Showing 1–50 of 119 results for author: Yin, W

Extreme Nanoconfinement Reshapes the Self-Dissociation of Water

Authors: Chenyu Wang, Wanjian Yin, Ke Zhou

Abstract: Water's ability to self-dissociate into H$_3$O$^+$ and OH$^-$ ions is central to acid-base chemistry and bioenergetics. Recent experimental advances have enabled the confinement of water down to the nanometre scale, even to the single-molecule limit, yet how this process is altered at the extreme nanoconfinement remains unclear. Using \emph{ab-initio} calculations and enhanced-sampling machine-lea… ▽ More Water's ability to self-dissociate into H$_3$O$^+$ and OH$^-$ ions is central to acid-base chemistry and bioenergetics. Recent experimental advances have enabled the confinement of water down to the nanometre scale, even to the single-molecule limit, yet how this process is altered at the extreme nanoconfinement remains unclear. Using \emph{ab-initio} calculations and enhanced-sampling machine-learning potential molecular dynamics, we show that monolayer-confined water exhibits a markedly lower barrier to auto-dissociation than bulk water. Confinement restructures both intramolecular bonding and the intermolecular hydrogen-bond network, while enforcing quasi-2D dipolar correlations that amplify dielectric fluctuations. Our results imply that two-dimensional confined water could act as a \emph{superdielectric} medium and may exhibit \emph{superionic} behavior, as observed in recent experiments. These findings reveal confinement as a powerful route to enhanced proton activity, shedding light on geochemical niches, biomolecular environments, and nanofluidic systems where water's chemistry is fundamentally reshaped. △ Less

Submitted 21 December, 2025; originally announced December 2025.

Fully Compensated Ferrimagnetic Properties of (Cr,Fe)S Compound with a Pyrrhotite-type Structure

Authors: Weida Yin, Masato Miyakawa, Satoshi Semboshi, Noriharu Yodoshi, Akira Masago, Yosuke Kawahito, Tetsuya Fukushima, Hisazumi Akai, Rie Y. Umetsu

Abstract: To optimize the processing conditions for the (Cr,Fe)S non-equilibrium phase with a pyrrhotite-type structure, the phase states and magnetic properties of the specimens obtained at various sintering temperatures were investigated. A slightly off-stoichiometric composition of Cr23Fe23S54 (approximately (Cr,Fe)7S8) sintered and quenched from 1323 K indicates a single-phase pyrrhotite-type structure… ▽ More To optimize the processing conditions for the (Cr,Fe)S non-equilibrium phase with a pyrrhotite-type structure, the phase states and magnetic properties of the specimens obtained at various sintering temperatures were investigated. A slightly off-stoichiometric composition of Cr23Fe23S54 (approximately (Cr,Fe)7S8) sintered and quenched from 1323 K indicates a single-phase pyrrhotite-type structure with a layered-NiAs-type structure in which vacancies occupy every two layers (C12/c1; the space number is 15). The compound shows fully compensated ferrimagnetic behavior at a magnetization compensated temperature of approximately 200 K. The magnetic behavior exhibits a typical N-type ferrimagnet, as predicted by Néel. From X-ray photoelectron spectroscopy analyses, it is found that the compound is composed of Fe2+ and Cr3+. The large magnetic coercivity of 38 kOe at 5 K is also unique and can be applied to spintronic devices. Furthermore, changing the quenching temperature enables control of the degree of order of the vacancies in the interlayer and results in tuning of the magnetization compensated temperature. First-principles calculations show a pseudo-gap located at the Fermi level in the up-spin band, suggesting high spin polarization as well as the NiAs-type structure indicated in the previous our report. △ Less

Submitted 16 December, 2025; originally announced December 2025.

Comments: 8 figures, 3 tables

Competing magnetic phases in Cr$_{3+δ}$Te$_4$ are spatially segregated

Authors: Vivek Bhartiya, Anirban Goswami, Nicholas Ng, Wei Tian, Matthew G. Tucker, Niraj Aryal, Lijun Wu, Weiguo Yin, Yimei Zhu, Milinda Abeykoon, Emmanuel Yakubu, Samaresh Guchhait, J. M. Tranquada

Abstract: Cr$_{1+x}$Te$_2$ is a self-intercalated vdW system that is of current interest for its room-temperature FM phases and tunable topological properties. Early NPD measurements on the monoclinic phase Cr$_3$Te$_4$ ($x=0.5$) presented evidence for competing FM and AFM phases. Here we apply neutron diffraction to a single crystal of Cr$_{3+δ}$Te$_4$ with $δ=-0.10$ and discover that it consists of two di… ▽ More Cr$_{1+x}$Te$_2$ is a self-intercalated vdW system that is of current interest for its room-temperature FM phases and tunable topological properties. Early NPD measurements on the monoclinic phase Cr$_3$Te$_4$ ($x=0.5$) presented evidence for competing FM and AFM phases. Here we apply neutron diffraction to a single crystal of Cr$_{3+δ}$Te$_4$ with $δ=-0.10$ and discover that it consists of two distinct monoclinic phases, one with FM order below $T_{\rm C} \approx 321$ K and another that develops AFM order below $T_{\rm N} \approx 86$ K. In contrast, we find that a crystal with $δ=-0.26$ exhibits only FM order. The single-crystal analysis is complemented by results obtained with NPD, XPD, and TEM measurements on the $δ=-0.10$ composition. From observations of spontaneous magnetostriction of opposite sign at $T_{\rm C}$ and $T_{\rm N}$, along with the TEM evidence for both monoclinic phases in a single thin ($\approx$ 100 nm) grain, we conclude that the two phases must have a fine-grained ($\lesssim$ 100 nm) intergrowth character, as might occur from high-temperature spinodal decomposition during the growth process. Calculations of the relaxed lattice structures for the FM and AFM phases with DFT provide a rationalization of the observed spontaneous magnetostrictions. Correlations between the magnitude and orientation of the magnetic moments with lattice parameter variation demonstrate that the magnetic orders are sensitive to strain, thus explaining why magnetic ordering temperatures and anisotropies can be different between bulk and thin-film samples, when the latter are subject to epitaxial strain. Our results point to the need to investigate the supposed coexistence FM and AFM phases reported elsewhere in the Cr$_{1+x}$Te$_2$ system, such as in the Cr$_5$Te$_8$ phase ($x=0.25$). △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 15 pages, 16 figures

Metamagnetic Transition in Low-Dimensional Site-Decorated Quantum Heisenberg Ferrimagnets

Authors: Weiguo Yin, A. M. Tsvelik

Abstract: The prohibition of finite-temperature phase transition in one-dimensional (1D) Ising models and 1D/2D quantum Heisenberg models with short-range interactions fundamentally constrains the application potentials of low-dimensional magnetic materials. Recently, ultranarrow phase crossover (UNPC), which can approach a transition at a desirable finite temperature $T_0$ arbitrarily closely, was discover… ▽ More The prohibition of finite-temperature phase transition in one-dimensional (1D) Ising models and 1D/2D quantum Heisenberg models with short-range interactions fundamentally constrains the application potentials of low-dimensional magnetic materials. Recently, ultranarrow phase crossover (UNPC), which can approach a transition at a desirable finite temperature $T_0$ arbitrarily closely, was discovered in 1D decorated Ising chains and ladders. Here we present a theoretical study of similarly decorated, yet much more challenging, quantum Heisenberg ferrimagnets in a magnetic field, which features ferromagnetic backbone exchange $J$, antiferromagnetic site-decoration coupling $J_{AF}$, and different magnetic moments for the backbone and decorating spins $μ_aS_a<μ_bS_b$. We exactly solved the model in the large $J$ limit -- as a central-macrospin model -- and found two finite-temperature second-order transitions; just above $T_{c2}$ a ``half-ice, half-fire'' regime appears. Finite-$J$ weak-field results follow from an effective-field mapping, suggesting the emergence of UNPC at finite $T_0$ in 2D square lattices thanks to its exponentially strong initial magnetic susceptibility $χ_0\propto e^{4πS_a^2 J/T_0}$, though less likely in 1D chains where $χ_0\propto J/T_0$. These results may shed light on new technological applications of low-dimensional quantum spin systems and attract experimental and computational tests. △ Less

Submitted 9 November, 2025; originally announced November 2025.

Comments: 5 pages, 3 figures. Supplemental Material (2 pages, 2 figures)

Lattice distortions and non-sluggish diffusion in BCC refractory high entropy alloys

Authors: Jingfeng Zhang, Xiang Xu, Fritz Körmann, Wen Yin, Xi Zhang, Christian Gadelmeier, Uwe Glatzel, Blazej Grabowski, Runxia Li, Gang Liu, Biao Wang, Gerhard Wilde, Sergiy V. Divinski

Abstract: Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for extreme high-temperature applications, for example, in next-generation turbines and nuclear reactors. In such applications, atomic diffusion critically governs essential properties including creep resistance and microstructural stability. The present study systematically investigates impurity diffusion of Co, Mn, and Z… ▽ More Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for extreme high-temperature applications, for example, in next-generation turbines and nuclear reactors. In such applications, atomic diffusion critically governs essential properties including creep resistance and microstructural stability. The present study systematically investigates impurity diffusion of Co, Mn, and Zn in single phase (BCC solid solution) HfTiZrNbTa and HfTiZrNbV RHEAs applying the radiotracer technique. A neutron total scattering technique is used to evaluate the pair distribution functions and element-specific lattice distortions in these alloys. \textit{Ab initio}-based calculations give access to lattice distortions and solubilities of the impurities under investigation, including the impact of short-range order. The diffusion results are discussed in relation to calculated substitutional and interstitial solution energies, local lattice distortions, and short-range order effects. Co diffusion is found to be dominated by the interstitial mechanism, exhibiting fast diffusion. These findings reveal important structure-property relationships between local atomic environments and diffusion kinetics in BCC RHEAs, providing critical insights for designing alloys with enhanced high-temperature performance through targeted control of impurity diffusion processes. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Comments: 18 pages, 8 figures

Journal ref: Acta Materialia 297 (2025) 121283

Orthorhombic nitride perovskite CeTaN3-δ with switchable and robust ferroelectric polarization

