arXiv subjects

Gia-Wei Chern

Publications and source records attributed to Gia-Wei Chern.

At least 109 records · Page 6Linked to original sources

arXiv2018

Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice

We present an extensive numerical study of a new type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a cross-linking network of Kondo or double-exchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n = 1/2$ and 2/3. In particular, we show that an intriguing glassy magnetic state characterized by ordering wavevectors $\mathbf q = \left( \frac{1}{3},\frac{1}{3}, 1\right)$ gives a rather satisfactory description of the low temperature phase recently observed in spinel GeFe$_2$O$_4$.

cond-mat.str-el↗

arXiv2018

Machine learning electron correlation in a disordered medium

Learning from data has led to a paradigm shift in computational materials science. In particular, it has been shown that neural networks can learn the potential energy surface and interatomic forces through examples, thus bypassing the computationally expensive density functional theory calculations. Combining many-body techniques with a deep learning approach, we demonstrate that a fully-connected neural network is able to learn the complex collective behavior of electrons in strongly correlated systems. Specifically, we consider the Anderson-Hubbard (AH) model, which is a canonical system for studying the interplay between electron correlation and strong localization. The ground states of the AH model on a square lattice are obtained using the real-space Gutzwiller method. The obtained solutions are used to train a multi-task multi-layer neural network, which subsequently can accurately predict quantities such as the local probability of double occupation and the quasiparticle weight, given the disorder potential in the neighborhood as the input.

cond-mat.str-el↗

arXiv2018

Topological Insulator in an Atomic Liquid

We demonstrate theoretically an atomic liquid phase that supports topologically nontrivial electronic structure. A minimum two-orbital model of liquid topological insulator in two dimensions is constructed within the framework of tight-binding molecular dynamics. As temperature approaches zero, our simulations show that the atoms crystallize into a triangular lattice with nontrivial band topology at high densities. Thermal fluctuations at finite temperatures melt the lattice, giving rise to a liquid state which inherits the nontrivial topology from the crystalline phase. The electronic structure of the resultant atomic liquid is characterized by a nonzero Bott index. Our work broadens the notion of topological materials, and points to a new systematic approach for searching topological phases in amorphous and liquid systems.

cond-mat.mes-hall↗

arXiv2018

Memory effect and phase transition in a hierarchical trap model for spin glass

We introduce an efficient dynamical tree method that enables us, for the first time, to explicitly demonstrate thermo-remanent magnetization memory effect in a hierarchical energy landscape. Our simulation nicely reproduces the nontrivial waiting-time and waiting-temperature dependences in this non-equilibrium phenomenon. We further investigate the condensation effect, in which a small set of micro-states dominates the thermodynamic behavior, in the multi-layer trap model. Importantly, a structural phase transition of the tree is shown to coincide with the onset of condensation phenomenon. Our results underscore the importance of hierarchical structure and demonstrate the intimate relation between glassy behavior and structure of barrier trees.

cond-mat.dis-nn↗

arXiv2018

Gradient-based stochastic estimation of the density matrix

Fast estimation of the single-particle density matrix is key to many applications in quantum chemistry and condensed matter physics. The best numerical methods leverage the fact that the density matrix elements $f(H)_{ij}$ decay rapidly with distance $r_{ij}$ between orbitals. This decay is usually exponential. However, for the special case of metals at zero temperature, algebraic decay of the density matrix appears and poses a significant numerical challenge. We introduce a gradient-based probing method to estimate all local density matrix elements at a computational cost that scales linearly with system size. For zero-temperature metals the stochastic error scales like $S^{-(d+2)/2d}$, where $d$ is the dimension and $S$ is a prefactor to the computational cost. The convergence becomes exponential if the system is at finite temperature or is insulating.

cond-mat.stat-mech↗

arXiv2018

Plaquette order in classical spin liquid stabilized by strong off-diagonal exchange

We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spin-orbit magnets where a dominant off-diagonal exchange, the so-called $Γ$ term, results in a macroscopic ground-state degeneracy at the classical level. We demonstrate that the system undergoes a phase transition driven by thermal order-by-disorder at a critical temperature $T_c \approx 0.04 |Γ|$. At first sight, this transition reduces an emergent spherical spin-symmetry to a cubic one: spins point predominantly toward the cubic axes at $T < T_c$. However, this seems to simply restore the cubic symmetry of the $Γ$ model, and the non-coplanar spins remain disordered below $T_c$. We show that the phase transition actually corresponds to plaquette ordering of hexagonal fluxes and the cubic symmetry is indeed broken, a scenario that is further confirmed by our extensive Monte Carlo simulations.

