arXiv subjects

Gregory A. Fiete

Publications and source records attributed to Gregory A. Fiete.

At least 19 recordsLinked to original sources

arXiv2025

Photostriction-Driven Phase Transition in Layered Chiral NbOX$_2$ Crystals: Electrical-Field-Controlled Enantiomer Selectivity

Chiral crystals offer an unique platform for controlling structural handedness through external stimuli. However, the ability to select between structural enantiomers remains challenging, both theoretically and experimentally. In this work, we demonstrate a two-step pathway for enantiomer selectivity in layered chiral NbOX$_2$ (X = Cl, Br, I) crystals based on photostriction-driven phase transitions. Ab-initio simulations reveal that optical excitation is capable of inducing a structural phase transition in NbOX$_2$ from the monoclinic ($C2$) ground state to the higher-symmetry ($C2/m$) structure. In the resulting transient high-symmetry state, an applied electric field breaks the residual inversion-symmetry degeneracy, selectively stabilizing one enantiomeric final state configuration over the other. Our results establish a combined optical-electrical control scheme for chiral materials, enabling reversible and non-contact enantiomer selection with potential applications in ultrafast switching, optoelectronics, and chiral information storage.

cond-mat.mtrl-sci↗

arXiv2025

Light-induced pseudo-magnetic fields in three-dimensional topological semimetals

In this work, we show that suitably designed spatially varying linearly polarized light provides a versatile route to generate and control pseudo-magnetic fields in Weyl semimetals through Floquet engineering. Within a high-frequency expansion, we derive an effective axial gauge potential $\mathbf{A}_5(\mathbf{r})$ whose curl gives the pseudo-magnetic field $\mathbf{B}_5(\mathbf{r})$. By mapping the light profile to $\mathbf{A}_5(\mathbf{r})$, we establish design principles for pseudo-magnetic field textures that mimic strain-induced gauge fields while offering key advantages like dynamic control, full reversibility, spatial selectivity, and absence of material deformation. We compare the Landau-level spectra produced by uniform real and pseudo-magnetic fields and also analyze both their linear optical conductivity and the second-order dc responses. Our results enable real-time manipulation of pseudo-magnetic fields and predict clear experimental signatures for optically engineered gauge fields in topological semimetals.

cond-mat.mes-hall↗

arXiv2025

Giant Shift Current in Electrically-Tunable Superlattice Bilayer Graphene

Recent introduction of superlattice potentials has opened new avenues for engineering tunable electronic band structures featuring topologically nontrivial moiré-like bands. Here we consider optoelectronic properties of Bernal-stacked graphene subjected to a superlattice potential either electrostatically or through lattice twisting to show that it exhibits a giant shift current response that is orders of magnitude larger than existing predictions in twisted mulitlayer systems. Effects of gate voltage and the strength and phase of the superlattice potential on the shift current are delineated systematically across various topological regimes. Our study gives insight into the nature of nonlinear responses of materials and how these responses could be optimized by tuning the superlattice potential.

cond-mat.mes-hall↗

arXiv2025

Magnetocrystalline anisotropy of FeNi and FeCo along the Bain path

We theoretically investigate magnetic anisotropy in materials with non-critical elements to determine which symmetry conditions and atomic shell filling favor enhanced magnetic anisotropy. We study the magnetocrystalline anisotropies (MCA) of the equiatomic ferrous compounds FeCo and FeNi using ab initio calculations and analytical approaches via the diatomic pair model. We find that when these materials undergo a Bain transformation, that is, the variation of the a and c lattice parameters adjust to interpolate between the B2 and L10 structural phases while keeping the unit cell volume constant, the MCA versus r = c/a ratio varies differently for FeCo and FeNi despite Co and Ni differing only by one valence electron. To uncover the physics governing these trends, we use a diatomic pair model to perform a theoretical analysis of the ab initio results. We find that the MCA variation along the Bain path is correlated with the structural phase of the material as well as the occupation of (l, m)-resolved states for each equiatomic ferrous compound. Accordingly, the MCA was found to differ depending on the element paired with Fe to form the Fe-X compound (X = Co, Ni). Our work could help guide the scientific community in solving the supply crisis of hard/strong permanent magnets that are crucial for various technological applications such as those depending on motors and generators for energy conversion and clean energy applications.

