Showing 1–50 of 143 results for author: Jin, R

Atomic-scale visualization of the toroidal order in a trimeric Dy(III) single-molecule toroic

Authors: Michael J. Jenkins, Madalynn G. Marshall, Jie Xing, Muthu Satheeshkumar, Brandon D. Watson-Sanders, Xiaoping Wang, Christina M. Hoffmann, Gopalan Rajaraman, Huibo Cao, Rongying Jin, Zi-Ling Xue

Abstract: Single-molecule toroics (SMTs) offer a unique platform for next-generation quantum devices utilizing head-to-tail spin alignments in the compounds. Presence of toroidal moments in SMTs has been essentially based on magnetometry and ab initio calculations. Here, we report observation and probe of the toroidal moment in [Dy3(OH)(teaH2)3(paa)3]Cl(OMe) [teaH3: triethanolamine; paaH: N-(2-pyridyl)-acet… ▽ More Single-molecule toroics (SMTs) offer a unique platform for next-generation quantum devices utilizing head-to-tail spin alignments in the compounds. Presence of toroidal moments in SMTs has been essentially based on magnetometry and ab initio calculations. Here, we report observation and probe of the toroidal moment in [Dy3(OH)(teaH2)3(paa)3]Cl(OMe) [teaH3: triethanolamine; paaH: N-(2-pyridyl)-acetoacetamide] from mapping of Dy3+ magnetic susceptibility tensors by polarized neutron diffraction (PND). Neutron diffraction under variable magnetic fields demonstrates field-induced magnetization along the c-axis with toroidal moments anti-parallelly stacked, providing definite proof of the toroidal moment. Magnetometry studies confirm the toroidal ground state. For the first time, the combined use of PND, variable-field neutron diffraction, ab initio calculations, and magnetometry is introduced as a robust and quantitative methodology to probe molecular-scale toroidal magnetism. This integrated approach overcomes limitations of earlier indirect methods, establishes a benchmark framework for investigating SMTs, and provides valuable insights for the design of molecular quantum materials. △ Less

Submitted 8 June, 2026; originally announced June 2026.

Metastable MnBi$_2$Te$_4$ enabled by magnetic-field-assisted synthesis

Authors: Abhinna Rajbanshi, G. M. Zills, Alexander M. Donald, Daniel Duong, David Graf, James J. Hamlin, Mark W. Meisel, I. Vekhter, Williams A. Shelton, Rongying Jin

Abstract: Magnetic topological insulators provide a unique platform to explore the interplay between magnetism and topology. MnBi$_2$Te$_4$, known for its A-type antiferromagnetic (AFM) ground state, undergoes a striking transformation when single crystals are grown in an applied magnetic field. Despite retaining the same crystal structure, field-grown MnBi$_2$Te$_4$ exhibits a ferromagnetic (FM) ground sta… ▽ More Magnetic topological insulators provide a unique platform to explore the interplay between magnetism and topology. MnBi$_2$Te$_4$, known for its A-type antiferromagnetic (AFM) ground state, undergoes a striking transformation when single crystals are grown in an applied magnetic field. Despite retaining the same crystal structure, field-grown MnBi$_2$Te$_4$ exhibits a ferromagnetic (FM) ground state with a Curie temperature of $\sim$ 12.5 K, confirmed by magnetization, magnetic torque, electrical resistivity, and specific heat measurements. First-principles calculations support these findings, revealing that magnetic-field-assisted synthesis can effectively reconfigure the ground-state spin order and thereby modify the material's electronic properties, as reflected in the de Haas-van Alphen oscillation seen in the magnetic torque. △ Less

Submitted 3 May, 2026; originally announced May 2026.

Emulation of the dynamics of bound electron exposed to strong oscillatory laser field with Bose-Einstein Condensates

Authors: Ziheng Ma, Jia Li, Rui Jin, Yajiang Hao

Abstract: This paper employs a Bose-Einstein condensates to simulate the dynamical response of bound electrons in a strongly oscillating pulsed laser field. We investigate the excitation dynamics of Bose-Einstein condensates with repulsive interaction confined in a potential well with finite depth and width driven by a strong oscillatory pulse field. By numerically solving the Gross-Pitaevskii equation with… ▽ More This paper employs a Bose-Einstein condensates to simulate the dynamical response of bound electrons in a strongly oscillating pulsed laser field. We investigate the excitation dynamics of Bose-Einstein condensates with repulsive interaction confined in a potential well with finite depth and width driven by a strong oscillatory pulse field. By numerically solving the Gross-Pitaevskii equation with Crank-Nicolson method and split operator method, we obtain the time-dependent wavefunction and therefore the evolution of density distribution in real space and that in momentum space, and the occupation distribution in energy space. It is shown that cold atoms with weak interaction oscillate as a whole body in a finite space when the amplitude of pulse drive is not strong enough. During the evolution atoms occupy the bound states with larger probability. Increasing the driving strength or atomic interactions promotes the excitation of atoms into continuum states and their diffusion out of the potential well, leading to complex structures or even interference-like patterns in the momentum distribution. The number of cycles in the pulse envelope plays a crucial role in the dynamical behavior: High-frequency driving can suppress diffusion and maintain localization. Furthermore, repulsive atomic interactions can enhance high-harmonic generation yields by several orders of magnitude. This study offers a new perspective for quantum simulations of ultrafast dynamics in strong fields and reveals the regulatory role of interactions in condensates on non-equilibrium dynamical processes. △ Less

Submitted 6 February, 2026; originally announced February 2026.

Comments: 8 pages, 7 figures

Ensemble-Based Data Assimilation for Material Model Characterization in High-Velocity Impact

Authors: Rong Jin, Guangyao Wang, Xingsheng Sun

Abstract: High-fidelity simulations are essential for predicting material behavior under high-velocity impact (HVI), but their accuracy depends on material models and parameters that are often calibrated by manual fitting to multiple costly experiments. In this study, we develop an ensemble-based data assimilation framework for automatic calibration of selected plasticity, fracture, and equation-of-state (E… ▽ More High-fidelity simulations are essential for predicting material behavior under high-velocity impact (HVI), but their accuracy depends on material models and parameters that are often calibrated by manual fitting to multiple costly experiments. In this study, we develop an ensemble-based data assimilation framework for automatic calibration of selected plasticity, fracture, and equation-of-state (EOS) parameters in HVI simulations using data from a single HVI test. The framework combines Smoothed Particle Hydrodynamics, the ensemble Kalman filter (EnKF), and adaptive covariance inflation to mitigate variance collapse. A simple benchmark first shows that the framework is at least one order of magnitude more computationally efficient than Markov chain Monte Carlo at comparable identification accuracy. We then use synthetic back-face deflection data for an AZ31B magnesium plate to identify representative parameters in the Johnson-Cook plasticity and fracture models and the Mie-Gruneisen EOS. Results under under-biased, over-biased, and limited-observation cases show that parameters to which the data are sufficiently sensitive can be accurately recovered within about five iterations, with convergent ensemble standard deviations. In contrast, insensitive parameters tend to converge to incorrect values and retain large ensemble spreads. With fewer observations, convergence is still achieved for sensitive parameters but requires more iterations. Under extreme prior bias, a parameter rejuvenation strategy drives sensitive parameters toward the true values even when the truth lies outside the initial ensemble spread. These results show that ensemble standard deviation provides a practical diagnostic for parameter sensitivity, identifiability, and potential non-uniqueness. Overall, the proposed framework offers an efficient and robust approach for material model characterization in HVI problems. △ Less

Submitted 31 March, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Comments: 33 pages, 10 figures

Journal ref: International Journal of Impact Engineering 215, 105738 (2026)

Role of Random Interaction Connection in the Order Transition of Active Matter Based on the Vicsek Model

Authors: Ruizhi Jin, Kejun Dong

Abstract: Randomness plays a key role in the order transition of active matter but has not yet been explicitly considered in pairwise interaction connection. In this letter, we introduce the perception rate P into the Vicsek model as the probability of the interaction connections and model the connections as superposition states. We show that with increasing P, the polar order number undergoes an order tran… ▽ More Randomness plays a key role in the order transition of active matter but has not yet been explicitly considered in pairwise interaction connection. In this letter, we introduce the perception rate P into the Vicsek model as the probability of the interaction connections and model the connections as superposition states. We show that with increasing P, the polar order number undergoes an order transition and then saturation. The order transition is a first-order phase transition with band formation, and the effect of P is different from density. The change of the order number is linked with the interaction structure. The order transition, order saturation, and phase separation correspond to different critical changes in the local interaction number. The global interaction structure is further analyzed as a network. The decrease of P is comparable to random edge removal, under which the network experiences modal transitions near the critical points of the order number, and the network exhibits surprising robustness. Our results suggest that random interaction can be a new important factor in active matter models, with potential applications in robotic swarms and social activities. △ Less

Submitted 18 January, 2025; originally announced January 2025.

Comments: 8 pages, 5 figures

Deep Learning Based Superconductivity: Prediction and Experimental Tests

Authors: Daniel Kaplan, Adam Zhang, Joanna Blawat, Rongying Jin, Robert J. Cava, Viktor Oudovenko, Gabriel Kotliar, Anirvan M. Sengupta, Weiwei Xie

Abstract: The discovery of novel superconducting materials is a longstanding challenge in materials science, with a wealth of potential for applications in energy, transportation, and computing. Recent advances in artificial intelligence (AI) have enabled expediting the search for new materials by efficiently utilizing vast materials databases. In this study, we developed an approach based on deep learning… ▽ More The discovery of novel superconducting materials is a longstanding challenge in materials science, with a wealth of potential for applications in energy, transportation, and computing. Recent advances in artificial intelligence (AI) have enabled expediting the search for new materials by efficiently utilizing vast materials databases. In this study, we developed an approach based on deep learning (DL) to predict new superconducting materials. We have synthesized a compound derived from our DL network and confirmed its superconducting properties in agreement with our prediction. Our approach is also compared to previous work based on random forests (RFs). In particular, RFs require knowledge of the chemical properties of the compound, while our neural net inputs depend solely on the chemical composition. With the help of hints from our network, we discover a new ternary compound $\textrm{Mo}_{20} \textrm{Re}_{6} \textrm{Si}_{4}$, which becomes superconducting below 5.4 K. We further discuss the existing limitations and challenges associated with using AI to predict and, along with potential future research directions. △ Less

Submitted 17 December, 2024; originally announced December 2024.