cond-mat.str-el↗

arXiv2017

Theory of magnetotransport in artificial kagome spin ice

Magnetic nanoarray with special geometries exhibits nontrivial collective behaviors similar to those observed in the spin ice materials. Here we present a novel circuit model to describe the complex magnetotransport phenomena in artificial kagome spin ice. In this picture, the system can be viewed as a resistor network driven by voltage sources that are located at vertices of the honeycomb array. The differential voltages across different terminals of these sources are related to the ice-rules that govern the local magnetization ordering. The circuit model relates the transverse Hall voltage of kagome ice to the underlying spin correlations. Treating the magnetic nanoarray as metamaterials, we present a mesoscopic constitutive equation relating the Hall resistance to magnetization components of the system. We further show that the Hall signal is significantly enhanced when the kagome ice undergoes a magnetic-charge ordering transition. Our analysis can be readily generalized to other lattice geometry, providing a quantitative method for the design of magnetoresistance devices based on artificial spin ices.

cond-mat.mes-hall↗

arXiv2017

Semiclassical dynamics of spin density waves

We present a theoretical framework for equilibrium and nonequilibrium dynamical simulation of quantum states with spin-density-wave (SDW) order. Within a semiclassical adiabatic approximation that retains electron degrees of freedom, we demonstrate that the SDW order parameter obeys a generalized Landau-Lifshitz equation. With the aid of an enhanced kernel polynomial method, our linear-scaling quantum Landau-Lifshitz dynamics (QLLD) method enables dynamical SDW simulations with $N \simeq 10^5$ lattice sites. Our real-space formulation can be used to compute dynamical responses, such as dynamical structure factor, of complex and even inhomogeneous SDW configurations at zero or finite temperatures. Applying the QLLD to study the relaxation of a noncoplanar topological SDW under the excitation of a short pulse, we further demonstrate the crucial role of spatial correlations and fluctuations in the SDW dynamics.

cond-mat.str-el↗

arXiv2017

Nonlinear Pauli Susceptibilities in Sr$_3$Ru$_2$O$_7$ and Universal Features of Itinerant Metamagnetism

We report, for the first time, measurements of the third order, $χ_3$ and fifth order, $χ_5$, susceptibilities in an itinerant oxide metamagnet, Sr$_3$Ru$_2$O$_7$ for magnetic fields both parallel and perpendicular to the c-axis. These susceptibilities exhibit maxima in their temperature dependence such that $T_1 \approx 2T_3 \approx 4T_5$ where the $T_i$ are the position in temperature where a peak in the $i$-th order susceptibility occurs. These features taken together with the scaling of the critical field with the temperature $T_1$ observed in a diverse variety of itinerant metamagnets find a natural explanation in a single band model with one Van Hove singularity (VHS) and onsite repulsion $U$. The separation of the VHS from the Fermi energy $Δ$, sets a single energy scale, which is the primary driver for the observed features of itinerant metamagnetism at low temperatures.

cond-mat.str-el↗

arXiv2017

Dynamic inhomogeneities and phase separation after quantum quenches in strongly correlated systems

We present a Gutzwiller von Neumann dynamics (GvND) method for simulating equilibrium and nonequilibrium phenomena in strongly correlated electron systems. Our approach is a real-space formulation of the time-dependent Gutzwiller approximation method. Applying the GvND method to simulate interaction quenches in the Peierls-Hubbard model, we demonstrate the amplification of initial inhomogeneities, which in turn results in the collapse of quench-induced synchronized oscillation. Moreover, we find a dynamical phase transition separating two quasi-stationary regimes with rather distinct spatial distributions of physical quantities after the collapsed oscillation. In particular, in the strong-coupling regime, the system exhibits a dynamic phase separation in the quasi-stationary state. Our results thus underscore the importance of spatial fluctuations in the nonequilibrium dynamics of strongly correlated systems.

cond-mat.str-el↗

arXiv2017

Elastic and electronic tuning of magnetoresistance in MoTe$_2$

Quasi-two dimensional transition metal dichalcogenides (TMD) exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe$_2$, a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ~ 30 % at low temperatures and high magnetic fields, when uniaxial strain is applied along the $a$-crystallographic direction and reduced by about the same amount when strain is applied along the $b$-direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. A shift of the Td - 1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain suggests the possibility of a nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations indeed show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains.

cond-mat.mtrl-sci↗

arXiv2017

Understanding magnetotransport signatures in networks of connected permalloy nanowires

The change in electrical resistance associated with the application of an external magnetic field is known as the magnetoresistance (MR). The measured MR is quite complex in the class of connected networks of single-domain ferromagnetic nanowires, known as "artificial spin ice", due to the geometrically-induced collective behavior of the nanowire moments. We have conducted a thorough experimental study of the MR of a connected honeycomb artificial spin ice, and we present a simulation methodology for understanding the detailed behavior of this complex correlated magnetic system. Our results demonstrate that the behavior, even at low magnetic fields, can be well-described only by including significant contributions from the vertices at which the legs meet, opening the door to new geometrically-induced MR phenomena.