Authors: Guozhu Song, Xiangliang Zheng, Xiaodong Yao, Xuefeng Zhou, Chao Gu, Qinghua Zhang, Jian Chen, Chenglu Huang, Tiancheng Yang, Leiming Fang, Ping Miao, Lingxiang Bao, Wen Yin, Xiaohui Yu, Jinlong Zhu, Wei Bao, Yusheng Zhao, Erjia Guo, Shanmin Wang

Abstract: Perovskite-type ternary nitrides with predicted exciting ferroelectricity and many other outstanding properties hold great promise to be an emerging class of advanced ferroelectrics for manufacturing diverse technologically important devices. However, such nitride ferroelectrics have not yet been experimentally identified, mainly due to the challenging sample synthesis by traditional methods at am… ▽ More Perovskite-type ternary nitrides with predicted exciting ferroelectricity and many other outstanding properties hold great promise to be an emerging class of advanced ferroelectrics for manufacturing diverse technologically important devices. However, such nitride ferroelectrics have not yet been experimentally identified, mainly due to the challenging sample synthesis by traditional methods at ambient pressure. Here we report the successful high-pressure synthesis of a high-quality ferroelectric nitride perovskite of CeTaN3-δ with nitrogen deficiency, adopting an orthorhombic Pmn21 polar structure. This material is electrically insulating and exhibits switchable and robust electric polarization for producing ferroelectricity. Furthermore, a number of other extraordinary properties are also revealed in this nitride such as excellent mechanical properties and chemical inertness, which would make it practically useful for many device-relevant applications and fundamentally important for the study of condensed-matter physics. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Single crystalline orthorhombic GdAlGe as a rare earth magnetic Dirac nodal-line metal

Authors: Antu Laha, Juntao Yao, Asish K. Kundu, Niraj Aryal, Anil Rajapitamahuni, Elio Vescovo, Fernando Camino, Kim Kisslinger, Lihua Zhang, Dmytro Nykypanchuk, J. Sears, J. M. Tranquada, Weiguo Yin, Qiang Li

Abstract: Crystal engineering is a method for discovering new quantum materials and phases, which may be achieved by external pressure or strain. Chemical pressure is unique in that it generates internal pressure perpetually to the lattice. As an example, GdAlSi from the rare-earth ($R$) $R$Al$X$ ($X =$ Si or Ge) family of Weyl semimetals is considered. Replacing Si with the larger isovalent element Ge crea… ▽ More Crystal engineering is a method for discovering new quantum materials and phases, which may be achieved by external pressure or strain. Chemical pressure is unique in that it generates internal pressure perpetually to the lattice. As an example, GdAlSi from the rare-earth ($R$) $R$Al$X$ ($X =$ Si or Ge) family of Weyl semimetals is considered. Replacing Si with the larger isovalent element Ge creates sufficiently large chemical pressure to induce a structural transition from the tetragonal structure of GdAlSi, compatible with a Weyl semimetallic state, to an orthorhombic phase in GdAlGe, resulting in an inversion-symmetry-protected nodal-line metal. We find that GdAlGe hosts an antiferromagnetic ground state with two successive orderings, at $T_\mathrm{N1}$ = 35 K and $T_\mathrm{N2}$ = 30 K. In-plane isothermal magnetization shows a magnetic field induced metamagnetic transition at 6.2 T for 2 K. Furthermore, electron-hole compensation gives rise to a large magnetoresistance of $\sim 100\%$ at 2 K and 14 T. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations reveal a Dirac-like linear band dispersion over an exceptionally large energy range of $\sim$ 1.5 eV with a high Fermi velocity of $\sim 10^6$ m/s, a rare feature not observed in any magnetic topological materials. △ Less

Submitted 20 June, 2025; originally announced June 2025.

Comments: 11 pages, 9 figures

Journal ref: Phys. Rev. B 112, 125155 (2025)

Half-ice, half-fire driven ultranarrow phase crossover in 1D decorated q-state Potts ferrimagnets: An AI-co-led exploration

Authors: Weiguo Yin

Abstract: OpenAI's reasoning model o3-mini-high was used to carry out an exact analytic study of onedimensional ferrimagnetic site- and bond-decorated q-state Potts models. We demonstrate that the finitetemperature ultranarrow phase crossover (UNPC), driven by a hidden "half-ice, half-fire" state recently discovered in the $q = 2$ case (Ising model), persists for $q > 2$. We identify unique novel features f… ▽ More OpenAI's reasoning model o3-mini-high was used to carry out an exact analytic study of onedimensional ferrimagnetic site- and bond-decorated q-state Potts models. We demonstrate that the finitetemperature ultranarrow phase crossover (UNPC), driven by a hidden "half-ice, half-fire" state recently discovered in the $q = 2$ case (Ising model), persists for $q > 2$. We identify unique novel features for $q > 2$, including the dome structure in the field-temperature phase diagram and for large $q$ a secondary high-temperature UNPC to the fully disordered paramagnetic state. Moreover, while the crossover temperature $T_0$ in the site-decorated Potts model is independent of the spin interaction $J$ between the backbone spins and thus remains unchanged as the UNPC quickly approaches a genuine transition -- the crossover width is narrowed exponentially -- by enhancing $J$ (referred to as Type-I UNPC), $T_0$ in the bond-decorated Potts model with $q > 2$ depends on $J$ and quickly shifts toward a finite temperature as $J$ increases (referred to as Type-II UNPC). These novel results establish a versatile framework for engineering controlled fast state-flipping switches in low-dimensional systems. Our nine-level AI-contribution rating assigns AI the meritorious status of AI-co-led discovery in this work. △ Less

Submitted 3 December, 2025; v1 submitted 4 May, 2025; originally announced May 2025.

Comments: 8 pages, 7 figures (Fig. 5 and Fig. S1 are new). A sequel to arXiv:2502.11270 and arXiv:2503.23758. The code and data that support the findings of this article are openly available at https://community.wolfram.com/groups/-/m/t/3489942

EuAuSb: An odd-parity helical variation on altermagnetism

Authors: J. Sears, Juntao Yao, Zhixiang Hu, Wei Tian, Niraj Aryal, Weiguo Yin, A. M. Tsvelik, I. A. Zaliznyak, Qiang Li, J. M. Tranquada

Abstract: EuAuSb is a triangular-lattice Dirac semimetal in which a topological Hall effect has been observed to develop in association with a magnetically-ordered phase. Our single-crystal neutron diffraction measurements have identified an incommensurate helical order in which individual ferromagnetic Eu$^{2+}$ layers rotate in-plane by $\sim$120$^{\circ}$ from one layer to the next. An in-plane magnetic… ▽ More EuAuSb is a triangular-lattice Dirac semimetal in which a topological Hall effect has been observed to develop in association with a magnetically-ordered phase. Our single-crystal neutron diffraction measurements have identified an incommensurate helical order in which individual ferromagnetic Eu$^{2+}$ layers rotate in-plane by $\sim$120$^{\circ}$ from one layer to the next. An in-plane magnetic field distorts the incommensurate order, eventually leading to a first order transition to a state that is approximately commensurate and that is continuously polarized as the bulk magnetization approaches saturation. From an analysis of the magnetic diffraction intensities versus field, we find evidence for a dip in the ordered in-plane moment at the same field where the topological Hall effect is a maximum, and we propose that this is due to field-induced quantum spin fluctuations. Our electronic structure calculations yield exchange constants compatible with the helical order and show that the bands near the Fermi level lose their spin degeneracy via a mechanism similar to that in the collinear altermagnets. We find that, unlike the even symmetry seen in the altermagnets, the spin-splitting in EuAuSb has odd-wave symmetry similar to that recently found in a number of coplanar magnetic materials. △ Less

Submitted 8 September, 2025; v1 submitted 30 April, 2025; originally announced May 2025.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. B 112, 094455 (2025)

Exact solution of the frustrated Potts model with next-nearest-neighbor interactions in one dimension via AI bootstrapping

Authors: Weiguo Yin

Abstract: The one-dimensional (1D) $J_1$-$J_2$ $q$-state Potts model is solved exactly for arbitrary $q$ by analytically block-diagonalizing the original $q^2\times q^2$ transfer matrix into a simple $2\times 2$ maximally symmetric subspace, based on using OpenAI's reasoning model o3-mini-high to exactly solve the $q=3$ case. Furthermore, by matching relevant subspaces, we map the Potts model onto a simpler… ▽ More The one-dimensional (1D) $J_1$-$J_2$ $q$-state Potts model is solved exactly for arbitrary $q$ by analytically block-diagonalizing the original $q^2\times q^2$ transfer matrix into a simple $2\times 2$ maximally symmetric subspace, based on using OpenAI's reasoning model o3-mini-high to exactly solve the $q=3$ case. Furthermore, by matching relevant subspaces, we map the Potts model onto a simpler effective 1D $q$-state Potts model, where $J_2$ acts as the nearest-neighbor interaction and $J_1$ as an effective magnetic field, nontrivially generalizing a 56-year-old theorem previously limited to the simplest case ($q=2$, the Ising model). Our exact results provide insights to phenomena such as atomic or electronic order stacking in layered materials and the emergence of dome-shaped phases in complex phase diagrams. This work is anticipated to fuel both research in 1D frustrated magnets for recently discovered finite-temperature application potentials and the fast moving topic area of AI in science. △ Less

Submitted 26 September, 2025; v1 submitted 31 March, 2025; originally announced March 2025.

Comments: The version accepted for publication. 6 pages, 4 figures, plus a 4-page Supplemental Material. The codes and data that support the findings of this article are openly available at https://community.wolfram.com/groups/-/m/t/3466026

Journal ref: Phys. Rev. B 112, 094424 (2025)

Site-Decorated Model for Unconventional Frustrated Magnets: Ultranarrow Phase Crossover and Spin Reversal Transition

Authors: Weiguo Yin

Abstract: The site-decorated Ising model is introduced to advance the understanding and experimental realization of the recently discovered one-dimensional finite-temperature ultranarrow phase crossover in an external magnetic field, while mitigating the geometric complexities of traditional bond-decorated models. Furthermore, although higher-dimensional Ising models in an external field remain unsolved, an… ▽ More The site-decorated Ising model is introduced to advance the understanding and experimental realization of the recently discovered one-dimensional finite-temperature ultranarrow phase crossover in an external magnetic field, while mitigating the geometric complexities of traditional bond-decorated models. Furthermore, although higher-dimensional Ising models in an external field remain unsolved, an exact solution for a novel spin-reversal transition -- driven by an exotic, hidden ``half-ice, half-fire'' state induced by site decoration -- is derived. This transition, triggered by a slight variation in temperature or magnetic field even in the weak-field limit, offers a promising route toward energy-efficient applications such as data storage and processing. The results establish site decoration as a compelling new avenue for materials and device design, particularly in systems such as mixed $d$-$f$ compounds, optical lattices, and neural networks. △ Less

Submitted 3 March, 2025; v1 submitted 16 February, 2025; originally announced February 2025.