Comments: 14 pages + 2 appendices + references. EPJ submission

Journal ref: Eur. Phys. J. Plus (2025) 140:58

Measuring Chemical Shifts with Energy-Dispersive X-ray Spectroscopy

Authors: Yueyun Chen, Rebekah Jin, Yarin Heffes, Brian Zutter, Tristan P. O'Neill, Jared J. Lodico, B. C. Regan, Matthew Mecklenburg

Abstract: Electron microscopy prevalently uses energy-dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) for elemental analysis. EDS and EELS energy resolutions are commonly between 30-100 eV or 0.01-1 eV, respectively. Large solid angle EDS detector technology has increased collection efficiency to enable precision spectroscopy via averaging of 0.02-0.1 eV. This improved preci… ▽ More Electron microscopy prevalently uses energy-dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) for elemental analysis. EDS and EELS energy resolutions are commonly between 30-100 eV or 0.01-1 eV, respectively. Large solid angle EDS detector technology has increased collection efficiency to enable precision spectroscopy via averaging of 0.02-0.1 eV. This improved precision gives access to chemical shifts; examples are shown in compounds of Al, Ti, and W. EDS can now detect chemical information in a complementary parameter space (accelerating voltage, thickness, atomic number) to that covered by EELS. △ Less

Submitted 23 September, 2024; originally announced September 2024.

Comments: 6 main text pages, 18 supplementary text pages

Large Bandgap Observed on the Surfaces of EuZn2As2 Single Crystals

Authors: Dejia Kong, Siavash Karbasizadeh, Ganesh Narasimha, Paras Regmi, Chenggang Tao, Sai Mu, Rama Vasudevan, Ian Harrison, Rongying Jin, Zheng Gai

Abstract: EuM2As2 (M = Zn, Cd, In, Sn etc.) is an excellent material system for studying topological properties, which can be easily tuned by magnetism involved. Theoretical calculations predict gapped and flat bands in EuZn2As2 but gapless structure in EuCd2As2. In this work, low-temperature (77 K) cleaved EuZn2As2 crystals are studied using scanning tunneling microscopy/spectroscopy (STM/S) and density fu… ▽ More EuM2As2 (M = Zn, Cd, In, Sn etc.) is an excellent material system for studying topological properties, which can be easily tuned by magnetism involved. Theoretical calculations predict gapped and flat bands in EuZn2As2 but gapless structure in EuCd2As2. In this work, low-temperature (77 K) cleaved EuZn2As2 crystals are studied using scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations. Defects-induced local density of states (LDOS) modification with a triangular shape helps identify the surface terminations: Eu versus AsZn surface. While large bandgaps (~1.5 eV at 77 K) are observed on both pristine surfaces, the bandgap width is found to be very sensitive to local heterogeneity, such as defects and step edges, with the tendency of reduction. Combining experimental data with DFT simulations, we conclude that the modified bandgap in the heterogeneous area arises from Zn vacancies and/or substitution by As atoms. Our investigation offers important information for reevaluating the electron topology of the EuM2As2 family. △ Less

Submitted 16 August, 2024; originally announced August 2024.

Comments: 17 pages, 4 figures

Size and Shape Dependence of Hydrogen-Induced Phase Transformation and Sorption Hysteresis in Palladium Nanoparticles

Authors: Xingsheng Sun, Rong Jin

Abstract: We establish a computational framework to explore the atomic configuration of a metal-hydrogen (M-H) system when in equilibrium with a H environment. This approach combines Diffusive Molecular Dynamics with an iteration strategy, aiming to minimize the system's free energy and ensure uniform chemical potential across the system that matches that of the H environment. Applying this framework, we in… ▽ More We establish a computational framework to explore the atomic configuration of a metal-hydrogen (M-H) system when in equilibrium with a H environment. This approach combines Diffusive Molecular Dynamics with an iteration strategy, aiming to minimize the system's free energy and ensure uniform chemical potential across the system that matches that of the H environment. Applying this framework, we investigate H chemical potential-composition isotherms during the hydrogenation and dehydrogenation of palladium nanoparticles, ranging in size from $3.9$ nm to $15.6$ nm and featuring various shapes including cube, rhombic dodecahedron, octahedron, and sphere. Our findings reveal an abrupt phase transformation in all examined particles during both H loading and unloading processes, accompanied by a distinct hysteresis gap between absorption and desorption chemical potentials. Notably, as particle size increases, absorption chemical potential rises while desorption chemical potential declines, consequently widening the hysteresis gap across all shapes. Regarding shape effects, we observe that, at a given size, cubic particles exhibit the lowest absorption chemical potentials during H loading, whereas octahedral particles demonstrate the highest. Moreover, octahedral particles also exhibit the highest desorption chemical potentials during H unloading. These size and shape effects are elucidated by statistics of atomic volumetric strains resulting from specific facet orientations and inhomogeneous H distributions. Prior to phase transformation in absorption, a H-rich surface shell induces lattice expansion in the H-poor core, while before phase transformation in desorption, surface stress promotes lattice compression in the H-rich core. The magnitude of the volumetric strains correlates well with the size and shape dependence, underlining their pivotal role in the observed phenomena. △ Less

Submitted 14 May, 2024; originally announced May 2024.

Multiscale structure-property discovery via active learning in scanning tunneling microscopy

Authors: Ganesh Narasimha, Dejia Kong, Paras Regmi, Rongying Jin, Zheng Gai, Rama Vasudevan, Maxim Ziatdinov

Abstract: Atomic arrangements and local sub-structures fundamentally influence emergent material functionalities. The local structures are conventionally probed using spatially resolved studies and the property correlations are usually deciphered by a researcher based on sequential explorations and auxiliary information, thus limiting the throughput efficiency. Here we demonstrate a Bayesian deep learning b… ▽ More Atomic arrangements and local sub-structures fundamentally influence emergent material functionalities. The local structures are conventionally probed using spatially resolved studies and the property correlations are usually deciphered by a researcher based on sequential explorations and auxiliary information, thus limiting the throughput efficiency. Here we demonstrate a Bayesian deep learning based framework that automatically correlates material structure with its electronic properties using scanning tunneling microscopy (STM) measurements in real-time. Its predictions are used to autonomously direct exploration toward regions of the sample that optimize a given material property. This autonomous method is deployed on the low-temperature ultra-high vacuum STM to understand the structure-property relationship in a europium-based semimetal, EuZn2As2, one of the promising candidates for studying the magnetism-driven topological properties. The framework employs a sparse sampling approach to efficiently construct the scalar-property space using a minimal number of measurements, about 1 - 10 % of the data required in standard hyperspectral imaging methods. We further demonstrate a target-property-guided active learning of structures within a multiscale framework. This is implemented across length scales in a hierarchical fashion for the autonomous discovery of structural origins for an observed material property. This framework offers the choice to select and derive a suitable scalar property from the spectroscopic data to steer exploration across the sample space. Our findings reveal correlations of the electronic properties unique to surface terminations, local defect density, and point defects. △ Less

Submitted 10 April, 2024; originally announced April 2024.

Thermoelectric Transport in Weyl Semimetal BaMnSb2: a First-Principles Study

Authors: Yubi Chen, Rongying Jin, Bolin Liao, Sai Mu

Abstract: Topological materials are often associated with exceptional thermoelectric properties. Orthorhombic BaMnSb2 is a topological semimetal consisting of alternating layers of Ba, Sb, and MnSb. A recent experiment demonstrates that BaMnSb2 has a low thermal conductivity and modest thermopower, promising as a thermoelectric material. Through first-principles calculations with Coulomb repulsion and spin-… ▽ More Topological materials are often associated with exceptional thermoelectric properties. Orthorhombic BaMnSb2 is a topological semimetal consisting of alternating layers of Ba, Sb, and MnSb. A recent experiment demonstrates that BaMnSb2 has a low thermal conductivity and modest thermopower, promising as a thermoelectric material. Through first-principles calculations with Coulomb repulsion and spin-orbit coupling included, we studied the electronic structure, phononic structure, and thermoelectric transport properties of BaMnSb2 in depth. We find that BaMnSb2 exhibits a low lattice thermal conductivity, owing to the scattering of the acoustic phonons with low-frequency optical modes. Using the linearized Boltzmann transport theory with a constant relaxation time approximation, the thermopower is further calculated and an intriguing goniopolar transport behavior, which is associated with both n-type and p-type conduction along separate transport directions simultaneously, is observed. We propose that the figure of merit can be enhanced via doping in which electrical conductivity is decreased while the thermopower remains undiminished. BaMnSb2 is a potential platform for elucidating complex band structure effects and topological phenomena, paving the way to explore rich physics in low-dimensional systems. △ Less

Submitted 1 March, 2024; originally announced March 2024.

