with magnetization data, and transport studies confirm a topological Hall effect, highlighting the system's nontrivial spin textures and Berry curvature. Compared to GdRu2Si2, electronic structure and exchange interaction evaluations reveal the more extended Ge-4p orbitals enhance competing exchange interactions in GdRu2Ge2, thereby manifesting the rich skyrmion behavior. This work demonstrates how modifying exchange interactions at the atomic level enables the tunability of topologically nontrivial electronic states while advancing our understanding of skyrmion formation mechanisms for future spintronics. △ Less

Submitted 12 August, 2025; v1 submitted 28 February, 2025; originally announced February 2025.

Susceptibility anisotropy and absence of ferroelectric order in the Kitaev spin liquid candidate Na$_2$Co$_2$TeO$_6$

Authors: C. Dhanasekhar, Monika Jawale, Rahul Kumar, D. Chandrasekhar Kakarla, Sagar Mahapatra, Mitch M. C. Chou, A. Sundaresan, H. D. Yang, A. V. Mahajan

Abstract: We report the magnetic, magnetodielectric, and electric polarization properties of single crystals of the Co-based Kitaev Spin Liquid (KSL) candidate Na$_2$Co$_2$TeO$_6$ (NCTO). The sample shows magnetic transitions at 26 K, 16 K, and 5 K, consistent with the literature. The magnetic measurements along and perpendicular to the Co-honeycomb planes show a strong anisotropy in susceptibility and in C… ▽ More We report the magnetic, magnetodielectric, and electric polarization properties of single crystals of the Co-based Kitaev Spin Liquid (KSL) candidate Na$_2$Co$_2$TeO$_6$ (NCTO). The sample shows magnetic transitions at 26 K, 16 K, and 5 K, consistent with the literature. The magnetic measurements along and perpendicular to the Co-honeycomb planes show a strong anisotropy in susceptibility and in Curie-Weiss (C-W) temperatures. The experimental anisotropic C-W temperatures of NCTO qualitatively match with the theoretical C-W temperatures, calculated using the HKTF model [C. Kim \textit{et al.}, J. Phys.: Condens. Matter \textbf{34}, 045802 (2021)]. We find from our temperature- and field-dependent dielectric and pyroelectric ($I_p$) current studies ($H\parallel ab$ and $E\perp ab$) that our single crystal NCTO samples do not have a finite electric polarization below 100 K. These $I_p$ studies confirm the absence of a magnetoelectric coupling and electric polarization properties in the title compound and suggest that the zig-zag AFM structure is more favorable than the triple-$Q$ structure with AFM Kitaev interactions. △ Less

Submitted 3 February, 2025; v1 submitted 3 February, 2025; originally announced February 2025.

Comments: 7 pages, 6 figures, 2 tables

Anomalous temperature-dependent magnetization in the nearly collinear antiferromagnet Y$_2$Co$_3$

Authors: Yunshu Shi, Huibo Cao, Hung-Cheng Wu, Li Yin, Neil Harrison, David S. Parker, Tushar Bhowmick, Tessa McNamee, Fatemeh Safari, Sergey L. Budko, James C. Fettinger, Susan M. Kauzlarich, Peter Klavins, Dmitry Popov, Ravhi Kumar, Russell J. Hemley, Shanti Deemyad, Taku J. Sato, Paul. C. Canfield, Valentin Taufour

Abstract: Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of Y$_2$Co$_3$ to be an A-type AFM str… ▽ More Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of Y$_2$Co$_3$ to be an A-type AFM structure with ferromagnetic (FM) interactions within the distorted kagome plane and an interplane antiferromagnetic interaction, as determined by single-crystal neutron diffraction. The magnetic moments align along the $b$ axis, with minimal canting towards the $c$ axis, at odds with the previous interpretation of bulk magnetization measurements. The magnetic moments on the two distinct Co sites are [0, -0.68(3), 0] $μ_B$ and [0, 1.25(4), 0.07(1)] $μ_B$. We attribute the previously reported "noncollinear" behavior to the considerable temperature dependence of itinerant AFM exchange interactions, induced by thermal contraction along the $b$ axis. Additionally, our examination of lattice constants through pressure studies reveals compensating effects on FM and AFM interactions, resulting in negligible pressure dependence of $T_\textrm{N}$. △ Less

Submitted 26 January, 2025; originally announced January 2025.