Comments: Equation (7), the critical equation that determines $T_0$, has been updated with the version derived by the OpenAI o3-mini-high reasoning model. While equivalent to the original, this equation is more elegant and numerically stable (see the final section of End Matter for more details). 6 pages, 6 figures

Ultralow-temperature heat transport evidence for residual density of states in the superconducting state of CsV3Sb5

Authors: C. C. Zhao, L. S. Wang, W. Xia, Q. W. Yin, H. B. Deng, G. W. Liu, J. J. Liu, X. Zhang, J. M. Ni, Y. Y. Huang, C. P. Tu, Z. C. Tao, Z. J. Tu, C. S. Gong, Z. W. Wang, H. C. Lei, Y. F. Guo, X. F. Yang, J. X. Yin, S. Y. Li

Abstract: The V-based kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) host charge density wave (CDW) and a topological nontrivial band structure, thereby provide a great platform to study the interplay of superconductivity (SC), CDW, frustration, and topology. Here, we report ultralow-temperature thermal conductivity measurements on CsV$_3$Sb$_5$ and Ta-doped Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$… ▽ More The V-based kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) host charge density wave (CDW) and a topological nontrivial band structure, thereby provide a great platform to study the interplay of superconductivity (SC), CDW, frustration, and topology. Here, we report ultralow-temperature thermal conductivity measurements on CsV$_3$Sb$_5$ and Ta-doped Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$ and scanning tunneling microscopy (STM) measurements on CsV$_3$Sb$_5$. The finite residual linear term of thermal conductivity at zero magnetic field suggests the existence of a residual density of states (DOS) in the superconducting state of CsV$_3$Sb$_5$. This is supported by the observation of non-zero conductance at zero bias in STM spectrum at an electronic temperature of 90 mK. However, in Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, which does not have CDW order, there is no evidence for residual DOS. These results show the importance of CDW order for the residual DOS, and a nodal $s$-wave gap or residual Fermi arc may be the origin of the residual DOS in such an unusual multiband kagome superconductor, CsV$_3$Sb$_5$. △ Less

Submitted 24 December, 2024; originally announced December 2024.

Comments: A small part of the contents overlaps with arXiv:2102.08356

Journal ref: Chinese Physics Letters 41, 127303 (2024)

Machine learning approach for vibronically renormalized electronic band structures

Authors: Niraj Aryal, Sheng Zhang, Weiguo Yin, Gia-Wei Chern

Abstract: We present a machine learning (ML) method for efficient computation of vibrational thermal expectation values of physical properties from first principles. Our approach is based on the non-perturbative frozen phonon formulation in which stochastic Monte Carlo algorithm is employed to sample configurations of nuclei in a supercell at finite temperatures based on a first-principles phonon model. A d… ▽ More We present a machine learning (ML) method for efficient computation of vibrational thermal expectation values of physical properties from first principles. Our approach is based on the non-perturbative frozen phonon formulation in which stochastic Monte Carlo algorithm is employed to sample configurations of nuclei in a supercell at finite temperatures based on a first-principles phonon model. A deep-learning neural network is trained to accurately predict physical properties associated with sampled phonon configurations, thus bypassing the time-consuming {\em ab initio} calculations. To incorporate the point-group symmetry of the electronic system into the ML model, group-theoretical methods are used to develop a symmetry-invariant descriptor for phonon configurations in the supercell. We apply our ML approach to compute the temperature dependent electronic energy gap of silicon based on density functional theory (DFT). We show that, with less than a hundred DFT calculations for training the neural network model, an order of magnitude larger number of sampling can be achieved for the computation of the vibrational thermal expectation values. Our work highlights the promising potential of ML techniques for finite temperature first-principles electronic structure methods. △ Less

Submitted 2 September, 2024; originally announced September 2024.

Comments: 17 pages, 7 figures

C-type antiferromagnetic structure of topological semimetal CaMnSb$_2$

Authors: Bo Li, Xu-Tao Zeng, Qianhui Xu, Fan Yang, Junsen Xiang, Hengyang Zhong, Sihao Deng, Lunhua He, Juping Xu, Wen Yin, Xingye Lu, Huiying Liu, Xian-Lei Sheng, Wentao Jin

Abstract: Determination of the magnetic structure and confirmation of the presence or absence of inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$) symmetry is imperative for correctly understanding the topological magnetic materials. Here high-quality single crystals of the layered manganese pnictide CaMnSb$_2$ are synthesized using the self-flux method. De Haas-van Alphen oscillations indicate a… ▽ More Determination of the magnetic structure and confirmation of the presence or absence of inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$) symmetry is imperative for correctly understanding the topological magnetic materials. Here high-quality single crystals of the layered manganese pnictide CaMnSb$_2$ are synthesized using the self-flux method. De Haas-van Alphen oscillations indicate a nontrivial Berry phase of $\sim$ $π$ and a notably small cyclotron effective mass, supporting the Dirac semimetal nature of CaMnSb$_2$. Neutron diffraction measurements identify a C-type antiferromagnetic (AFM) structure below $T\rm_{N}$ = 303(1) K with the Mn moments aligned along the $a$ axis, which is well supported by the density functional theory (DFT) calculations. The corresponding magnetic space group is $Pn'm'a'$, preserving a $\mathcal{P}\times\mathcal{T}$ symmetry. Adopting the experimentally determined magnetic structure, band crossings near the Y point in momentum space and linear dispersions of the Sb $5p_{y,z}$ bands are revealed by the DFT calculations. Furthermore, our study predicts the possible existence of an intrinsic second-order nonlinear Hall effect in CaMnSb$_2$, offering a promising platform to study the impact of topological properties on nonlinear electrical transports in antiferromagnets. △ Less

Submitted 1 April, 2024; originally announced April 2024.

Comments: 7 Pages, 6 figures

Journal ref: Chinese Physics Letters 41, 037104 (2024)

Structural, magnetic and magnetocaloric properties of triangular-lattice transition-metal phosphates

Authors: Chuandi Zhang, Junsen Xiang, Quanliang Zhu, Longfei Wu, Shanfeng Zhang, Juping Xu, Wen Yin, Peijie Sun, Wei Li, Gang Su, Wentao Jin

Abstract: The recent discovery of the spin supersolid candidate Na$_2$BaCo(PO$_4$)$_2$ stimulates numerous research interest on the triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic and magnetocaloric properties of polycrystalline Na$_2$$A$$T$(PO$_4$)$_2$ ($A$ = Ba, Sr; $T$ = Co, Ni, Mn). X-ray and neutron diffraction measurements confirm that N… ▽ More The recent discovery of the spin supersolid candidate Na$_2$BaCo(PO$_4$)$_2$ stimulates numerous research interest on the triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic and magnetocaloric properties of polycrystalline Na$_2$$A$$T$(PO$_4$)$_2$ ($A$ = Ba, Sr; $T$ = Co, Ni, Mn). X-ray and neutron diffraction measurements confirm that Na$_2$Ba$T$(PO$_4$)$_2$ (NB$T$P) crystallizes in a trigonal structure, while Na$_2$Sr$T$(PO$_4$)$_2$ (NS$T$P) forms a monoclinic structure with a slight distortion of the triangular network of $T^{2+}$ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the Néel temperatures of NS$T$P are consistently higher than those of NB$T$P for $T$ = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinic distortions. Further magnetocaloric measurements show that trigonal NB$T$P can reach a lower temperature in the quasi-adiabatic demagnetization process and thus shows a better performance in the magnetic refrigeration, compared with monoclinic NS$T$P. Our findings highlight the outstanding magnetocaloric performances of the trigonal transition-metal phosphates, and disclose two necessary ingredients for a superior magnetic coolant that can reach an ultra-low temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions. △ Less

Submitted 1 April, 2024; originally announced April 2024.

Comments: 10 Pages, 6 figures, accepted for publication in Physical Review Materials

Journal ref: Physical Review Materials 8, 044409 (2024)

Origin of light-induced metastability in ZrTe$_5$

Authors: D. Nevola, N. Aryal, G. D. Gu, P. D. Johnson, W. -G. Yin, Q. Li

Abstract: We study the non-equilibrium electronic structure of a model Dirac semimetal ZrTe$_5$ by using time-and-angle resolved photoemission spectroscopy and density functional theory-based electron and phonon calculations. By measuring the electronic dispersion near the $Γ$ point at time delays up to 10 picoseconds, we discovered that the band spectral weight does not recover during the measured temporal… ▽ More We study the non-equilibrium electronic structure of a model Dirac semimetal ZrTe$_5$ by using time-and-angle resolved photoemission spectroscopy and density functional theory-based electron and phonon calculations. By measuring the electronic dispersion near the $Γ$ point at time delays up to 10 picoseconds, we discovered that the band spectral weight does not recover during the measured temporal window, revealing the existence of light induced metastable state in the electronic structure of this material. Our calculations find that the photoexcited $A_{1g}$ phonon mode lead to a band renormalization that both supports our experimental observations at the zone center and predicts changes to the band structure outside of our experimental window, ultimately showing the evolution from a direct to an indirect gap semimetal; such band renormalization dramatically reduces the electron-hole recombination rate giving rise to the metastability in this system. △ Less

Submitted 13 March, 2024; originally announced March 2024.