Multigap nodeless superconductivity in the topological semimetal PdTe

Authors: Chengcheng Zhao, Xiangqi Liu, Jinjin Wang, Chunqiang Xu, Baomin Wang, Wei Xia, Zhenhai Yu, Xiaobo Jin, Xu Zhang, Jing Wang, Dongzhe Dai, Chengpeng Tu, Jiaying Nie, Hanru Wang, Yihan Jiao, Daniel Duong, Silu Huang, Rongying Jin, Zhu'an Xu, Yanfeng Guo, Xiaofeng Xu, Shiyan Li

Abstract: Recently PdTe was identified as a spin-orbit coupled topological Dirac semimetal and was claimed to exhibit both bulk-nodal and surface-nodeless superconducting gaps. Here we report the ultralow-temperature thermal conductivity measurements on PdTe single crystals with $T_c$ = 4.5 K to investigate its superconducting gap structure. It is found that the residual linear term $κ_0/T$ is negligible in… ▽ More Recently PdTe was identified as a spin-orbit coupled topological Dirac semimetal and was claimed to exhibit both bulk-nodal and surface-nodeless superconducting gaps. Here we report the ultralow-temperature thermal conductivity measurements on PdTe single crystals with $T_c$ = 4.5 K to investigate its superconducting gap structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0(H)/T$ exhibits an $\sf S$-shaped curve. These results suggest that PdTe has multiple nodeless superconducting gaps, which is at odds with the claimed bulk-nodal gap. The reason for the discrepancy is likely that previous angle-resolved photoemission spectroscopy measurements were only performed down to 2 K and cannot observe the smaller nodeless gap. The fully gapped superconducting state in PdTe is compatible with it being a topological superconductor candidate. △ Less

Submitted 12 October, 2023; originally announced October 2023.

Evidence for Unconventional Superconductivity and Nontrivial Topology in PdTe

Authors: Ramakanta Chapai, P. V. Sreenivasa Reddy, Lingyi Xing, David E. Graf, Amar B. Karki, Tay-Rong Chang, Rongying Jin

Abstract: PdTe is a superconductor with Tc ~4.25 K. Recently, evidence for bulk-nodal and surface-nodeless gap features has been reported in PdTe [Yang et al., Phys. Rev. Lett. 130, 046402 (2023)]. Here, we investigate the physical properties of PdTe in both the normal and superconducting states via specific heat and magnetic torque measurements and first-principles calculations. Below Tc, the electronic sp… ▽ More PdTe is a superconductor with Tc ~4.25 K. Recently, evidence for bulk-nodal and surface-nodeless gap features has been reported in PdTe [Yang et al., Phys. Rev. Lett. 130, 046402 (2023)]. Here, we investigate the physical properties of PdTe in both the normal and superconducting states via specific heat and magnetic torque measurements and first-principles calculations. Below Tc, the electronic specific heat initially decreases in T3 behavior (1.5 K < T < Tc) then exponentially decays. Using the two-band model, the superconducting specific heat can be well described with two energy gaps: one is 0.372 meV and another 1.93 meV. The calculated bulk band structure consists of two electron bands (α and \b{eta}) and two hole bands (γ and η) at the Fermi level. Experimental detection of the de Haas-van Alphen (dHvA) oscillations allows us to identify four frequencies (Fα = 65 T, F\b{eta} = 658 T, Fγ = 1154 T, and Fη = 1867 T for H // a), consistent with theoretical predictions. Nontrivial α and \b{eta} bands are further identified via both calculations and the angle dependence of the dHvA oscillations. Our results suggest that PdTe is a candidate for unconventional superconductivity. △ Less

Submitted 27 April, 2023; originally announced April 2023.

Comments: 23 pages, 11 figures (including supplementary material)

Journal ref: Scientific Reports 13, 6824 (2023)

Quantum-limit phenomena and bandstructure in the magnetic topological semimetal EuZn2As2

Authors: Joanna Blawat, Smita Speer, John Singleton, Weiwei Xie, Rongying Jin

Abstract: We have experimentally investigated the low-temperature (0.6 K) electronic and magnetic properties of the layered antiferromagnet EuZn2As2 in pulsed magnetic fields of up to 60 T at a temperature of 0.6 K, giant positive magnetoresistance (MR) is observed above μ_{0}H ~ 20 T, a regime in which the spins are already fully polarized. Both magnetic torque and proximity detector oscillator (PDO) data… ▽ More We have experimentally investigated the low-temperature (0.6 K) electronic and magnetic properties of the layered antiferromagnet EuZn2As2 in pulsed magnetic fields of up to 60 T at a temperature of 0.6 K, giant positive magnetoresistance (MR) is observed above μ_{0}H ~ 20 T, a regime in which the spins are already fully polarized. Both magnetic torque and proximity detector oscillator (PDO) data show no corresponding anomaly at or close to this field. By analyzing the quantum oscillations observed in the MR and PDO frequency, we find that (1) the oscillation frequency F = 46 \pm 6 T for H // c and 42 \pm 2 T for H // ab; (2) the corresponding Berry phase is close to πfor H // c, implying a nontrivial topology; and (3) the large linear MR at high fields corresponds to the quantum limit (i.e., only the last Landau level being occupied). From these observations we conclude that the linear MR can be understood by considering diffusing cyclotron centers in the quantum limit. Our findings help understand the intimate relationship between magnetism and electronic topology in EuZn2As2 under extremely high fields and suggest reasons for the emergent behavior in the quantum limit. △ Less

Submitted 24 April, 2023; originally announced April 2023.

Multi-k magnetic structure and large anomalous Hall effect in candidate magnetic Weyl semimetal NdAlGe

Authors: C. Dhital, R. L. Dally, R. Ruvalcaba, R. Gonzalez-Hernandez, J. Guerrero-Sanchez, H. B. Cao, Q. Zhang, W. Tian, Y. Wu, M. D. Frontzek, S. K. Karna, A. Meads, B. Wilson, R. Chapai, D. Graf, J. Bacsa, R. Jin, J. F. DiTusa

Abstract: The magnetic structure, magnetoresistance, and Hall effect of non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this non-centrosymmetric compound. The magnetization and magnetoresistance measurements are both highly anisotropic and indicate an Ising-like magnetic… ▽ More The magnetic structure, magnetoresistance, and Hall effect of non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this non-centrosymmetric compound. The magnetization and magnetoresistance measurements are both highly anisotropic and indicate an Ising-like magnetic system. The magnetic structure is complex in that it involves three magnetic ordering vectors including an incommensurate spin density wave and commensurate ferrimagnetic state in zero field. We have discovered a large anomalous Hall conductivity that reaches = 430 Ω-1cm-1 implying that it originates from an intrinsic Berry curvature effect stemming from Weyl nodes found in the electronic structure. These electronic structure calculations indicate the presence of nested Fermi surface pockets with nesting wave vectors similar to the measured magnetic ordering wavevector and the presence of Weyl nodes in proximity to the Fermi surface. We associate the incommensurate magnetic structure with the large anomalous Hall response to be the result of the combination of Fermi surface nesting and the Berry curvature associated with Weyl nodes. △ Less

Submitted 29 June, 2023; v1 submitted 10 February, 2023; originally announced February 2023.

Comments: 46 pages, 12 figures

Journal ref: PRB 2023

Coexistence of bulk-nodal and surface-nodeless Cooper pairings in a superconducting Dirac semimetal

Authors: Xian P. Yang, Yigui Zhong, Sougata Mardanya, Tyler A. Cochran, Ramakanta Chapai, Akifumi Mine, Junyi Zhang, Jaime Sánchez-Barriga, Zi-Jia Cheng, Oliver J. Clark, Jia- Xin Yin, Joanna Blawat, Guangming Cheng, Ilya Belopolski, Tsubaki Nagashima, Najafzadeh Sahand, Shiyuan Gao, Nan Yao, Arun Bansil, Rongying Jin, Tay-Rong Chang, Shik Shin, Kozo Okazaki, M. Zahid Hasan

Abstract: The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoe… ▽ More The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling. △ Less

Submitted 3 January, 2023; originally announced January 2023.

Comments: accepted by PRL

Unusual electrical and magnetic properties in layered EuZn2As2

Authors: Joanna Blawat, Madalynn Marshall, John Singleton, Erxi Feng, Huibo Cao, Weiwei Xie, Rongying Jin

Abstract: Eu-based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd2As2. We investigate the structure and physical properties of EuZn2As2 through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Our data show that EuZn2As2 orders antiferromagnetically with an A-type spin configura… ▽ More Eu-based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd2As2. We investigate the structure and physical properties of EuZn2As2 through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Our data show that EuZn2As2 orders antiferromagnetically with an A-type spin configuration below TN = 19 K. Surprisingly, there is strong evidence for dominant ferromagnetic fluctuations above TN, as reflected by positive Curie-Weiss temperature and extremely large negative magnetoresistance (MR) between TN and Tfl » 200 K. Furthermore, the angle dependence of the MRab indicates field-induced spin reorientation from the ab-plane to a direction approximately 45° from the ab plane. Compared to EuCd2As2, the doubled TN and Tfl make EuZn2As2 a better platform for exploring topological properties in both magnetic fluctuation (TN < T < Tfl) and ordered (T < TN) regimes. △ Less

Submitted 11 February, 2022; originally announced February 2022.

Comments: 17 pages, 4 figures

Magnetic Texture in Insulating Single Crystal High Entropy Oxide Spinel Films

Authors: Yogesh Sharma, Alessandro R. Mazza, Brianna L. Musico, Elizabeth Skoropata, Roshan Nepal, Rongying Jin, Anton V. Ievlev, Liam Collins, Zheng Gai, Aiping Chen, Matthew Brahlek, Veerle Keppens, Thomas Z. Ward

Abstract: Magnetic insulators are important materials for a range of next generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators which can be synthesized with surface… ▽ More Magnetic insulators are important materials for a range of next generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators which can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single crystal (Mg0.2Ni0.2Fe0.2Co0.2Cu0.2)Fe2O4 films on substrates spanning a range of strain states. These films show smooth surfaces, high resistivity, and strong magnetic responses at room temperature. Local and global magnetic measurements further demonstrate how strain can be used to manipulate magnetic texture and anisotropy. These findings provide insight into how precise magnetic responses can be designed using compositionally complex materials that may find application in next generation magnetic devices. △ Less

Submitted 30 March, 2021; originally announced March 2021.