cond-mat.mes-hall↗

arXiv2016

Kitaev-Heisenberg model in a magnetic field: order-by-disorder and commensurate-incommensurate transitions

We present a theoretical study of field-induced magnetic phases in the honeycomb Kitaev-Heisenberg model, which is believed to describe the essential physics of Mott insulators with strong spin-orbit coupling such as $A_2$IrO$_3$ and $α$-RuCl$_3$. We obtain a finite temperature phase diagram based on extensive Monte Carlo simulations and analytical calculations. We show that, while Zeeman coupling favors a symmetric non-coplanar magnetic order, thermal fluctuations enhances the stability of a collinear zigzag phase that breaks the rotational symmetry of the lattice. Our large-scale simulations also uncover intriguing commensurate-incommensurate transitions and multiple-$\mathbf Q$ incommensurate phases at high field. Experimental implications are also discussed.

cond-mat.str-el↗

arXiv2016

Resistivity Minimum in Highly Frustrated Itinerant Magnets

We study the transport properties of frustrated itinerant magnets comprising localized {\it classical} moments, which interact via exchange with the conduction electrons. Strong frustration stabilizes a liquidlike spin state, which extends down to temperatures well below the effective Ruderman-Kittel-Kasuya-Yosida interaction scale. The crossover into this state is characterized by spin structure factor enhancement at wave vectors smaller than twice the Fermi wave vector magnitude. The corresponding enhancement of electron scattering generates a resistivity upturn at decreasing temperatures.

cond-mat.str-el↗

arXiv2016

Magnetic avalanches in granular ferromagnets: Thermal activated collective behavior

We present a numerical study on the thermal activated avalanche dynamics in granular materials composed of ferromagnetic clusters embedded in a non-magnetic matrix. A microscopic dynamical simulation based on the reaction-diffusion process is developed to modeling the magnetization process of such systems. The large-scale simulations presented here explicitly demonstrate inter-granular collective behavior induced by thermal activation of spin tunneling. In particular, we observe an intriguing criticality controlled by the rate of energy dissipation. We show that thermal activated avalanches can be understood in the framework of continuum percolation and the emergent dissipation induced criticality is in the universality class of three-dimensional percolation transition. Implications of these results to the phase-separated states of colossal magnetoresistance materials and other artificial granular magnetic systems are also~discussed.

cond-mat.dis-nn↗

arXiv2016

Matter-wave propagation in optical lattices: geometrical and flat-band effects

The geometry of optical lattices can be engineered allowing the study of atomic transport along paths arranged in patterns that are otherwise difficult to probe in the solid state. A question readily accessible to atomic systems is related to the speed of propagation of matter-waves as a function of the lattice geometry. To address this issue, we have investigated theoretically the quantum transport of non-interacting and weakly-interacting ultracold fermionic atoms in several 2D optical lattice geometries. We find that the triangular lattice has a higher propagation velocity compared to the square lattice, despite supporting longer paths. The body-centered square lattice has even longer paths, nonetheless the propagation velocity is yet faster. This apparent paradox arises from the mixing of the momentum states which leads to different group velocities in quantum systems. Standard band theory provides an explanation and allows for a systematic way to search and design systems with controllable matter-wave propagation. Moreover, the presence of a flat band such as in a two-leg ladder geometry leads to a dynamical density discontinuity, which contrasts the behavior of mobile and localized atoms in quantum transport. Our predictions are realizable with present experimental capability.

cond-mat.quant-gas↗

arXiv2015

Vortex Crystals with Chiral Stripes in Itinerant Magnets

We study noncoplanar magnetic ordering in frustrated itinerant magnets. For a family of Kondo square lattice models with classical local moments, we find that a double-$Q$ noncoplanar vortex crystal has lower energy than the single-$Q$ helical order expected from the Ruderman-Kittel-Kasuya-Yosida interaction whenever the lattice symmetry dictates four global maxima in the bare magnetic susceptibility. By expanding in the small Kondo exchange and the degree of noncoplanarity, we demonstrate that this noncoplanar state arises from a Fermi surface instability, and it is generic for a wide range of electron filling fractions whenever the two ordering wave vectors connect independent sections of the Fermi surface.

cond-mat.str-el↗

arXiv2015

Mott transition in a metallic liquid - Gutzwiller molecular dynamics simulations

We present a formulation of quantum molecular dynamics that includes electron correlation effects via the Gutzwiller method. Our new scheme enables the study of the dynamical behavior of atoms and molecules with strong electron interactions. The Gutzwiller approach goes beyond the conventional mean-field treatment of the intra-atomic electron repulsion and captures crucial correlation effects such as band narrowing and electron localization. We use Gutzwiller quantum molecular dynamics to investigate the Mott transition in the liquid phase of a single-band metal and uncover intriguing structural and transport properties of the atoms.

cond-mat.str-el↗