Comments: 6 pages, 4 figures

Design of 2D Skyrmionic Metamaterial Through Controlled Assembly

Authors: Qichen Xu, Zhuanglin Shen, Alexander Edström, I. P. Miranda, Zhiwei Lu, Anders Bergman, Danny Thonig, Wanjian Yin, Olle Eriksson, Anna Delin

Abstract: Despite extensive research on magnetic skyrmions and antiskyrmions, a significant challenge remains in crafting nontrivial high-order skyrmionic textures with varying, or even tailor-made, topologies. We address this challenge, by focusing on a construction pathway of skyrmionic metamaterials within a monolayer thin film and suggest several skyrmionic metamaterials that are surprisingly stable, i.… ▽ More Despite extensive research on magnetic skyrmions and antiskyrmions, a significant challenge remains in crafting nontrivial high-order skyrmionic textures with varying, or even tailor-made, topologies. We address this challenge, by focusing on a construction pathway of skyrmionic metamaterials within a monolayer thin film and suggest several skyrmionic metamaterials that are surprisingly stable, i.e., long-lived, due to a self-stabilization mechanism. This makes these new textures promising for applications. Central to our approach is the concept of 'simulated controlled assembly', in short, a protocol inspired by 'click chemistry' that allows for positioning topological magnetic structures where one likes, and then allowing for energy minimization to elucidate the stability. Utilizing high-throughput atomistic-spin-dynamic simulations alongside state-of-the-art AI-driven tools, we have isolated skyrmions (topological charge Q=1), antiskyrmions (Q=-1), and skyrmionium (Q=0). These entities serve as foundational 'skyrmionic building blocks' to form the here reported intricate textures. In this work, two key contributions are introduced to the field of skyrmionic systems. First, we present a a novel combination of atomistic spin dynamics simulations and controlled assembly protocols for the stabilization and investigation of new topological magnets. Second, using the aforementioned methods we report on the discovery of skyrmionic metamaterials. △ Less

Submitted 13 January, 2025; v1 submitted 16 February, 2024; originally announced February 2024.

On the origin of metal-insulator transitions in the parent compounds of ruthenium-pnictide superconductors

Authors: Niraj Aryal, Emil S. Bozin, Weiguo Yin

Abstract: We study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombi… ▽ More We study the interplay of the structural phase transition, flat electronic band dispersion, and metal-to-insulator transition (MIT) in the parent compounds of the Ru-pnictide superconductors by using first-principles calculations. Our electron and phonon calculations reveal that Ru(P,As) undergo MIT accompanied by orthorhombic to monoclinic distortion at low temperature, but RuSb stays orthorhombic and metallic in agreement with the experimental findings. We find that although small monoclinic distortion can remove the van Hove singularity at the Fermi level, it does not immediately gap out the Fermi surface and a large value of monoclinic distortion is necessary for a clear MIT suggesting the possibility of an intermediate pseudogapped monoclinic metallic phase. Furthermore, we predict a light-induced two-step insulator-to-metal and structural transitions in the monoclinic phases of RuP and RuAs, which can be tested in future ultrafast pump-probe experiments as an alternative ideal play ground to VO$_2$. △ Less

Submitted 27 January, 2024; originally announced January 2024.

Phase Switch Driven by the Hidden Half-Ice, Half-Fire State in a Ferrimagnet

Authors: Weiguo Yin, A. M. Tsvelik

Abstract: The notion of "half fire, half ice" was recently introduced to describe an exotic macroscopic ground-state degeneracy emerging in a ferrimagnet under the critical magnetic field, in which the "hot" spins are fully disordered on the sublattice with smaller magnetic moments and the "cold" spins are fully ordered on the sublattice with larger magnetic moments. Here we further point out that this stat… ▽ More The notion of "half fire, half ice" was recently introduced to describe an exotic macroscopic ground-state degeneracy emerging in a ferrimagnet under the critical magnetic field, in which the "hot" spins are fully disordered on the sublattice with smaller magnetic moments and the "cold" spins are fully ordered on the sublattice with larger magnetic moments. Here we further point out that this state has a twin named "half ice, half fire" in which the hot and cold spins switch positions. The new state is an excited state -- thus hidden in the ground-state phase diagram -- and is robust with respect to the interactions that destroy the half fire, half ice state. We demonstrate with exact results how this hidden state can drive phase switching at desirable finite temperature, even for the one-dimensional Ising model where phase transition at finite temperature is forbidden. We suggest that our findings may open a new door to the understanding and controlling of phase competition and transition in unconventional frustrated systems. △ Less

Submitted 29 December, 2024; v1 submitted 1 January, 2024; originally announced January 2024.

Comments: The third paper in a trilogy. 6 pages, 3 figures. SM.pdf enclosed in the zip file contains Supplemental Material with 2 figures

Report number: Code and data are available at https://community.wolfram.com/groups/-/m/t/3530595

Journal ref: Phys. Rev. Lett. 133, 266701 (2024)

Electronic structure, magnetic and transport properties of antiferromagnetic Weyl semimetal GdAlSi

Authors: Antu Laha, Asish K. Kundu, Niraj Aryal, Emil S. Bozin, Juntao Yao, Sarah Paone, Anil Rajapitamahuni, Elio Vescovo, Tonica Valla, Milinda Abeykoon, Ran Jing, Weiguo Yin, Abhay N. Pasupathy, Mengkun Liu, Qiang Li

Abstract: We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric unde… ▽ More We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of ~ 1310 $Ω^{-1}$cm$^{-1}$ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (~ 155 $Ω^{-1}$cm$^{-1}$). Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy. △ Less

Submitted 21 December, 2023; originally announced December 2023.

Comments: 10 pages, 6 figures, Accepted for publication in Phys. Rev. B

Paradigm for approaching the forbidden phase transition in the one-dimensional Ising model at fixed finite temperature: Single chain in a magnetic field

Authors: Weiguo Yin

Abstract: In a previous paper [Weiguo Yin, Phys. Rev. Res. 6, 013331 (2024)], the forbidden spontaneous phase transition in the one-dimensional Ising model was found to be approachable arbitrarily closely in decorated ladders by ultranarrow phase crossover (UNPC) at a given finite temperature $T_0$ with the crossover width $2δT$ reduced exponentially, which resembles a genuine first-order transition with la… ▽ More In a previous paper [Weiguo Yin, Phys. Rev. Res. 6, 013331 (2024)], the forbidden spontaneous phase transition in the one-dimensional Ising model was found to be approachable arbitrarily closely in decorated ladders by ultranarrow phase crossover (UNPC) at a given finite temperature $T_0$ with the crossover width $2δT$ reduced exponentially, which resembles a genuine first-order transition with large latent heat. Here, I reveal that the forbidden phase transition can be approached at fixed $T_0$ as well in decorated single-chain Ising models in the presence of a magnetic field, in which $T_0$ is determined by the interactions involving only the decorated parts and the magnetic field, while $2δT$ is independently, exponentially reduced ($δT=0$ means a genuine transition) by restoring the ferromagnetic interaction between the ordinary spins on the chain backbone -- which was neglected in the previous studies of pseudotransition -- thus manifesting that this asymptoticity to the forbidden transition is essentially the buildup of coherence in preformed crossover of local states. Furthermore, I show that the UNPC can be realized even in the absence of the conventional geometric frustration because the magnetic field itself can induce previously unnoticed hidden spin frustration. These findings make the doors wide open to the engineering and utilization of UNPC as a new paradigm for exploring exotic phenomena and 1D device applications. △ Less

Submitted 10 June, 2024; v1 submitted 18 December, 2023; originally announced December 2023.

Comments: 23 pages, 8 figures

Journal ref: Phys. Rev. B 109, 214413 (2024)

Dilution induced magnetic localization in Rb(Co$_{1-x}$Ni$_{x}$)$_{2}$Se$_{2}$ single crystals

Authors: H. Liu, M. W. Huo, C. X. Huang, X. Huang, H. L. Sun, L. Chen, J. P. Xu, W. Yin, R. X. Li, M. Wang

Abstract: We report experimental studies on a series of Rb(Co$_{1-x}$Ni$_{x}$)$_{2}$Se$_{2}$ (0.02 $\leq x \leq $ 0.9) powder and single crystal samples using x-ray diffraction, neutron diffraction, magnetic susceptibility, and electronic transport measurements. All compositions are metallic and adopt the body-centered tetragonal structure with $I4/mmm$ space group. Anisotropic magnetic susceptibilities mea… ▽ More We report experimental studies on a series of Rb(Co$_{1-x}$Ni$_{x}$)$_{2}$Se$_{2}$ (0.02 $\leq x \leq $ 0.9) powder and single crystal samples using x-ray diffraction, neutron diffraction, magnetic susceptibility, and electronic transport measurements. All compositions are metallic and adopt the body-centered tetragonal structure with $I4/mmm$ space group. Anisotropic magnetic susceptibilities measured on single crystal samples suggest that Rb(Co$_{1-x}$Ni$_{x}$)$_{2}$Se$_{2}$ undergo an evolution from ferromagnetism to antiferromagnetism, and finally to paramagnetism with increasing Ni concentration. Neutron diffraction measurements on the samples with $x$ = 0.1, 0.4, and 0.6 reveal an $A$-type antiferromagnetic order with moments lying in the $ab$ plane. The moment size changes from 0.69 ($x=0.1$) to 2.80$μ_B$ ($x=0.6$) per Co ions. Our results demonstrate that dilution of the magnetic Co ions by substitution of nonmagnetic Ni ions induces magnetic localization and evolution from itinerant to localized magnetism in Rb(Co$_{1-x}$Ni$_{x}$)$_{2}$Se$_{2}$. △ Less

Submitted 1 March, 2023; originally announced March 2023.

Global optimization in the discrete and variable-dimension conformational space: The case of crystal with the strongest atomic cohesion

Authors: Guanjian Cheng, Xin-Gao Gong, Wan-Jian Yin

Abstract: We introduce a computational method to optimize target physical properties in the full configuration space regarding atomic composition, chemical stoichiometry, and crystal structure. The approach combines the universal potential of the crystal graph neural network and Bayesian optimization. The proposed approach effectively obtains the crystal structure with the strongest atomic cohesion from all… ▽ More We introduce a computational method to optimize target physical properties in the full configuration space regarding atomic composition, chemical stoichiometry, and crystal structure. The approach combines the universal potential of the crystal graph neural network and Bayesian optimization. The proposed approach effectively obtains the crystal structure with the strongest atomic cohesion from all possible crystals. Several new crystals with high atomic cohesion are identified and confirmed by density functional theory for thermodynamic and dynamic stability. Our method introduces a novel approach to inverse materials design with additional functional properties for practical applications. △ Less

Submitted 27 February, 2023; originally announced February 2023.