Helical magnetic order and Fermi surface nesting in non-centrosymmetric ScFeGe

Authors: Sunil K. Karna, D. Tristant, J. K. Hebert, G. Cao, R. Chapai, W. A. Phelan, Q. Zhang, Y. Wu, C. Dhital, Y. Li, H. B. Cao, W. Tian, C. R. Dela Cruz, A. A. Aczel, O. Zaharko, A. Khasanov, M. A. McGuire, A. Roy, W. Xie, D. A. Browne, I. Vekhter, V. Meunier, W. A. Shelton, P. W. Adams, P. T. Sprunger , et al. (3 additional authors not shown)

Abstract: An investigation of the structural, magnetic, thermodynamic, and charge transport properties of non-centrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below $T_N = 36$ K. Neutron diffraction measurements discovered a temperature and field independent helical wavevector \textbf{\textit{k}} = (0 0 0.193) with… ▽ More An investigation of the structural, magnetic, thermodynamic, and charge transport properties of non-centrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below $T_N = 36$ K. Neutron diffraction measurements discovered a temperature and field independent helical wavevector \textbf{\textit{k}} = (0 0 0.193) with magnetic moments of 0.53 $μ_{B}$ per formula unit confined to the {\it ab}-plane. Density functional theory calculations are consistent with these measurements and find several bands that cross the Fermi level along the {\it c}-axis with a nearly degenerate set of flat bands just above the Fermi energy. The anisotropy found in the electrical transport is reflected in the calculated Fermi surface, which consists of several warped flat sheets along the $c$-axis with two regions of significant nesting, one of which has a wavevector that closely matches that found in the neutron diffraction. The electronic structure calculations, along with a strong anomaly in the {\it c}-axis conductivity at $T_N$, signal a Fermi surface driven magnetic transition, similar to that found in spin density wave materials. Magnetic fields applied in the {\it ab}-plane result in a metamagnetic transition with a threshold field of $\approx$ 6.7 T along with a sharp, strongly temperature dependent, discontinuity and a change in sign of the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to investigate the effect of in-plane magnetic fields on an easy-plane magnetic system, where the relative strength of the magnetic interactions and anisotropies determine the topology and magnetic structure. △ Less

Submitted 29 September, 2020; originally announced September 2020.

Comments: 15 pages, 13 figures

Journal ref: Phys. Rev. B 103, 014443 (2021)

Crystal and Magnetic Structure of Polar Oxide HoCrWO$_6$

Authors: C. Dhital, D. Pham, T. Lawal, C. Bucholz, A. Poyraz, Q. Zhang, R. Nepal, R. Jin, R. Rai

Abstract: Polar magnetic oxide HoCrWO$_6$ is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO$_6$ forms the polar crystal structure (space group Pna2$_1$ (#33)) due to the cation ordering of W$^{6+}$ and Cr$^{3+}$. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.$^{-1}$K$^{-1}$ at 70 kOe).… ▽ More Polar magnetic oxide HoCrWO$_6$ is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO$_6$ forms the polar crystal structure (space group Pna2$_1$ (#33)) due to the cation ordering of W$^{6+}$ and Cr$^{3+}$. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.$^{-1}$K$^{-1}$ at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)$μ_B$ and 8.7(4)$μ_B$ at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the $b$ axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO$_6$ and DyCrWO$_6$ indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of eg electrons in the transition metal sublattice. △ Less

Submitted 16 July, 2020; originally announced July 2020.

Comments: 7 figures, 22 pages. Journal of magnetism and magnetic materials, 2020

Observation of sixfold degenerate fermions in PdSb$_2$

Authors: Xiàn Yáng, Tyler A. Cochran, Ramakanta Chapai, Damien Tristant, Jia-Xin Yin, Ilya Belopolski, Zǐjiā Chéng, Daniel Multer, Songtian S. Zhang, Nana Shumiya, Maksim Litskevich, Yuxiao Jiang, Guoqing Chang, Qi Zhang, Ilya Vekhter, William A. Shelton, Rongying Jin, Su-Yang Xu, M. Zahid Hasan

Abstract: Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerat… ▽ More Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerate bands in PdSb$_2$. The ultrahigh energy resolution we are able to achieve allows for the confirmation of all the sixfold degenerate bands at the R point, in remarkable consistency with first-principles calculations. Moreover, we find that this sixfold degenerate crossing has quadratic dispersion as predicted by theory. Finally, we compare sixfold degenerate fermions with previously confirmed fermions to demonstrate the importance of this work: our study indicates a topological fermion beyond the constraints of high energy physics. △ Less

Submitted 20 May, 2020; originally announced May 2020.

Journal ref: Phys Rev B. 101, 201105(R) (2020)

Observation of a mesoscopic magnetic modulation in chiral Mn1/3NbS2

Authors: Sunil K. Karna, F. N. Womack, R. Chapai, D. P. Young, M. Marshall, Weiwei Xie, D. Graf, Yan Wu, Huibo Cao, L. DeBeer-Schmitt, P. W. Adams, R. Jin, J. F. DiTusa

Abstract: We have investigated the structural, magnetic, thermodynamic, and charge transport properties of Mn1/3NbS2 single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall effect measurements. Mn1/3NbS2 displays a magnetic transition at TC ~ 45 K with highly anisotropic behavior expected for a hexagonal structured material. Below TC, neutron diffract… ▽ More We have investigated the structural, magnetic, thermodynamic, and charge transport properties of Mn1/3NbS2 single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall effect measurements. Mn1/3NbS2 displays a magnetic transition at TC ~ 45 K with highly anisotropic behavior expected for a hexagonal structured material. Below TC, neutron diffraction reveals increased scattering near the structural Bragg peaks having a wider Q-dependence along the c-axis than the nuclear Bragg peaks. This indicates helimagnetism with a long pitch length of ~250 nm (or a wavevector q~0.0025 Å-1) along the c-axis. This q is substantially smaller than that found for the helimagnetic state in isostructural Cr1/3NbS2 (0.015 Å-1). Specific heat capacity measurements confirm a second-order magnetic phase transition with a substantial magnetic contribution that persists to low temperature. The large low-temperature specific heat capacity is consistent with a large density of low-lying magnetic excitations that are likely associated with topologically interesting magnetic modes. Changes to the magnetoresistance, the magnetization, and the magnetic neutron diffraction, which become more apparent below 20 K, imply a modification in the character of the magnetic ordering corresponding to the magnetic contribution to the specific heat capacity. These observations signify a more complex magnetic structure both at zero and finite fields for Mn1/3NbS2 than for the well-investigated Cr1/3NbS2. △ Less

Submitted 1 May, 2019; originally announced May 2019.

Comments: 22 pages, 7 figures

Journal ref: Phys. Rev. B 100, 184413 (2019)

Fermions and bosons in nonsymmorphic PdSb2 with sixfold degeneracy

Authors: Ramakanta Chapai, Yating Jia, W. A. Shelton, Roshan Nepal, Mohammad Saghayezhian, J. F. DiTusa, E. W. Plummer, Changqing Jin, Rongying Jin

Abstract: PdSb2 is a candidate for hosting 6-fold-degenerate exotic fermions (beyond Dirac and Weyl fermions).The nontrivial band crossing protected by the nonsymmorphic symmetry plays a crucial role in physical properties. We have grown high-quality single crystals of PdSb2 and characterized their physical properties under several stimuli (temperature, magnetic field, and pressure). While it is a diamagnet… ▽ More PdSb2 is a candidate for hosting 6-fold-degenerate exotic fermions (beyond Dirac and Weyl fermions).The nontrivial band crossing protected by the nonsymmorphic symmetry plays a crucial role in physical properties. We have grown high-quality single crystals of PdSb2 and characterized their physical properties under several stimuli (temperature, magnetic field, and pressure). While it is a diamagnetic Fermi-liquid metal under ambient pressure, PdSb2 exhibits a large magnetoresistance with continuous increase up to 14 T, which follows the Kohler's scaling law at all temperatures. This implies one-band electrical transport, although multiple bands are predicted by first principles calculations. By applying magnetic field along the [111] direction, de Haas-van Alphen oscillations are observed with frequency of 102 T. The effective mass is nearly zero (0.045m0) with the Berry phase close to π, confirming that the band close to the R point has a nontrivial character. Under quasihydrostatic pressure (p), evidence for superconductivity is observed in the resistivity below the critical temperature Tc. The dome-shaped Tc versus p is obtained with maximum Tc~2.9 K. We argue that the formation of Cooper pairs (bosons) is the consequence of the redistribution of the 6-fold-degenerate fermions under pressure. △ Less

Submitted 16 April, 2019; originally announced April 2019.

Journal ref: Phys. Rev. B 99, 161110(R) (2019)

Anomalous Magnetic Behavior in Ba2CoO4 with Isolated CoO4 Tetrahedra

Authors: Qiang Zhang, Guixin Cao, Feng Ye, Huibo Cao, Masaaki Matsuda, D. A. Tennant, Songxue Chi, S. E. Nagler, W. A. Shelton, Rongying Jin, E. W. Plummer, Jiandi Zhang

Abstract: The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in… ▽ More The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to TN = 26 K. More unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate that Ba2CoO4 is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and novel spin states in oxides. △ Less

Submitted 6 March, 2019; originally announced March 2019.