Journal ref: Physical Review B 110,235159 (2024)

Evidence for spin liquid behavior in the frustrated three-dimensional $S = 1/2$ Heisenberg garnet NaCa$_{2}$Cu$_{2}$(VO$_{4}$)$_{3}$

Authors: Y. Alexanian, R. Kumar, H. Zeroual, B. Bernu, L. Mangin-Thro, J. R. Stewart, J. M. Wilkinson, S. Bhattacharya, P. L. Paulose, F. Bert, P. Mendels, B. Fåk, E. Kermarrec

Abstract: Three-dimensional quantum spin liquids have remained elusive, hindered by reduced quantum fluctuations from larger lattice connectivity inherent to high-dimensional systems. Here, we investigate the remarkable persistence of dynamical short-range magnetic correlations in the nearly body-centered cubic garnet NaCa$_{2}$Cu$_{2}$(VO$_{4}$)$_{3}$ down to $T = 50$ mK, two orders of magnitude below its… ▽ More Three-dimensional quantum spin liquids have remained elusive, hindered by reduced quantum fluctuations from larger lattice connectivity inherent to high-dimensional systems. Here, we investigate the remarkable persistence of dynamical short-range magnetic correlations in the nearly body-centered cubic garnet NaCa$_{2}$Cu$_{2}$(VO$_{4}$)$_{3}$ down to $T = 50$ mK, two orders of magnitude below its Curie-Weiss temperature. Using a combination of neutron and muon spectroscopies plus numerical simulations, we demonstrate that a dynamical regime emerges, characterized by a dual response in the inelastic spectrum composed of short-live dispersive excitations and a quasi-elastic component. Strongly frustrated exchange interactions combined with subtle temperature-dependent Jahn-Teller spin-lattice effects are a plausible mechanism to explain the origin of this spin-liquid behavior. △ Less

Submitted 3 September, 2025; v1 submitted 8 January, 2025; originally announced January 2025.

Comments: 10 pages, 4 figures. Supplemental Material: 5 pages, 6 figures

Journal ref: Physical Review Materials 9, 074411 (2025)

Photon-photon coupling induced bound state in the continuum and transparency

Authors: Ekta Tunwal, Kuldeep Kumar Shrivastava, Rakesh Kumar Nayak, Ravi Kumar, Somak Bhattacharyya, Rajeev Singh, Biswanath Bhoi

Abstract: This study presents the coherent and dissipative coupling realized in the hybrid photonic resonators that have been achieved via the constructive and destructive interference of the photonic resonator fields with the radiation of a common transmission line fed with microwave photons. In the dissipative coupling regime we have found the coexistence of a peculiar phenomenon bound state in the contin… ▽ More This study presents the coherent and dissipative coupling realized in the hybrid photonic resonators that have been achieved via the constructive and destructive interference of the photonic resonator fields with the radiation of a common transmission line fed with microwave photons. In the dissipative coupling regime we have found the coexistence of a peculiar phenomenon bound state in the continuum (BIC) near the crossing of frequency of the uncoupled resonators by satisfying the Friedrich-Wintgen BICs condition. Again just by rotating one of the samples and with the dynamic adjustment of a parameter we have achieved coupling induced transparency between the photonic resonators. This transition from BIC in the absorption regime to transparency opens avenues for different sorts of plain or programmable oscillators, filters, quantum information processors, sensors etc. △ Less

Submitted 1 January, 2025; originally announced January 2025.

Aharonov-Bohm Interference in Even-Denominator Fractional Quantum Hall States

Authors: Jehyun Kim, Himanshu Dev, Amit Shaer, Ravi Kumar, Alexey Ilin, André Haug, Shelly Iskoz, Kenji Watanabe, Takashi Taniguchi, David F. Mross, Ady Stern, Yuval Ronen

Abstract: Position exchange of non-Abelian anyons affects the quantum state of their system in a topologically-protected way. Their expected manifestations in even-denominator fractional quantum Hall (FQH) systems offer the opportunity to directly study their unique statistical properties in interference experiments. In this work, we present the observation of coherent Aharonov-Bohm interference at two even… ▽ More Position exchange of non-Abelian anyons affects the quantum state of their system in a topologically-protected way. Their expected manifestations in even-denominator fractional quantum Hall (FQH) systems offer the opportunity to directly study their unique statistical properties in interference experiments. In this work, we present the observation of coherent Aharonov-Bohm interference at two even-denominator states in high-mobility bilayer graphene-based van der Waals heterostructures by employing the Fabry-Pérot interferometry (FPI) technique. Operating the interferometer at a constant filling factor, we observe an oscillation period corresponding to two flux quanta inside the interference loop, $ΔΦ=2Φ_0$, at which the interference does not carry signatures of non-Abelian statistics. The absence of the expected periodicity of $ΔΦ=4Φ_0$ may indicate that the interfering quasiparticles carry the charge $e^* = \frac{1}{2}e$ or that interference of $e^* = \frac{1}{4}e$ quasiparticles is thermally smeared. Interestingly, at two hole-conjugate states, we also observe oscillation periods of half the expected value, indicating interference of $e^* = \frac{2}{3}e$ quasiparticles instead of $e^* = \frac{1}{3}e$. To probe statistical phase contributions, we operated the FPI with controlled deviations of the filling factor, thereby introducing fractional quasiparticles inside the interference loop. The resulting changes to the interference patterns at both half-filled states indicate that the additional bulk quasiparticles carry the fundamental charge $e^*=\frac{1}{4}e$, as expected for non-Abelian anyons. △ Less

Submitted 27 December, 2024; originally announced December 2024.

Report number: Nature 649, 323--329 (2026)