Machine-learning the spectral function of a hole in a quantum antiferromagnet

Authors: Jackson Lee, Matthew R. Carbone, Weiguo Yin

Abstract: Understanding charge motion in a background of interacting quantum spins is a fundamental problem in quantum many-body physics. The most extensively studied model for this problem is the so-called $t$-$t'$-$t''$-$J$ model, where the determination of the parameter $t'$ in the context of cuprate superconductors is challenging. Here we present a theoretical study of the spectral functions of a mobile… ▽ More Understanding charge motion in a background of interacting quantum spins is a fundamental problem in quantum many-body physics. The most extensively studied model for this problem is the so-called $t$-$t'$-$t''$-$J$ model, where the determination of the parameter $t'$ in the context of cuprate superconductors is challenging. Here we present a theoretical study of the spectral functions of a mobile hole in the $t$-$t'$-$t''$-$J$ model using two machine learning techniques: K-nearest Neighbors regression (KNN) and a feed-forward neural network (FFNN). We employ the self-consistent Born approximation to generate a dataset of about $1.3 \times 10^5$ spectral functions. We show that for the forward problem, both methods allow for the accurate and efficient prediction of spectral functions, allowing for e.g. rapid searches through parameter space. Furthermore, we find that for the inverse problem (inferring Hamiltonian parameters from spectra), the FFNN can, but the KNN cannot, accurately predict the model parameters using merely the density-of-state. Our results suggest that it may be possible to use deep learning methods to predict materials parameters from experimentally measured spectral functions. △ Less

Submitted 19 January, 2023; originally announced January 2023.

Comments: 10 pages, 10 figures, 1 table

Journal ref: Phys. Rev. B 107, 205132 (2023)

Thermal transport properties of IrSbSe

Authors: Yu Liu, Milinda Abeykoon, Niraj Aryal, David Graf, Zhixiang Hu, Weiguo Yin, C. Petrovic

Abstract: We report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the $P2_13$ space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [$E_ρ$ = 128(2) meV] and for thermopower [$E_S$ = 17.7(9) meV] from 200 to 300 K indicates the polaronic transport mechanism. Electrical resistivity… ▽ More We report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the $P2_13$ space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [$E_ρ$ = 128(2) meV] and for thermopower [$E_S$ = 17.7(9) meV] from 200 to 300 K indicates the polaronic transport mechanism. Electrical resistivity varies as $exp(T_0/T)^{1/4}$ and thermopower varies as $T^{1/2}$ at low temperatures, indicating that it evolves into the Mott's variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows relatively low value of thermal conductivity ($\sim$ 1.65 W/K$\cdot$m) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, high vacancy defect concentration on both Ir and Sb atomic sites of up to 15\%, suggests high defect tolerance and points to possibility of future improvement of carrier density by chemical substitution or defect optimization. △ Less

Submitted 28 October, 2022; originally announced October 2022.

Journal ref: Phys. Rev. B 106, 195116 (2022)

Glassy crystals with colossal multi-baroresponsivities

Authors: Kun Zhang, Zhe Zhang, Hailong Pan, Xueting Zhao, Ji Qi, Zhao Zhang, Ruiqi Song, Chenyang Yu, Biaohong Huang, Xujing Li, Huaican Chen, Changlong Tan, Wen Yin, Weijin Hu, Michael Wübbenhorst, Jiangshui Luo, Dehong Yu, Zhidong Zhang, Bing Li

Abstract: As a nontrivial solid state of matter, the glassy-crystal state embraces physical features of both crystalline and amorphous solids, where a long-range ordered periodic structure formed by the mass centers of constituent molecules accommodates orientational glasses. Here, we discover and validate a glassy-crystal state in 2-amino-2-methyl-1,3-propanediol (AMP, C4H11NO2) by neutron scattering and c… ▽ More As a nontrivial solid state of matter, the glassy-crystal state embraces physical features of both crystalline and amorphous solids, where a long-range ordered periodic structure formed by the mass centers of constituent molecules accommodates orientational glasses. Here, we discover and validate a glassy-crystal state in 2-amino-2-methyl-1,3-propanediol (AMP, C4H11NO2) by neutron scattering and complementary broadband dielectric spectroscopy (BDS) measurements. The freezing process of the dynamic orientational disorder is manifested at relaxation times well described by the Vogel-Fulcher-Tammann (VFT) law and the strongly frequency-dependent freezing temperature ranging from around 225 K at 0.1 Hz to above room temperature in the GHz region. At room temperature, the supercooled state is extremely sensitive to pressure such that a few MPa pressure can induce crystallization to the ordered crystal state, eventually leading to a temperature increase by 48 K within 20 s, a significant reduction of visible light transmittance from about 95% to a few percentages, and a remarkable decrease of electrical conductivity by three orders of magnitude. These ultrasensitive baroresponsivities might find their applications in low-grade waste heat recycling, pressure sensors and non-volatile memory devices. It is expected that glassy crystals serve as an emerging platform for exploiting exotic states of matter and the associated fantastic applications. △ Less

Submitted 10 September, 2022; originally announced September 2022.

Comments: Totally 13 figures and 1 table

Role of Local Ru Hexamers in Superconductivity of Ruthenium Phosphide

Authors: Robert J. Koch, Niraj Aryal, Oleh Ivashko, Yu Liu, Milinda Abeykoon, Eric D. Bauer, Martin v. Zimmermann, Weiguo Yin, Cedomir Petrovic, Emil S. Bozin

Abstract: Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a mysterious non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T$_{c}$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became "usual… ▽ More Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a mysterious non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T$_{c}$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became "usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, the parent of the maximum-Tc branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering experiments. Large local Ru$_{6}$ hexamer distortions associated with orbital-charge trimerization form above the two-stage electronic transition in RuP. While hexamer ordering enables the nonmagnetic ground state and presumed complex oligomerization, the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials. △ Less

Submitted 19 August, 2022; originally announced August 2022.

Comments: 10 pages 6 figures

Journal ref: Phys. Rev. B 106, 214516 (2022)

Atomically engineered cobaltite layers for robust ferromagnetism

Authors: Shengru Chen, Qinghua Zhang, Xujing Li, Jiali Zhao, Shan Lin, Qiao Jin, Haitao Hong, Amanda Huon, Timothy Charlton, Qian Li, Wensheng Yan, Jiaou Wang, Chen Ge, Can Wang, Baotian Wang, Michael R. Fitzsimmons, Haizhong Guo, Lin Gu, Wen Yin, Kuijuan Jin, Er Jia Guo

Abstract: Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit thick syntactic layers of cobaltites within a strongly tilted octahedr… ▽ More Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit thick syntactic layers of cobaltites within a strongly tilted octahedral matrix via atomically precise synthesis. The octahedral tilt patterns of adjacent layers propagate into cobaltites, leading to a continuation of octahedral tilting while maintaining significant misfit tensile strain. These effects induce severe rumpling within an atomic plane of neighboring layers triggers the electronic reconstruction between the splitting orbitals. First-principles calculations reveal that the cobalt ions transits to a higher spin state level upon octahedral tilting, resulting in robust ferromagnetism in ultrathin cobaltites. This work demonstrates a design methodology for fine-tuning the lattice and spin degrees of freedom in correlated quantum heterostructures by exploiting epitaxial geometric engineering. △ Less

Submitted 7 July, 2022; originally announced July 2022.

Comments: 18 pages, 5 figures

Topological Antiferromagnetic Semimetal for Spintronics: A Case Study of a Layered Square Net System EuZnSb$_2$

Authors: Niraj Aryal, Qiang Li, A. M. Tsvelik, Weiguo Yin

Abstract: We use the first principles and effective Hamiltonian methods to study the electronic structure and magnetic properties of a recently synthesized layered antiferromagnetic square net topological semimetal EuZnSb$_2$ [1]. The main message of the paper is that effects of small changes in the band structure produced by the magnetic ordering and changes in the orientation of the \Neel vector are ampli… ▽ More We use the first principles and effective Hamiltonian methods to study the electronic structure and magnetic properties of a recently synthesized layered antiferromagnetic square net topological semimetal EuZnSb$_2$ [1]. The main message of the paper is that effects of small changes in the band structure produced by the magnetic ordering and changes in the orientation of the \Neel vector are amplified in such transport properties as the spin Hall conductivity. We predict that the effects of the broken symmetry introduced by the ordering of the \Neel vector, being very weak in the bulk, are pronounced in the surface electronic dispersion, suggesting that surface probes may be more suited to measure them. The coexistence of the magnetism with many other competing phases make this material interesting and possibly useful for quantum spintronics applications. △ Less

Submitted 8 June, 2022; originally announced June 2022.

Comments: 16 pages

The Role of Permanent and Induced Electrostatic Dipole Moments for Schottky Barriers in Janus MXY/Graphene Heterostructures: a First Principles Study

Authors: Yuqi Chen, Huanhuan Zhang, Bo Wen, Xibo Li, Yifeng Chai, Ying Xu, Xiaolin Wei, Wen-Jin Yin, Gilberto Teobaldi

Abstract: The Schottky barrier height ($E_{SBH}$) is a crucial factor in determining the transport properties of semiconductor materials as it directly regulates the carrier mobility in opto-electronics devices. In principle, van der Waals (vdW) Janus heterostructures offer an appealing avenue to controlling the ESBH. However, the underlying atomistic mechanisms are far from understood conclusively, which p… ▽ More The Schottky barrier height ($E_{SBH}$) is a crucial factor in determining the transport properties of semiconductor materials as it directly regulates the carrier mobility in opto-electronics devices. In principle, van der Waals (vdW) Janus heterostructures offer an appealing avenue to controlling the ESBH. However, the underlying atomistic mechanisms are far from understood conclusively, which prompts for further research in the topic. To this end, here, we carry out an extensive first principles study of the electronic properties and $E_{SBH}$ of several vdW Janus MXY/Graphene (M=Mo, W; X, Y=S, Se, Te) heterostructures. The results of the simulations show that by changing the composition and geometry of the heterostructure's interface, it is possible to control its electrical contact, thence electron transport properties, from Ohmic to Schottky with nearly one order of magnitude variations in the $E_{SBH}$. Detailed analysis of the simulations enables rationalization of this highly attractive property on the basis of the interplay between the permanent dipole moment of the Janus MXY sheet and the induced one due to interfacial charge redistribution at the MXY/Gr interface. Such an interplay is shown to be highly effective in altering the electrostatic potential difference across the vdW Janus heterostructure, determining its ESBH, thence Schottky (Ohmic) contact type. These computational findings contribute guidelines to control electrical contacts in Janus heterostructures towards rational design of electrical contacts in nanoscale devices. △ Less

Submitted 9 March, 2022; originally announced March 2022.