Comments: to be published in Physical Review B

Structure-Property Relationship in Layered BaMn2Sb2 and Ba2Mn3Sb2O2

Authors: Qiang Zhang, Zhenyu Diao, Huibo Cao, Ahmad Saleheen, Ramakanta Chapai, Dongliang Gong, Shane Stadler, R. Jin

Abstract: Layered transition-metal compounds have received great attention owing to their novel physical properties. Here, we present the structural, electronic, thermal, and magnetic properties of BaMn2Sb2 and Ba2Mn3Sb2O2 single crystals, both with the layered structure analogous to high-temperature superconductors. While the Mn moment in the MnSb4 tetrahedral environment forms G-type antiferromagnetic (AF… ▽ More Layered transition-metal compounds have received great attention owing to their novel physical properties. Here, we present the structural, electronic, thermal, and magnetic properties of BaMn2Sb2 and Ba2Mn3Sb2O2 single crystals, both with the layered structure analogous to high-temperature superconductors. While the Mn moment in the MnSb4 tetrahedral environment forms G-type antiferromagnetic (AFM) ordering in both BaMn2Sb2 (TN1~443 K) and Ba2Mn3Sb2O2 (TN1~314 K), a short-range AFM order is found in the intercalated MnO2 layer at a much lower temperature (TN2~60 K) in Ba2Mn3Sb2O2. The directions of the ordered moments in these two magnetic sub-lattices of Ba2Mn3Sb2O2 are perpendicular to each other, even though the system is electrically conductive. This indicates that the large magnetic moments in these compounds are highly localized, leading to negligible coupling between MnSb4 and MnO2 layers in Ba2Mn3Sb2O2. These findings provide an insight into the structure-magnetism-based design principle for new superconductors. △ Less

Submitted 4 January, 2019; originally announced January 2019.

Comments: 15 pages, 4 figures. 2 tables

Journal ref: Phys. Rev. B 99, 184416 (2019)

Topological phases in the TaSe3 compound

Authors: Simin Nie, Lingyi Xing, Rongying Jin, Weiwei Xie, Zhijun Wang, Fritz B. Prinz

Abstract: Based on first-principles calculations, we show that stoichiometric TaSe3, synthesized in space group P21/m, belongs to a three-dimensional (3D) strong topological insulator (TI) phase with Z2 invariants (1;100). The calculated surface spectrum shows clearly a single Dirac cone on surfaces, with helical spin texture at a constant energy contour. To check the stability of the topological phase, str… ▽ More Based on first-principles calculations, we show that stoichiometric TaSe3, synthesized in space group P21/m, belongs to a three-dimensional (3D) strong topological insulator (TI) phase with Z2 invariants (1;100). The calculated surface spectrum shows clearly a single Dirac cone on surfaces, with helical spin texture at a constant energy contour. To check the stability of the topological phase, strain effects have been systematically investigated, showing that many topological phases survive in a wide range of the strains along both the a- and c-axes, such as strong TI (STI), weak TI (WTI) and Dirac semimetal phases. TaSe3 provides us an ideal platform for experimental study of topological phase transitions. More interestingly, since superconductivity in TaSe3 has been reported for a long time, the co-existence of topological phases and superconducting phase suggests that TaSe3 is a realistic system to study the interplay between topological and superconducting phases in the future. △ Less

Submitted 6 November, 2018; originally announced November 2018.

Comments: 7 pages, 6 figures

Journal ref: Phys. Rev. B 98, 125143 (2018)

Mechanisms of DNA Hybridization: Transition Path Analysis of a Simulation-Informed Markov Model

Authors: Raymond Jin, Lutz Maibaum

Abstract: Complementary DNA strands in solution reliably hybridize to form stable duplexes. We study the kinetics of the hybridization process and the mechanisms by which two initially isolated strands come together to form a stable double helix. We adopt a multi-step computational approach. First, we perform a large number of Brownian dynamics simulations of the hybridization process using the coarse-grain… ▽ More Complementary DNA strands in solution reliably hybridize to form stable duplexes. We study the kinetics of the hybridization process and the mechanisms by which two initially isolated strands come together to form a stable double helix. We adopt a multi-step computational approach. First, we perform a large number of Brownian dynamics simulations of the hybridization process using the coarse-grained oxDNA2 model. Second, we use these simulations to construct a Markov State Model of DNA dynamics that uses a state decomposition based on the inter-strand hydrogen bonding pattern. Third, we take advantage of Transition Path Theory to obtain quantitative information about the thermodynamic and dynamic properties of the hybridization process. We find that while there is a large ensemble of possible hybridization pathways there is a single dominant mechanism in which an initial base pair forms close to either end of the nascent double helix, and the remaining bases pair sequentially in a zipper-like fashion. We also show that the number of formed base pairs by itself is insufficient to describe the transition state of the hybridization process. △ Less

Submitted 31 August, 2018; v1 submitted 11 July, 2018; originally announced July 2018.

Electron Counts, Structural Stability, and Magnetism in BaCuSn$_2$-CeNi$_1$$_-$$_x$Si$_2$-type YT$_x$Ge$_2$ (T= Cr, Mn, Fe, Co, and Ni)

Authors: Lea Gustin, Lingyi Xing, Max T. Pan, Rongying Jin, Weiwei Xie

Abstract: Results of crystallographic refinement, the relationship between electron counts and structural stability, and magnetic characterization of YT$_x$Ge$_2$ (T= Cr, Mn, Fe, Co, and Ni) prepared using the arc melting method are presented. These YT$_x$Ge$_2$ compounds crystallize in the BaCuSn$_2$-CeNi$_1$$_-$$_x$Si$_2$-type structure with space group Cmcm, and the site occupancies of 3d transition meta… ▽ More Results of crystallographic refinement, the relationship between electron counts and structural stability, and magnetic characterization of YT$_x$Ge$_2$ (T= Cr, Mn, Fe, Co, and Ni) prepared using the arc melting method are presented. These YT$_x$Ge$_2$ compounds crystallize in the BaCuSn$_2$-CeNi$_1$$_-$$_x$Si$_2$-type structure with space group Cmcm, and the site occupancies of 3d transition metals range from x = 0.22(1) for Cr to x = 0.66(1) for Ni. Based on a combination of single crystal and powder X-ray diffraction and scanning electron microscopy, the trends are clearly established that the smaller transition metal atoms exhibit larger occupancies on T (Cu) site. Our investigation into the relationship between electron count and site defect reveals that a stable configuration is obtained when reaching 10.3e- per transition metal (Y+T), which strongly correlates with the defect observed in the case of T metals. Magnetic properties measurements indicate paramagnetism for T = Cr, Fe, and Co, but ferromagnetism for T = Mn with a Curie temperature at 293 K and effective moment ~ 3.6 uB/Mn. The absence of superconductivity in this series is surprising because they consist of similar building blocks and electron counts to superconducting YGe$_1$$_.$$_5$$_+$$_d$Si$_2$ except for 3d transition metals. Introducing 3d transition metals into the system plays a critical role in suppressing superconductivity, offering new insights into the interplay between superconductivity and magnetism in layered intermetallics. △ Less

Submitted 22 December, 2017; originally announced December 2017.

Comments: 17 pages, 4 figures

Pt-Bi Antibonding Interaction: The Key Factor for Superconductivity in Monoclinic BaPt2Bi2

Authors: Xin Gui, Lingyi Xing, Xiaoxiong Wang, Guang Bian, Rongying Jin, Weiwei Xie

Abstract: In the search for superconductivity in BaAu2Sb2-type monoclinic structure, we have successfully synthesized a new compound BaPt2Bi2, which crystallizes in the space group P21/m (S.G. 11; Pearson symbol mP10) according to a combination of powder and single crystal X-ray diffraction and scanning electron microscopy. Sharp electrical resistivity drop and large diamagnetic magnetization below 2.0 K in… ▽ More In the search for superconductivity in BaAu2Sb2-type monoclinic structure, we have successfully synthesized a new compound BaPt2Bi2, which crystallizes in the space group P21/m (S.G. 11; Pearson symbol mP10) according to a combination of powder and single crystal X-ray diffraction and scanning electron microscopy. Sharp electrical resistivity drop and large diamagnetic magnetization below 2.0 K indicates it owns the superconducting ground state. This makes BaPt2Bi2 the first reported superconductor in mono-clinic BaAu2Sb2-type structure, a previously unappreciated structure for superconductivity. First-principles calculations considering the spin-orbit coupling indicate that Pt-Bi anti-bonding interaction plays a critical role in inducing superconductivity. △ Less

Submitted 25 November, 2017; originally announced November 2017.

Observation of Giant Exchange Bias and Topological Hall Effect in Manganese Nitride Films

Authors: Meng Meng, Shuwei Li, Mohammad Saghayezhian, E. W. Plummer, Rongying Jin

Abstract: Magnetic and magneto-transport properties of manganese nitride films grown by molecular beam epitaxy have been investigated. Due to the mixed ferrimagnetic (FI) phase ($ε$-phase with TFI ~ 738 K) and the antiferromagnetic phase ($ζ$-phase with TN ~ 273 K), we observe magnetization hysteresis loops with non-zero exchange bias below TN, reaching ~ 0.22 T at 5 K. This indicates that noncollinear spin… ▽ More Magnetic and magneto-transport properties of manganese nitride films grown by molecular beam epitaxy have been investigated. Due to the mixed ferrimagnetic (FI) phase ($ε$-phase with TFI ~ 738 K) and the antiferromagnetic phase ($ζ$-phase with TN ~ 273 K), we observe magnetization hysteresis loops with non-zero exchange bias below TN, reaching ~ 0.22 T at 5 K. This indicates that noncollinear spins exist at the interfaces between two phases, creating a competition between interfacial Dzyaloshinskii-Moriya (DM) and exchange interactions. Strikingly, in addition to the normal Hall effect by Lorentz force and anomalous Hall effect by magnetization, we observe new contribution namely topological Hall effect below 75 K. This verifies the existence of topological spin texture, which is the consequence of competing interactions controlled by both applied field and temperature. Our work demonstrates that spintronic devices may be fabricated exploiting rich magnetic properties of different phases. △ Less

Submitted 8 August, 2017; originally announced August 2017.