Ultrafast Melting of Superconductivity in an Iron-Based Superconductor

Authors: Dan Nevola, Nader Zaki, John M. Tranquada, Weiguo Yin, Genda Gu, Qiang Li, Peter D. Johnson

Abstract: Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in… ▽ More Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in prior to the destruction of the superconducting peak, followed by a metastable response. We propose that the former is due to pair phase decoherence and speculate that the latter is due to the increase in double stripe correlations that are known to compete with superconductivity. Our results add to exciting new developments on the iron-based superconductors, indicating that the photoinduced metastable state possibly competes with superconductivity. △ Less

Submitted 17 September, 2022; v1 submitted 1 March, 2022; originally announced March 2022.

Conjoined Charge Density Waves in the Kagome Superconductor CsV3Sb5

Authors: Haoxiang Li, G. Fabbris, A. H. Said, Y. Y. Pai, Q. W. Yin, C. S. Gong, Z. J. Tu, H. C. Lei, J. P. Sun, J. -G. Cheng, Ziqiang Wang, Binghai Yan, R. Thomale, H. N. Lee, H. Miao

Abstract: The intricate interplay between novel lattice geometry and spontaneous symmetry-breaking states is at the forefront of contemporary research on quantum materials. Recently, the observation of unconventional charge and pairing density waves in a kagome metal CsV3Sb5 brings out a new showcase for intertwined orders. While electronic instabilities in CsV3Sb5 are widely believed to originate from the… ▽ More The intricate interplay between novel lattice geometry and spontaneous symmetry-breaking states is at the forefront of contemporary research on quantum materials. Recently, the observation of unconventional charge and pairing density waves in a kagome metal CsV3Sb5 brings out a new showcase for intertwined orders. While electronic instabilities in CsV3Sb5 are widely believed to originate from the V 3d-electrons residing on the 2-dimensional kagome sublattice, the pivotal role of Sb 5p-electrons for 3-dimensional orders is yet to be understood. Here, using resonant tender x-ray scattering and high-pressure X-ray scattering, we report a rare realization of conjoined charge density waves (CDW) in CsV3Sb5. At ambient pressure, we discover a resonant enhancement at Sb L1-edge (2s-5p) at the 2$\times$2$\times$2 CDW wavevectors. The resonance, however, is absent at the 2$\times$2 CDW wavevectors. Applying hydrostatic pressure, we find the CDW transition temperatures to separate, where the 2$\times$2$\times$2 CDW emerges 4 K above the 2$\times$2 CDW at 1GPa. Our results establish the coexistence of the 2$\times$2 CDW and the 5p-electron assisted 2$\times$2$\times$2 CDW in CsV3Sb5. The evolution of the conjoined CDWs under pressure suggests the joint importance of electronic and phononic fluctuations for the double dome superconductivity. △ Less

Submitted 27 February, 2022; originally announced February 2022.

Journal ref: Nature Communications 13:6348 (2022)

Verwey transition as evolution from electronic nematicity to trimerons via electron-phonon coupling

Authors: Wei Wang, Jun Li, Zhixiu Liang, Lijun Wu, Pedro M. Lozano, Alexander C. Komarek, Xiaozhe Shen, Alex H. Reid, Xijie Wang, Qiang Li, Weiguo Yin, Kai Sun, Yimei Zhu, Ian K. Robinson, Mark P. M. Dean, Jing Tao

Abstract: Understanding the driving mechanisms behind metal-insulator transitions (MITs) is a critical step towards controlling material's properties. Since the proposal of charge-order-induced MIT in magnetite Fe3O4 in 1939 by Verwey, the nature of the charge order and its role in the transition have remained elusive-a longstanding challenge in the studies of complex oxides. Recently, a trimeron order was… ▽ More Understanding the driving mechanisms behind metal-insulator transitions (MITs) is a critical step towards controlling material's properties. Since the proposal of charge-order-induced MIT in magnetite Fe3O4 in 1939 by Verwey, the nature of the charge order and its role in the transition have remained elusive-a longstanding challenge in the studies of complex oxides. Recently, a trimeron order was discovered in the low-temperature monoclinic structure of Fe3O4; however, the expected transition entropy change in forming trimeron at the Verwey transition is greater than the observed value, which arises a reexamination of the ground state in the high-temperature phase. Here we use electron diffraction to unveil that a nematic charge order on particular Fe sites emerges in the high-temperature cubic structure of bulk Fe3O4, and that upon cooling, a competitive intertwining of charge and lattice orders leads to the emergence of the Verwey transition. Moreover, MeV ultrafast electron diffraction (UED) provides a dynamic measure of the strong coupling between photoexcited electrons and the X3 phonon modes. Our findings discover a new type of electronic nematicity in correlated materials and offer novel insights into the Verwey transition mechanism in Fe3O4 via the electron-phonon coupling. △ Less

Submitted 30 April, 2023; v1 submitted 17 February, 2022; originally announced February 2022.

Comments: Supplementary materials are added

Emergence of spinons in layered trimer iridate Ba4Ir3O10

Authors: Y. Shen, J. Sears, G. Fabbris, A. Weichselbaum, W. Yin, H. Zhao, D. G. Mazzone, H. Miao, M . H. Upton, D. Casa, R. Acevedo-Esteves, C. Nelson, A. M. Barbour, C. Mazzoli, G. Cao, M. P. M. Dean

Abstract: Spinons are well-known as the elementary excitations of one-dimensional antiferromagnetic chains, but means to realize spinons in higher dimensions is the subject of intense research. Here, we use resonant x-ray scattering to study the layered trimer iridate Ba4Ir3O10, which shows no magnetic order down to 0.2 K. An emergent one-dimensional spinon continuum is observed that can be well-described b… ▽ More Spinons are well-known as the elementary excitations of one-dimensional antiferromagnetic chains, but means to realize spinons in higher dimensions is the subject of intense research. Here, we use resonant x-ray scattering to study the layered trimer iridate Ba4Ir3O10, which shows no magnetic order down to 0.2 K. An emergent one-dimensional spinon continuum is observed that can be well-described by XXZ spin-1/2 chains with magnetic exchange of ~55 meV and a small Ising-like anisotropy. With 2% isovalent Sr doping, magnetic order appears below TN=130 K along with sharper excitations, indicating that the spinons become more confined in (Ba1-xSrx)4Ir3O10. We propose that the frustrated intra-trimer interactions effectively reduce the system into decoupled spin chains, the subtle balance of which can be easily tipped by perturbations such as chemical doping. Our results put Ba4Ir3O10 between the one-dimensional chain and two-dimensional quantum spin liquid scenarios, illustrating a new way to suppress magnetic order and realize fractional spinons. △ Less

Submitted 17 October, 2022; v1 submitted 7 January, 2022; originally announced January 2022.

Comments: Accepted in Physical Review Letters; 5 pages not including references or supplementary

Anomalous Hall effect at the Lifshitz transition in ZrTe5

Authors: Pedro M. Lozano, Gabriel Cardoso, Niraj Aryal, Daniel Nevola, Genda Gu, Alexei Tsvelik, Weiguo Yin, Qiang Li

Abstract: Zirconium pentatelluride ZrTe5 is a topological semimetal. The presence of a temperature induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined exp… ▽ More Zirconium pentatelluride ZrTe5 is a topological semimetal. The presence of a temperature induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined experimental and theoretical study and show that a low energy model can be used to understand the complicated Hall response and large anomalous Hall effect observed in ZrTe5 over a wide range of temperature and magnetic field. We found that the anomalous Hall contribution dominates the Hall response in a narrow temperature window around the Lifshitz transition, away from which the orbital contribution dominates. Moreover, our results indicate that a topological phase transition coexists with the Lifshitz transition. Our model provides a unifying framework to understand the Hall effect in semimetals with large Zeeman splitting and non-trivial topology. △ Less

Submitted 30 December, 2021; originally announced December 2021.

Comments: 6 pages, 3 figures

Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

Authors: Wei Wang, Lijun Wu, Junjie Li, Niraj Aryal, Xilian Jin, Yu Liu, Mikhail Fedurin, Marcus Babzien, Rotem Kupfer, Mark Palmer, Cedomir Petrovic, Weiguo Yin, Mark P. M. Dean, Ian Robinson, Jing Tao, Yimei Zhu

Abstract: Identifying and understanding the mechanisms behind strong phonon-phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combin… ▽ More Identifying and understanding the mechanisms behind strong phonon-phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c-axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices. △ Less

Submitted 24 October, 2021; originally announced October 2021.

Journal ref: npj Quantum Mater. 6, 97 (2021)

Revealing the immediate formation of two-fold rotation symmetry in charge-density-wave state of Kagome superconductor CsV$_3$Sb$_5$ by optical polarization rotation measurement

Authors: Qiong Wu, Z. X. Wang, Q. M. Liu, R. S. Li, S. X. Xu, Q. W. Yin, C. S. Gong, Z. J. Tu, H. C. Lei, T. Dong, N. L. Wang

Abstract: We report the observation of two-fold rotation symmetry in charge density wave (CDW) state in the newly discovered Kagome superconductor CsV$_3$Sb$_5$. Below its CDW transition temperature ($T_{CDW}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows two-fold rotation symmetry. With femtosecond laser pulse pump… ▽ More We report the observation of two-fold rotation symmetry in charge density wave (CDW) state in the newly discovered Kagome superconductor CsV$_3$Sb$_5$. Below its CDW transition temperature ($T_{CDW}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows two-fold rotation symmetry. With femtosecond laser pulse pumping, the rotation angle can be easily suppressed and then recovers in several picoseconds accompanied with coherent oscillations. Significantly, the oscillations in the signal also experience a 180 degree periodic change. Our investigation provides clear optical evidence for the formation of nematic order with two-fold rotation symmetry just below $T_{CDW}$. The results imply a immediate development of nematicity and possible time-reversal symmetry breaking in CDW state of CsV$_3$Sb$_5$. △ Less

Submitted 2 March, 2022; v1 submitted 21 October, 2021; originally announced October 2021.

Comments: 6 pages, 3 figures

Robust and tunable Weyl phases by coherent infrared phonons in ZrTe$_5$

Authors: Niraj Aryal, Xilian Jin, Qiang Li, Mengkun Liu, A. M. Tsvelik, Weiguo Yin

Abstract: Ultrafast optical control of the structural and electronic properties of various quantum materials has recently sparked great interest. In particular, photoinduced quantum phase transition between distinct topological phases has been considered as a promising route to realize ultrafast topological quantum computers. Here we use first-principles and effective Hamiltonian methods to show that in ZrT… ▽ More Ultrafast optical control of the structural and electronic properties of various quantum materials has recently sparked great interest. In particular, photoinduced quantum phase transition between distinct topological phases has been considered as a promising route to realize ultrafast topological quantum computers. Here we use first-principles and effective Hamiltonian methods to show that in ZrTe$_5$, a layered topological material, lattice distortions corresponding to all three types of zone-center infrared optical phonon modes can drive the system from the strong or weak topological insulating phase to a Weyl semimetal by breaking the global inversion symmetry. Thus achieved Weyl phases are robust, highly tunable and one of the cleanest ones due to the proximity of the Weyl points to the Fermi level and a lack of other carriers. We further show that the amount of infrared-mode pumping necessary to induce such Weyl phases can be reduced if used in conjunction with an A$_g$ Raman-mode pumping that first drives the system to the Dirac semimetal state. We also find that Berry curvature dipole moment (BCDM), induced by the dynamical inversion symmetry breaking, gives rise to various nonlinear effects that oscillate with the amplitude of the phonon modes. These nonlinear effects present a novel switch for controlling the Weyltronics enabled quantum system. △ Less

Submitted 13 October, 2021; originally announced October 2021.

Comments: 14 pages, 4 figures

Journal ref: npj Comput Mater 8, 113 (2022)

Spatial symmetry constraint of charge-ordered kagome superconductor CsV$_3$Sb$_5$

Authors: Haoxiang Li, Yu-Xiao Jiang, J. X. Yin, Sangmoon Yoon, Andrew R. Lupini, Y. Pai, C. Nelson, A. Said, Y. M. Yang, Q. W. Yin, C. S. Gong, Z. J. Tu, H. C. Lei, Binghai Yan, Ziqiang Wang, M. Z. Hasan, H. N. Lee, H. Miao

Abstract: Elucidating the symmetry of intertwined orders in exotic superconductors is at the quantum frontier. Recent surface sensitive studies of the topological kagome superconductor CsV$_3$Sb$_5$ discovered a cascade 4a$_0$ superlattice below the charge density wave (CDW) ordering temperature, which can be related to the pair density modulations in the superconducting state. If the 4a$_0$ phase is a bulk… ▽ More Elucidating the symmetry of intertwined orders in exotic superconductors is at the quantum frontier. Recent surface sensitive studies of the topological kagome superconductor CsV$_3$Sb$_5$ discovered a cascade 4a$_0$ superlattice below the charge density wave (CDW) ordering temperature, which can be related to the pair density modulations in the superconducting state. If the 4a$_0$ phase is a bulk and intrinsic property of the kagome lattice, this would form a striking analogy to the stripe order and pair density wave discovered in the cuprate high-temperature superconductors, and the cascade ordering found in twisted bilayer graphene. High-resolution X-ray diffraction has recently been established as an ultra-sensitive probe for bulk translational symmetry-breaking orders, even for short-range orders at the diffusive limit. Here, combining high-resolution X-ray diffraction, scanning tunneling microscopy and scanning transmission electron microscopy, we demonstrate that the 4a$_0$ superstructure emerges uniquely on the surface and hence exclude the 4a$_0$ phase as the origin of any bulk transport or spectroscopic anomaly. Crucially, we show that our detected 2$\times$2$\times$2 CDW order breaks the bulk rotational symmetry to C2, which can be the driver for the bulk nematic orders and nematic surface superlattices including the 4a$_0$ phase. Our high-resolution data impose decisive spatial symmetry constraints on emergent electronic orders in the kagome superconductor CsV$_3$Sb$_5$. △ Less

Submitted 23 September, 2021; v1 submitted 7 September, 2021; originally announced September 2021.

Journal ref: Nature Communications 13:6348 (2022)

Competition between charge-density-wave and superconductivity in the kagome metal RbV3Sb5

Authors: N. N. Wang, K. Y. Chen, Q. W. Yin, Y. N. N. Ma, B. Y. Pan, X. Yang, X. Y. Ji, S. L. Wu, P. F. Shan, S. X. Xu, Z. J. Tu, C. S. Gong, G. T. Liu, G. Li, Y. Uwatoko, X. L. Dong, H. C. Lei, J. P. Sun, J. -G. Cheng

Abstract: The interplay between charge-density-wave (CDW) order and superconductivity (SC) in the Kagome metal RbV3Sb5 is studied by tracking the evolutions of their transition temperatures, T* and Tc, as a function of pressure (P) via measurements of resistivity and magnetic susceptibility under various hydrostatic pressures up to ~ 5 GPa. It is found that the CDW order at T* experiences a subtle modificat… ▽ More The interplay between charge-density-wave (CDW) order and superconductivity (SC) in the Kagome metal RbV3Sb5 is studied by tracking the evolutions of their transition temperatures, T* and Tc, as a function of pressure (P) via measurements of resistivity and magnetic susceptibility under various hydrostatic pressures up to ~ 5 GPa. It is found that the CDW order at T* experiences a subtle modification at Pc1 ~ 1.5 GPa before it is completely suppressed around Pc2 ~ 2.4 GPa. Accordingly, the superconducting transition Tc(P) exhibits a shallow M-shaped double superconducting dome with two extrema of Tconset ~ 4.4 K and 3.9 K around Pc1 and Pc2, respectively, leading to a fourfold enhancement of Tc with respect to that at ambient pressure. The constructed T-P phase diagram of RbV3Sb5 resembles that of CsV3Sb5, and shares similar features as many other unconventional superconducting systems with intertwined competing electronic orders. The strong competition between CDW and SC is also evidenced by the broad superconducting transition width in the coexistent region. Our results shed more light on the intriguing physics involving intertwined electronic orders in this novel topological kagome metal family. △ Less

Submitted 1 July, 2021; originally announced July 2021.

Comments: 19 pages, 5 figures

Journal ref: Phys. Rev. Research 3, 043018 (2021)

Unconventional charge density wave and photoinduced lattice symmetry change in Kagome Metal CsV$_3$Sb$_5$ probed by time-resolved spectroscopy

Authors: Z. X. Wang, Q. Wu, Q. W. Yin, Z. J. Tu, C. S. Gong, T. Lin, Q. M. Liu, L. Y. Shi, S. J. Zhang, D. Wu, H. C. Lei, T. Dong, N. L. Wang

Abstract: Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has received wide attention due to its presence of superconductivity, charge density wave (CDW) and peculiar properties from topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by formation of energy gap below the phase… ▽ More Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has received wide attention due to its presence of superconductivity, charge density wave (CDW) and peculiar properties from topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by formation of energy gap below the phase transition being similar to a usual second-order CDW condensate, by contrast, the structure change is predominantly first order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from the traditional CDW condensate. We also find that weak pump pulse can non-thermally melt the CDW order and drive the sample into its high temperature phase, revealing the fact that the difference in lattice potential between those phases is small. △ Less

Submitted 24 May, 2021; originally announced May 2021.

Comments: 3 figures, 6 pages

Competing superconductivity and charge-density wave in Kagome metal CsV3Sb5: evidence from their evolutions with sample thickness

Authors: B. Q. Song, X. M. Kong, W. Xia, Q. W. Yin, C. P. Tu, C. C. Zhao, D. Z. Dai, K. Meng, Z. C. Tao, Z. J. Tu, C. S. Gong, H. C. Lei, Y. F. Guo, X. F. Yang, S. Y. Li

Abstract: Recently superconductivity and topological charge-density wave (CDW) were discovered in the Kagome metals $A$V$_3$Sb$_5$ ($A$ = Cs, Rb, and K), which have an ideal Kagome lattice of vanadium. Here we report resistance measurements on thin flakes of CsV$_3$Sb$_5$ to investigate the evolution of superconductivity and CDW with sample thickness. The CDW transition temperature ${\it T}_{\rm CDW}$ decre… ▽ More Recently superconductivity and topological charge-density wave (CDW) were discovered in the Kagome metals $A$V$_3$Sb$_5$ ($A$ = Cs, Rb, and K), which have an ideal Kagome lattice of vanadium. Here we report resistance measurements on thin flakes of CsV$_3$Sb$_5$ to investigate the evolution of superconductivity and CDW with sample thickness. The CDW transition temperature ${\it T}_{\rm CDW}$ decreases from 94 K in bulk to a minimum of 82 K at thickness of 60 nm, then increases to 120 K as the thickness is reduced further to 4.8 nm (about five monolayers). Since the CDW order in CsV$_3$Sb$_5$ is quite three-dimensional (3D) in the bulk sample, the non-monotonic evolution of ${\it T}_{\rm CDW}$ with reducing sample thickness can be explained by a 3D to 2D crossover around 60 nm. Strikingly, the superconducting transition temperature ${\it T}_{\rm c}$ shows an exactly opposite evolution, increasing from 3.64 K in the bulk to a maximum of 4.28 K at thickness of 60 nm, then decreasing to 0.76 K at 4.8 nm. Such exactly opposite evolutions provide strong evidence for competing superconductivity and CDW, which helps us to understand these exotic phases in $A$V$_3$Sb$_5$ Kagome metals. △ Less

Submitted 19 May, 2021; originally announced May 2021.