Comments: the main text has 18 pages, 4 figures. And the supporting information has 3 pages, 2 figures

Nontrivial Berry phase in magnetic BaMnSb2 semimetal

Authors: Silu Huang, Jisun Kim, W. A. Shelton, E. W. Plummer, Rongying Jin

Abstract: The subject of topological materials has attracted immense attention in condensed-matter physics, because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones eit… ▽ More The subject of topological materials has attracted immense attention in condensed-matter physics, because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here, we demonstrate experimentally that canted antiferromagnetic BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material. △ Less

Submitted 24 May, 2017; originally announced May 2017.

Journal ref: Proceedings of the National Academy of Sciences of the United States of America (2017)

Mn-induced magnetic symmetry breaking and its correlation with the metal-insulator transition in bilayered Sr3(Ru1-xMnx)2O7

Authors: Qiang Zhang, Feng Ye, Wei Tian, Huibo Cao, Songxue Chi, Biao Hu, Zhenyu Diao, David A. Tennant, Rongying Jin, Jiandi Zhang, Ward Plummer

Abstract: Bilayered Sr3Ru2O7 is an unusual metamagnetic metal with inherently antiferromagnetic (AFM) and ferromagnetic (FM) fluctuations. Partial substitution of Ru by Mn results in the establishment of metal-insulator transition (MIT) at TMIT and AFM ordering at TM in Sr3(Ru1-xMnx)2O7. Using elastic neutron scattering we determined the effect of Mn doping on the magnetic structure and in-plane magnetic co… ▽ More Bilayered Sr3Ru2O7 is an unusual metamagnetic metal with inherently antiferromagnetic (AFM) and ferromagnetic (FM) fluctuations. Partial substitution of Ru by Mn results in the establishment of metal-insulator transition (MIT) at TMIT and AFM ordering at TM in Sr3(Ru1-xMnx)2O7. Using elastic neutron scattering we determined the effect of Mn doping on the magnetic structure and in-plane magnetic correlation lengths in Sr3(Ru1-xMnx)2O7 (x = 0.06 and 0.12). With increasing Mn doping (x) from 0.06 to 0.12 or decreasing temperatures for x=0.12, an evolution from an in-plane short-range to long-range double-stripe AFM ground state occurs. For both compounds, the onset of magnetic correlation with an anisotropic behavior coincides with the sharp rise of the electrical resistivity and the specific heat. Since it does not induce measurable lattice distortion, the double-stripe magnetic order with anisotropic spin texture breaks the symmetry from C4v crystal lattice to C2v magnetic sublattice. These observations shed new light on an age-old question of Slater versus Mott-type MIT. △ Less

Submitted 16 May, 2017; originally announced May 2017.

Comments: 17 pages, 4 figures, accepted in Physical Review B (rapid communication)

Journal ref: Phys. Rev. B 95, 220403(R), 2017

Spin Density wave instability in a ferromagnet

Authors: Yan Wu, Zhenhua Ning, Huibo Cao, Guixin Cao, K. A. Benavides, Gregory T. McCandless, R. Jin, Julia Y. Chan, W. A. Shelton, J. F. DiTusa

Abstract: Ferromagnetic (FM) and incommensurate spin-density wave (ISDW) states are an unusual set of competing magnetic orders that are seldom observed in the same material without application of a polarizing magnetic field. We report, for the first time, the discovery of an ISDW state that is derived from a FM ground state through a Fermi surface (FS) instability in Fe$_3$Ga$_4$. This was achieved by comb… ▽ More Ferromagnetic (FM) and incommensurate spin-density wave (ISDW) states are an unusual set of competing magnetic orders that are seldom observed in the same material without application of a polarizing magnetic field. We report, for the first time, the discovery of an ISDW state that is derived from a FM ground state through a Fermi surface (FS) instability in Fe$_3$Ga$_4$. This was achieved by combining neutron scattering experiments with first principles simulations. Neutron diffraction demonstrates that Fe$_3$Ga$_4$ is in an ISDW state at intermediate temperatures and that there is a conspicuous re-emergence of ferromagnetism above 360 K. First principles calculations show that the ISDW ordering wavevector is in excellent agreement with a prominent nesting condition in the spin-majority FS demonstrating the discovery of a novel instability for FM metals; ISDW formation due to Fermi surface nesting in a spin-polarized Fermi surface. △ Less

Submitted 25 July, 2017; v1 submitted 21 April, 2017; originally announced April 2017.

Comments: 6 pages with 4 figures. Supplemental Materials Included

Delicate competing electronic states in ultrathin manganite films

Authors: Zhaoliang Liao, Rongying Jin, E. W. Plummer, Jiandi Zhang

Abstract: The coupling between the electrical transport properties of La2/3Sr1/3MnO3 (LSMO) thin films and structural phase transitions of SrTiO3 (STO) substrates at Ts = 105 K has been investigated. We found that the electrical resistivity of LSMO films exhibit a cusp at Ts, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate comp… ▽ More The coupling between the electrical transport properties of La2/3Sr1/3MnO3 (LSMO) thin films and structural phase transitions of SrTiO3 (STO) substrates at Ts = 105 K has been investigated. We found that the electrical resistivity of LSMO films exhibit a cusp at Ts, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate competing electronic states. Our results demonstrate that small amounts of strain can tip the subtle balance of competing interactions and tune the electronic properties in correlated electron materials. △ Less

Submitted 26 January, 2017; originally announced January 2017.

Comments: Accepted by Phys. Rev. B

Surface phases of the transition-metal dichalcogenide IrTe2

Authors: Chen Chen, Jisun Kim, Yifan Yang, Guixin Cao, Rongying Jin, E. W. Plummer

Abstract: Transition-metal dichalcogenide IrTe2 has attracted attention because of striped lattice, charge ordering and superconductivity. We have investigated the surface structure of IrTe2, using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). A complex striped lattice modulations as a function of temperature is observed, which shows hysteresis between cooling and warming.… ▽ More Transition-metal dichalcogenide IrTe2 has attracted attention because of striped lattice, charge ordering and superconductivity. We have investigated the surface structure of IrTe2, using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). A complex striped lattice modulations as a function of temperature is observed, which shows hysteresis between cooling and warming. While the bulk 5x1 and 8x1 phases appear at high temperatures, the surface ground state has the 6x1 phase, not seen in the bulk, and the surface transition temperatures are distinct from the bulk. The broken symmetry at the surface creates a quite different phase diagram, with the coexistence of several periodicities resembling devil's staircase behavior. △ Less

Submitted 30 December, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 95, 094118 (2017)

Effect of Negative Pressure on the Prototypical Itinerant Magnet MnSi

Authors: C. Dhital, M. A Khan, M. Saghayezhian, W. A. Phelan, D. P. Young, R. Y. Jin, J. F. DiTusa

Abstract: The evolution of the magnetic and charge transport properties of itinerant magnetic metal MnSi with the substitution of Al and Ga on the Si site is investigated. We observe an increase in unit cell volume indicating that both Al and Ga substitutions create negative chemical pressure. There are substantial increases in the Curie temperature and the ordered moment demonstrating that the substitution… ▽ More The evolution of the magnetic and charge transport properties of itinerant magnetic metal MnSi with the substitution of Al and Ga on the Si site is investigated. We observe an increase in unit cell volume indicating that both Al and Ga substitutions create negative chemical pressure. There are substantial increases in the Curie temperature and the ordered moment demonstrating that the substitutions give the magnetism a more local character. The substitutions also increase the range of temperature and field where the skyrmion phase is stable due to a change in the character of the magnetism. In contrast to the behavior of pure MnSi and expectations for the intrinsic anomalous Hall effect, we find a significant temperature dependence to the magnitude and sign of anomalous Hall conductivity constant in Al or Ga substituted samples. This temperature dependence likely reflects changes in the spin-orbit coupling strength with temperature, which may have significant consequences on the helical and skyrmion states. Overall, we observe a continuous evolution of magnetic and charge transport properties through positive to negative pressure △ Less

Submitted 15 January, 2017; v1 submitted 26 September, 2016; originally announced September 2016.

Comments: 22 pages, 9 figures

Journal ref: Phys. Rev. B 95, 024407 (2017)

Hidden Phases Revealed at the Surface of Double-Layered Sr3(Ru1-xMnx)2O7

Authors: Chen Chen, Jisun Kim, V. B. Nascimento, Zhenyu Diao, Jing Teng, Biao Hu, Guorong Li, Fangyang Liu, Jiandi Zhang, Rongying Jin, E. W. Plummer

Abstract: Double-layered Sr3Ru2O7 has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of Sr3(Ru1-… ▽ More Double-layered Sr3Ru2O7 has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of Sr3(Ru1-xMnx)2O7. Driven by the tilt distortion of RuO6 octahedra, the surface of Sr3Ru2O7 is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn-doping, the surface of Sr3(Ru0.84Mn0.16)2O7 is metallic while the bulk is insulating. Our result demonstrates that the electronic and structural properties at the surface are intimately coupled and consistent with quasi two-dimensional character. △ Less

Submitted 5 August, 2016; originally announced August 2016.

Interrogating the superconductor Ca10(Pt4As8)(Fe2-xPtxAs2)5 Layer-by-layer

Authors: Jisun Kim, Hyoungdo Nam, Guorong Li, A. B. Karki, Zhen Wang, Yimei Zhu, Chih-Kang Shih, Jiandi Zhang, Rongying Jin, E. W. Plummer

Abstract: Ever since the discovery of high-Tc superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca10(Pt4As8)(Fe2As2)5 provides opportunities to explore superconductivity layer by layer, because it contains both sup… ▽ More Ever since the discovery of high-Tc superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca10(Pt4As8)(Fe2As2)5 provides opportunities to explore superconductivity layer by layer, because it contains both superconducting building blocks (Fe2As2 layers) and intermediate Pt4As8 layers. Cleaving a single crystal under ultra-high vacuum results in multiple terminations: an ordered Pt4As8 layer, two reconstructed Ca layers on the top of a Pt4As8 layer, and disordered Ca layer on the top of Fe2As2 layer. The electronic properties of individual layers are studied using scanning tunneling microscopy/spectroscopy (STM/S), which reveals different spectra for each surface. Remarkably superconducting coherence peaks are seen only on the ordered Ca/Pt4As8 layer. Our results indicate that an ordered structure with proper charge balance is required in order to preserve superconductivity. △ Less

Submitted 27 July, 2016; originally announced July 2016.

Journal ref: Scientific Reports 6, 35365 (2016)

Surface terminations and layer-resolved spectroscopy in 122 iron pnictide superconductors

Authors: Ang Li, J. -X. Yin, Jihui Wang, Zheng Wu, Jihua Ma, Athena S. Sefat, Brian C. Sales, David G. Mandrus, Rongying Jin, Chenglin Zhang, Pengcheng Dai, Bing Lv, Xuejin Liang, P. -H. Hor, C. -S. Ting, Shuheng H. Pan

Abstract: The surface terminations of 122-type alkaline earth metal iron pnictides AEFe2As2 (AE = Ca, Ba) are investigated with scanning tunneling microscopy/spectroscopy (STM/STS). Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with atomically resolved square-root-two (rt2) or 1*2 lattice, as well as the very rare terminations with 1*1 symmetry. By means of latti… ▽ More The surface terminations of 122-type alkaline earth metal iron pnictides AEFe2As2 (AE = Ca, Ba) are investigated with scanning tunneling microscopy/spectroscopy (STM/STS). Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with atomically resolved square-root-two (rt2) or 1*2 lattice, as well as the very rare terminations with 1*1 symmetry. By means of lattice alignment and chemical marking, we identify these terminations as rt2-AE, 1*2-As, and rt2-Fe surfaces, respectively. Layer-resolved spectroscopy on these terminating surfaces reveals a well-defined superconducting gap on the As terminations, while the gap features become weaker and absent on AE and Fe terminations respectively. The local gap features are hardly affected by the surface reconstruction on As or AE surface, whereas a suppression of them along with the in-gap states can be induced by As vacancies. The emergence of two impurity resonance peaks at +-2 meV is consistent with the sign-reversal pairing symmetry. The definite identification of surface terminations and their spectroscopic signatures shall provide a more comprehensive understanding of the high-temperature superconductivity in multilayered iron pnictides. △ Less

Submitted 16 February, 2016; originally announced February 2016.

Journal ref: Phys. Rev. B 99, 134520 (2019)

Edge-Mediated Skyrmion Chain and Its Collective Dynamics in a Confined Geometry

Authors: Haifeng Du, Renchao Che, Lingyao Kong, Xuebing Zhao, Chiming Jin, Chao Wang, Jiyong Yang, Wei Ning, Runwei Li Changqing jin, Xianhui Chen, Jiadong Zang, Yuheng Zhang, Mingliang Tian

Abstract: The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. There, extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report the first experimental observation of the skyrmion chain in FeGe nanostripes by u… ▽ More The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. There, extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report the first experimental observation of the skyrmion chain in FeGe nanostripes by using high resolution Lorentz transmission electron microscopy. Under an applied field normal to the nanostripes plane, we observe that the helical ground states with distorted edge spins would evolves into individual skyrmions, which assemble in the form of chain at low field and move collectively into the center of nanostripes at elevated field. Such skyrmion chain survives even as the width of nanostripe is much larger than the single skyrmion size. These discovery demonstrates new way of skyrmion formation through the edge effect, and might, in the long term, shed light on the applications. △ Less

Submitted 18 October, 2015; v1 submitted 19 May, 2015; originally announced May 2015.

Comments: 7 pages, 3 figures

Journal ref: Nature Communications 6:8504, 2015

Weak pinning and vortex bundles in anisotropic Ca$_{10}$(Pt$_4$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ single crystals

Authors: O. E. Ayala-Valenzuela, N. Haberkorn, A. B. Karki, Jisun Kim, R. Jin, Jeehoon Kim

Abstract: We report the magnetic field -- temperature ($H-T$) phase diagram of Ca$_{10}$(Pt$_4$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ($x\approx 0.05$) single crystals, which consists of normal, vortex liquid, plastic creep and elastic creep phases. The upper critical field anisotropy is determined by a radio frequency technique via the measurements of magnetic penetration depth, $λ$. Both, irreversibili… ▽ More We report the magnetic field -- temperature ($H-T$) phase diagram of Ca$_{10}$(Pt$_4$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ($x\approx 0.05$) single crystals, which consists of normal, vortex liquid, plastic creep and elastic creep phases. The upper critical field anisotropy is determined by a radio frequency technique via the measurements of magnetic penetration depth, $λ$. Both, irreversibility line, $H_{irr}(T)$, and flux creep line, $H^{SPM}(T)$, are obtained by measuring the magnetization. We find that $H_{irr}(T)$ is well described by the Lindemann criterion with parameters similar to those for cuprates, while small $H^{SPM}(T)$ results in a wide plastic creep regime. The flux creep rates in the elastic creep regime are in qualitative agreement with the collective creep theory for random point defects. A gradual crossover from a single vortex to a bundles regime is observed. Moreover, we obtain $λ(4~ \text K) = 260(26)$ nm through the direct measurement of the London penetration depth by magnetic force microscopy. △ Less

Submitted 30 January, 2015; originally announced January 2015.

Competing magnetic states, disorder, and the magnetic character of Fe3Ga4

Authors: J. H. Mendez, C. E. Ekuma, Y. Wu, B. W. Fulfer, J. C. Prestigiacomo, W. A. Shelton, M. Jarrell, J. Moreno, D. P. Young, P. W. Adams, A. Karki, R. Jin, Julia Y. Chan, J. F. DiTusa

Abstract: The physical properties of metamagnetic Fe$_3$Ga$_4$ single crystals are investigated to explore the sensitivity of the magnetic states to temperature, magnetic field, and sample history. The data reveal a moderate anisotropy in the magnetization and the metamagnetic critical field along with features in the specific heat at the magnetic transitions $T_1=68$ K and $T_2=360$ K. Both $T_1$ and… ▽ More The physical properties of metamagnetic Fe$_3$Ga$_4$ single crystals are investigated to explore the sensitivity of the magnetic states to temperature, magnetic field, and sample history. The data reveal a moderate anisotropy in the magnetization and the metamagnetic critical field along with features in the specific heat at the magnetic transitions $T_1=68$ K and $T_2=360$ K. Both $T_1$ and $T_2$ are found to be sensitive to the annealing conditions of the crystals suggesting that disorder affects the competition between the ferromagnetic (FM) and antiferromagnetic (AFM) states. Resistivity measurements reveal metallic transport with a sharp anomaly associated with the transition at $T_2$. The Hall effect is dominated by the anomalous contribution which rivals that of magnetic semiconductors in magnitude ($-5 μΩ$ cm at 2 T and 350 K) and undergoes a change of sign upon cooling into the low temperature FM state. The temperature and field dependence of the Hall effect indicate that the magnetism is likely to be highly itinerant in character and that a significant change in the electronic structure accompanies the magnetic transitions. We observe a contribution from the topological Hall effect in the AFM phase suggesting a non-coplanar contribution to the magnetism. Electronic structure calculations predict an AFM ground state with a wavevector parallel to the crystallographic $c$-axis preferred over the experimentally measured FM state by $\approx$ 50 meV per unit cell. However, supercell calculations with a small density of Fe-antisite defects introduced tend to stabilize the FM over the AFM state indicating that antisite defects may be the cause of the sensitivity to sample synthesis conditions. △ Less

Submitted 20 April, 2015; v1 submitted 9 October, 2014; originally announced October 2014.

Comments: 13 pages, 14 figures, and 4 supplementary tables

Quantum Fisher information as signature of superradiant quantum phase transition

Authors: T. L. Wang, L. N. Wu, W. Yang, G. R. Jin, N. Lambert, F. Nori

Abstract: The single-mode Dicke model is well-known to undergo a quantum phase transition from the so-called normal phase to the supperradiant phase (hereinafter called the "superradiant quantum phase transition"). Normally, quantum phase transitions are closely related to the critical behavior of quantities such as entanglement, quantum fluctuations, and fidelity. In this paper, we study quantum Fisher inf… ▽ More The single-mode Dicke model is well-known to undergo a quantum phase transition from the so-called normal phase to the supperradiant phase (hereinafter called the "superradiant quantum phase transition"). Normally, quantum phase transitions are closely related to the critical behavior of quantities such as entanglement, quantum fluctuations, and fidelity. In this paper, we study quantum Fisher information (QFI) of the field mode and that of the atoms in the ground state of the Dicke Hamiltonian. For finite and large enough number of atoms, our numerical results show that near the critical atom-field coupling, the QFIs of the atomic and the field subsystems can surpass the classical limits, due to the appearance of nonclassical squeezed states. As the coupling increases far beyond the critical point, the two subsystems are in highly mixed states, which degrade the QFI and hence the ultimate phase sensitivity. In the thermodynamic limit, we present analytical results of the QFIs and their relationships with the reduced variances. For each subsystem, we find that there is a singularity in the derivative of the QFI at the critical point, a clear signature of quantum criticality in the Dicke model. △ Less

Submitted 9 December, 2013; v1 submitted 4 December, 2013; originally announced December 2013.