Comments: 4 pages, 4 figures

Journal ref: Nature Communications 14, 2492 (2023)

Double-dome superconductivity under pressure in the V-based Kagome metals AV3Sb5 (A = Rb and K)

Authors: C. C. Zhu, X. F. Yang, W. Xia, Q. W. Yin, L. S. Wang, C. C. Zhao, D. Z. Dai, C. P. Tu, B. Q. Song, Z. C. Tao, Z. J. Tu, C. S. Gong, H. C. Lei, Y. F. Guo, S. Y. Li

Abstract: We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to th… ▽ More We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$ displays a similar double-dome phase diagram, however, its two maximum $T_c$s are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb and K) is constructed, showing that the ionic radius of the intercalated alkali-metal atoms has a significant effect. Our work demonstrates that double-dome superconductivity under pressure is a common feature of these V-based Kagome metals. △ Less

Submitted 29 April, 2021; originally announced April 2021.

Comments: 5 pages, 4 figures

Journal ref: Physical Review B 105, 094507 (2022), Editors' Suggestion

Double superconducting dome and triple enhancement of Tc in the kagome superconductor CsV3Sb5 under high pressure

Authors: K. Y. Chen, N. N. Wang, Q. W. Yin, Z. J. Tu, C. S. Gong, J. P. Sun, H. C. Lei, Y. Uwatoko, J. -G. Cheng

Abstract: CsV3Sb5 is a newly discovered Z2 topological kagome metal showing the coexistence of a charge density wave (CDW)-like order at T* = 94 K and superconductivity (SC) at Tc = 2.5 K at ambient pressure. Here we study the interplay between CDW and SC in CsV3Sb5 via measurements of resistivity and magnetic susceptibility under hydrostatic pressures. We find that the CDW transition decreases with pressur… ▽ More CsV3Sb5 is a newly discovered Z2 topological kagome metal showing the coexistence of a charge density wave (CDW)-like order at T* = 94 K and superconductivity (SC) at Tc = 2.5 K at ambient pressure. Here we study the interplay between CDW and SC in CsV3Sb5 via measurements of resistivity and magnetic susceptibility under hydrostatic pressures. We find that the CDW transition decreases with pressure and experience a subtle modification at Pc1 = 0.6-0.9 GPa before it vanishes completely at Pc2 = 2 GPa. Correspondingly, Tc(P) displays an unusual M-shaped double dome character with two maxima around Pc1 and Pc2, respectively, leading to a tripled enhancement of Tc to about 8 K at 2 GPa. The obtained temperature-pressure phase diagram resembles those of many unconventional superconductors, illustrating an intimated competition between CDW-like order and SC. The competition is found to be particularly strong for the intermediate pressure range Pc1 <= P <= Pc2 as evidenced by the broad superconducting transition and reduced superconducting volume fraction. This work not only demonstrates the potential to raise the Tc of the V-based kagome superconductors, but also offers more insights into the rich physics related to the electronic correlations in this novel family of topological kagome metals. △ Less

Submitted 18 February, 2021; originally announced February 2021.

Journal ref: Phys. Rev. Lett. 126, 247001 (2021)

Nodal superconductivity and superconducting domes in the topological Kagome metal CsV3Sb5

Authors: C. C. Zhao, L. S. Wang, W. Xia, Q. W. Yin, J. M. Ni, Y. Y. Huang, C. P. Tu, Z. C. Tao, Z. J. Tu, C. S. Gong, H. C. Lei, Y. F. Guo, X. F. Yang, S. Y. Li

Abstract: Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K, Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based superconductors also host charge density wave (CDW) and topological nontrivial band structure. Here we report the ultralow-temperature thermal conductivity and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the highest among AV3Sb… ▽ More Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K, Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based superconductors also host charge density wave (CDW) and topological nontrivial band structure. Here we report the ultralow-temperature thermal conductivity and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the highest among AV3Sb5. A finite residual linear term of thermal conductivity at zero magnetic field and its rapid increase in fields suggest nodal superconductivity. By applying pressure, the Tc of CsV3Sb5 increases first, then decreases to lower than 0.3 K at 11.4 GPa, showing a clear first superconducting dome peaked around 0.8 GPa. Above 11.4 GPa, superconductivity re-emerges, suggesting a second superconducting dome. Both nodal superconductivity and superconducting domes point to unconventional superconductivity in this V-based superconductor. While our finding of nodal superconductivity puts a strong constrain on the pairing state of the first dome, which should be related to the CDW instability, the superconductivity of the second dome may present another exotic pairing state in this ideal Kagome lattice of vanadium. △ Less

Submitted 1 March, 2021; v1 submitted 16 February, 2021; originally announced February 2021.

Comments: 4 figures

Dimerization and spin-decoupling in two-leg Heisenberg ladder with frustrated trimer rungs

Authors: Andreas Weichselbaum, Weiguo Yin, Alexei M. Tsvelik

Abstract: We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin which is motivated and relevant also for the description of real two-dimensional materials such as the two-dimensional trimer magnet Ba$_4$Ir$_3$O$_{10}$. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent descripti… ▽ More We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin which is motivated and relevant also for the description of real two-dimensional materials such as the two-dimensional trimer magnet Ba$_4$Ir$_3$O$_{10}$. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. Overall, we find in the regime of strong rung coupling a gapped dimerized phase related to competing symmetry sectors in Hilbert space that ultimately results in frustration-driven spin-Peierls transition. In the weak rung-coupling regime, the system is uniform, yet shows a gapped spinon continuum together with a sharp coherent low-energy branch which renders the system critical overall. In either case, the additional rung spin quickly get sidelined and nearly decouple once their bare coupling to the ladder drops somewhat below the direct Heisenberg coupling of the legs. △ Less

Submitted 15 March, 2021; v1 submitted 6 January, 2021; originally announced January 2021.

Comments: 14 + 3 pages, 13 figures; fixes of typos and minor textual changes

Journal ref: Phys. Rev. B 103, 125120 (2021)

Crystal structure prediction via combining graph network and Bayesian optimization

Authors: Guanjian Cheng, Xin-Gao Gong, Wan-Jian Yin

Abstract: We developed a density functional theory-free approach for crystal structure prediction via combing graph network (GN) and Bayesian optimization (BO). GN is adopted to establish the correlation model between crystal structure and formation enthalpies. BO is to accelerate searching crystal structure with optimal formation enthalpy. The approach of combining GN and BO for crystal Structure Searching… ▽ More We developed a density functional theory-free approach for crystal structure prediction via combing graph network (GN) and Bayesian optimization (BO). GN is adopted to establish the correlation model between crystal structure and formation enthalpies. BO is to accelerate searching crystal structure with optimal formation enthalpy. The approach of combining GN and BO for crystal Structure Searching (GN-BOSS), in principle, can predict crystal structure at given chemical compositions without additional constraints on cell shapes and lattice symmetries. The applicability and efficiency of GN-BOSS approach is then verified via solving the classical Ph-vV challenge. It can correctly predict the crystal structures of 24 binary compounds from scratch with averaged computational cost ~ 30 minutes each by only one CPU core. GN-BOSS approach may open a new avenue to data-driven crystal structural prediction without using the expensive DFT calculations. △ Less

Submitted 22 November, 2020; originally announced November 2020.

Probing the pathway of an ultrafast structural phase transition to illuminate the transition mechanism in Cu2S

Authors: Junjie Li, Kai Sun, Jun Li, Qingping Meng, Xuewen Fu, Wei-Guo Yin, Deyu Lu, Yan Li, Marcus Babzien, Mikhail Fedurin, Christina Swinson, Robert Malone, Mark Palmer, Leanne Mathurin, Ryan Mason, Jingyi Chen, Robert M. Konik, Robert J. Cava, Yimei Zhu, Jing Tao

Abstract: Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of the transition mechanisms in strongly correlated systems and quantum materials. Here we present a study of structural phase transition pathways in superionic Cu2S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sens… ▽ More Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of the transition mechanisms in strongly correlated systems and quantum materials. Here we present a study of structural phase transition pathways in superionic Cu2S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sensitive in both momentum-space and the time-domain, we distinguish the dynamics of crystal symmetry breaking and lattice expansion in this system. We are able to follow the transient states along the transition pathway and so observe the dynamics of both the primary and secondary order parameters. Based on these observations we argue that the mechanism of the structural phase transition in Cu2S is dominated by the electron-phonon coupling. This mechanism advances the understanding from previous results where the focus was solely on dynamic observations of the lattice expansion. △ Less

Submitted 2 November, 2020; originally announced November 2020.

Journal ref: Appl. Phys. Lett. 113, 041904 (2018)

Photoinduced Dirac semimetal in ZrTe5

Authors: T. Konstantinova, L. Wu, W. -G. Yin, J. Tao, G. D. Gu, X. J. Wang, Jie Yang, I. A. Zaliznyak, Y. Zhu

Abstract: Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices compatible with optical communication and information technology. Here… ▽ More Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices compatible with optical communication and information technology. Here, we use MeV Ultrafast Electron Diffraction (UED) to show how a transient three-dimensional (3D) Dirac semimetal state can be induced by a femtosecond laser pulse in a topological insulator ZrTe$_5$. We observe marked changes in Bragg diffraction, which are characteristic of bond distortions in the photoinduced state. Using the atomic positions refined from the UED, we perform density functional theory (DFT) analysis of the electronic band structure. Our results reveal that the equilibrium state of ZrTe$_5$ is a topological insulator with a small band gap of $\sim$25 meV, consistent with angle-resolved photoemission (ARPES) experiments. However, the gap is closed in the presence of strong spin-orbit coupling (SOC) in the photoinduced transient state, where massless Dirac fermions emerge in the chiral band structure. The time scale of the relaxation dynamics to the transient Dirac semimetal state is remarkably long, $τ\sim$160 ps, which is two orders of magnitude longer than the conventional phonon-driven structural relaxation. The long relaxation is consistent with the vanishing density of states in Dirac spectrum and slow spin-repolarization of the SOC-controlled band structure accompanying the emergence of Dirac fermions. △ Less

Submitted 8 October, 2020; originally announced October 2020.

Comments: 19 pages, 4 figures plus 7 Supplementary pages, 5 Supplementary figures

Journal ref: npj Quantum Materials 5, Article number: 80 (2020)

Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System

Authors: R. J. Koch, R. Sinclair, M. T. McDonnell, R. Yu, M. Abeykoon, M. G. Tucker, A. M. Tsvelik, S. J. L. Billinge, H. D. Zhou, W. -G. Yin, E. S. Bozin

Abstract: The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up… ▽ More The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning $\sim$6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems. △ Less

Submitted 29 September, 2020; originally announced September 2020.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. Lett. 126, 186402 (2021)