Comments: 7.8 pages, 3 figures, typing errors are fixed and new references are added

Journal ref: New J. Phys. 16, 063039 (2014)

Quantum Fisher information of entangled coherent state in the presence of photon losses: exact solution

Authors: Y. M. Zhang, X. W. Li, W. Yang, G. R. Jin

Abstract: We investigate the performance of entangled coherent state for quantum enhanced phase estimation. An exact analytical expression of quantum Fisher information is derived to show the role of photon losses on the ultimate phase sensitivity. We find a transition of the sensitivity from the Heisenberg scaling to the classical scaling due to quantum decoherence of the photon state. This quantum-classic… ▽ More We investigate the performance of entangled coherent state for quantum enhanced phase estimation. An exact analytical expression of quantum Fisher information is derived to show the role of photon losses on the ultimate phase sensitivity. We find a transition of the sensitivity from the Heisenberg scaling to the classical scaling due to quantum decoherence of the photon state. This quantum-classical transition is uniquely determined by the number of photons being lost, instead of the number of incident photons or the photon loss rate alone. Our results also reveal that a crossover of the sensitivity between the entangled coherent state and the NOON state can occur even for very small photon loss rate. △ Less

Submitted 28 July, 2013; originally announced July 2013.

Comments: 7 page, 3 figures

Journal ref: Physical Review A 88, 043832 (2013)

Thermoelectric Properties of Intermetallic Semiconducting RuIn3 and Metallic IrIn3

Authors: N. Haldolaarachchige, W. A. Phelan, Y. M. Xiong, R. Jin, J. Y. Chan, S. Stadler, D. P. Young

Abstract: Low temperature (<400 K) thermoelectric properties of semiconducting RuIn3 and metallic IrIn3 are reported. RuIn3 is a narrow band gap semiconductor with a large n-type Seebeck coefficient at room temperature (S(290K)~400 μV/K), but the thermoelectric Figure of merit (ZT(290K) = 0.007) is small because of high electrical resistivity and thermal conductivity (κ(290 K) ~ 2.0 W/m K). IrIn3 is a metal… ▽ More Low temperature (<400 K) thermoelectric properties of semiconducting RuIn3 and metallic IrIn3 are reported. RuIn3 is a narrow band gap semiconductor with a large n-type Seebeck coefficient at room temperature (S(290K)~400 μV/K), but the thermoelectric Figure of merit (ZT(290K) = 0.007) is small because of high electrical resistivity and thermal conductivity (κ(290 K) ~ 2.0 W/m K). IrIn3 is a metal with low thermopower at room temperature (S(290K)~20 μV/K) . Iridium substitution on the ruthenium site has a dramatic effect on transport properties, which leads to a large improvement in the power factor and corresponding Figure of merit (ZT(380 K) = 0.053), improving the efficiency of the material by an over of magnitude. △ Less

Submitted 22 January, 2013; originally announced January 2013.

Comments: Submitted to JAP

Journal ref: J. Appl. Phys. 113, 083709 (2013)

Physical Properties of $Ba_2 Mn_2 Sb_2 O$ Single Crystals

Authors: J. Li, C. E. Ekuma, I. Vekhter, M. Jarrell, J. Moreno, S. Stadler, A. B. Karki, R. Jin

Abstract: We report both experimental and theoretical investigations of the physical properties of Ba$_\mathrm{2}$Mn$_\mathrm{2}$Sb$_\mathrm{2}$O single crystals. This material exhibits a hexagonal structure with lattice constants: a = 4.7029(15) Å and c = 19.9401(27) Å, as obtained from powder X-ray diffraction measurements, and in agreement with structural optimization through density functional theory (D… ▽ More We report both experimental and theoretical investigations of the physical properties of Ba$_\mathrm{2}$Mn$_\mathrm{2}$Sb$_\mathrm{2}$O single crystals. This material exhibits a hexagonal structure with lattice constants: a = 4.7029(15) Å and c = 19.9401(27) Å, as obtained from powder X-ray diffraction measurements, and in agreement with structural optimization through density functional theory (DFT) calculations. The magnetic susceptibility and specific heat show anomalies at T$_\mathrm{N}$ = 60 K, consistent with antiferromagnetic ordering. However, the magnitude of T$_\mathrm{N}$ is significantly smaller than the Curie-Weiss temperature ($\mid$$\mathrm{Θ_{CW}}$$\mid$ $\approx$ 560 K), suggesting a magnetic system of reduced dimensionality. The temperature dependence of both the in-plane and out-of-plane resistivity changes from an activated at $T$ $>$ T$_\mathrm{x}$ $\sim$ 200 K to a logarithmic at $T$ $<$ T$_\mathrm{x}$. Correspondingly, the magnetic susceptibility displays a bump at T$_\mathrm{x}$. DFT calculations at the DFT + U level support the experimental observation of an antiferromagnetic ground state. △ Less

Submitted 4 December, 2012; v1 submitted 30 November, 2012; originally announced December 2012.

Comments: 6 Pages, 5 Figures, and 1 Table

Journal ref: Phys. Rev. B 86, 195142 (2012)

Structural and magnetic properties of the new cobaltate series (BaSr)4-xLa2xCo4O15

Authors: V. O. Garlea, R. Jin, E. Garlea, G. Ehlers, E. Mamontov, D. B. Myers, F. Xie, R. Custelcean

Abstract: We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O1… ▽ More We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples. △ Less

Submitted 13 September, 2012; originally announced September 2012.

Evolution of Structural and Physical Properties of Sr3(Ru1-xMnx)2O7 with Mn Concentration

Authors: Biao Hu, Gregory T. McCandless, V. O. Garlea, S. Stadler, Yimin Xiong, Julia Y. Chan, E. W. Plummer, R. Jin

Abstract: Layered ruthenates are prototype materials with strong structure-property correlations. We report the structural and physical properties of double-layered perovskite Sr3(Ru1-xMnx)2O7 single crystals with 0<=x<=0.7. Single crystal x-ray diffraction refinements reveal that Mn doping on the Ru site leads to the shrinkage of unit-cell volume and disappearance of (Ru/Mn)O6 octahedron rotation when x>0.… ▽ More Layered ruthenates are prototype materials with strong structure-property correlations. We report the structural and physical properties of double-layered perovskite Sr3(Ru1-xMnx)2O7 single crystals with 0<=x<=0.7. Single crystal x-ray diffraction refinements reveal that Mn doping on the Ru site leads to the shrinkage of unit-cell volume and disappearance of (Ru/Mn)O6 octahedron rotation when x>0.16, while the crystal structure remains tetragonal. Correspondingly, the electric and magnetic properties change with x. The electrical resistivity reveals metallic character (d rho/d T>0) at high temperatures but insulating behavior (d rho/d T<0) below a characteristic temperature T_MIT. Interestingly, T_MIT is different from T_M, at which magnetic susceptibility reaches maximum. T_MIT monotonically increases with increasing x while T_M shows non-monotonic dependence with x. The difference between T_MIT and T_M (T_MIT>T_M) becomes larger when x>0.16. The constructed phase diagram consists of five distinct regions, demonstrating that the physical properties of such a system can easily be tuned by chemical doping. △ Less

Submitted 1 August, 2011; originally announced August 2011.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 84, 174411 (2011)

Spin squeezing: transforming one-axis-twisting into two-axis-twisting

Authors: Y. C. Liu, Z. F. Xu, G. R. Jin, L. You

Abstract: Squeezed spin states possess unique quantum correlation or entanglement that are of significant promises for advancing quantum information processing and quantum metrology. In recent back to back publications [C. Gross \textit{et al, Nature} \textbf{464}, 1165 (2010) and Max F. Riedel \textit{et al, Nature} \textbf{464}, 1170 (2010)], reduced spin fluctuations are observed leading to spin squeezin… ▽ More Squeezed spin states possess unique quantum correlation or entanglement that are of significant promises for advancing quantum information processing and quantum metrology. In recent back to back publications [C. Gross \textit{et al, Nature} \textbf{464}, 1165 (2010) and Max F. Riedel \textit{et al, Nature} \textbf{464}, 1170 (2010)], reduced spin fluctuations are observed leading to spin squeezing at -8.2dB and -2.5dB respectively in two-component atomic condensates exhibiting one-axis-twisting interactions (OAT). The noise reduction limit for the OAT interaction scales as $\propto 1/{N^{2/3}}$, which for a condensate with $N\sim 10^3$ atoms, is about 100 times below standard quantum limit. We present a scheme using repeated Rabi pulses capable of transforming the OAT spin squeezing into the two-axis-twisting type, leading to Heisenberg limited noise reduction $\propto 1/N$, or an extra 10-fold improvement for $N\sim 10^3$. △ Less

Submitted 2 May, 2011; originally announced May 2011.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett. 107, 013601 (2011)

Tuning the magnetic ground state of a triangular lattice system

Authors: V. Ovidiu Garlea, Andrei T. Savici, Rongying Jin

Abstract: The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is used as a basic model for studying the influence of spin disorder on the ground state properties of a two-dimensional frustrated antiferromagnet. Neutron diffraction measurements show that the undoped phase (x=0) undergoes a transition to antiferromagnetic long-range order that is stabilized by a frustration-relieving st… ▽ More The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is used as a basic model for studying the influence of spin disorder on the ground state properties of a two-dimensional frustrated antiferromagnet. Neutron diffraction measurements show that the undoped phase (x=0) undergoes a transition to antiferromagnetic long-range order that is stabilized by a frustration-relieving structural distortion. Small deviation from the stoichiometric composition alters the magnetoelastic characteristics and reduces the effective dimensionality of the magnetic lattice. Upon increasing the doping level, the interlayer coupling changes from antiferromagnetic to ferromagnetic. As the structural distortion is suppressed, the long-range magnetic order is gradually transformed into a two-dimensional order. △ Less

Submitted 30 March, 2011; originally announced March 2011.

Comments: 4 pages, 5 figures