cond-mat.mtrl-sci↗

arXiv2025

Long-lived Zone-boundary Magnons in an Antiferromagnet

Antiferromagnetic (AFM) insulators exhibit many desirable features for spintronic applications such as fast dynamics in the THz range and robustness to fluctuating external fields. However, large damping typically associated with THz magnons presents a serious challenge for THz magnonic applications. Here, we report long-lived short-wavelength zone boundary magnons in the honeycomb AFM insulator CoTiO3, recently found to host topological magnons. We find that its zone-boundary THz magnons exhibit longer lifetimes than its zone-center magnons. This unusual momentum-dependent long magnon lifetime originates from several factors including the antiferromagnetic order, exchange anisotropy, a finite magnon gap, and magnon band dispersion. Our work suggests that magnon-magnon interaction may not be detrimental to magnon lifetimes and should be included in future searches for topological magnons.

cond-mat.mes-hall↗

arXiv2025

High-accuracy evaluation of non-thermal magnetic states beyond spin-wave theory: applications to higher-energy states

We present an approximation scheme based on selective Hilbert space truncation for characterizing non-thermal states of magnetic systems beyond spin-wave theory. We study applications to states that are inaccessible through linear spin-wave theory, such as multi-magnon states and higher-energy states. Our approach is based on the existence of an exact representation of spin operators in terms of finite-order polynomials of bosonic operators. It can be applied to systems with and without a magnetically ordered ground state. The approximation exactly diagonalizes the bosonic Hamiltonian restricted to particular boson occupation subspaces, improving the conventional linear spin-wave approach and exponentially reducing the computing time relative to exact diagonalization schemes. As a test case, we apply the approach to a prototypical one-dimensional model - an XXZ spin chain with an applied magnetic field and antisymmetric exchange coupling. Here the antisymmetric coupling introduces a continuous parameter to tune the system away from its exactly solvable limit. We find excellent agreement between numerically exact eigenstates and eigenvalues and those found via the approximation scheme. Our approach applies not just to higher lying states but also to boson bound states, which could make them more accessible to theoretical predictions for comparison with experiment.

cond-mat.str-el↗

arXiv2025

Interlayer Dzyaloshinskii-Moriya interactions induced via non-linear phononics in bilayer van der Waals materials

We theoretically study the impact of light-driven structural changes via nonlinear phononics on the magnetic order of untwisted bilayer van der Waals materials. We consider an illustrative example of the AA-stacked bilayer honeycomb lattice and show that high-intensity light in resonance with selected phonons induces large amplitude phonon displacements that modify the magnetic Hamiltonian of the system. We performed a group theory analysis to identify the vibrational modes of the honeycomb bilayer and the nonlinear couplings among them in the strongly driven regime. We find that the structural changes in the strongly driven regime lower the symmetry relative to the equilibrium lattice and produce changes in the magnetic interactions between the local moments. In particular, the lattice symmetry changes permit a non-zero interlayer Dzyaloshinskii-Moriya interaction that induces a magnetic state with canted local moments. Using a spin-wave analysis about the new magnetic configuration we study the corresponding changes in the magnon spectrum and identify a protocol for engineering topological band transitions using a combination of nonlinear phononics and an external magnetic field. Our work suggests a strategy to induce and control interlayer Dyzaloshinskii-Moriya interactions in a class of layered van der Waals materials, the effect of which is to modify the magnetic ground state, magnon dispersions, and related band geometric properties, including topological invariants.

cond-mat.str-el↗

arXiv2025

Light-induced reorientation transition in an antiferromagnetic semiconductor

Due to the lack of a net magnetic moment, antiferromagnets possess a unique robustness to external magnetic fields and are thus predicted to play an important role in future magnetic technologies. However, this robustness also makes them quite difficult to control, and the development of novel methods to manipulate these systems with external stimuli is a fundamental goal of antiferromagnetic spintronics. In this work, we report evidence for a metastable reorientation of the order parameter in an antiferromagnetic semiconductor triggered by an ultrafast quench of the equilibrium order via photoexcitation above the band gap. The metastable state forms less than 10 ps after the excitation pulse, and persists for longer than 150 ps before decaying to the ground state via thermal fluctuations. Importantly, this transition cannot be induced thermodynamically, and requires the system to be driven out of equilibrium. Broadly speaking, this phenomenology is ultimately the result of large magnetoelastic coupling in combination with a relatively low symmetry of the magnetic ground state. Since neither of these properties are particularly uncommon in magnetic materials, the observations presented here imply a generic path toward novel device technology enabled by ultrafast dynamics in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2024

Metastability in Coexisting Competing Orders

The dynamical phase transition of a system with two coexisting competing order parameters is studied using the time-dependent-Ginzburg-Landau framework. The dynamics are induced by parameters capturing the physics of driving the system with an ultrafast laser pulse. A remarkable enhancement of the order parameter with a smaller mean-field value following the pump and the emergence of an induced metastable state is investigated through analytical and numerical studies. The effect of order parameter fluctuations on the exploration of the nonequilibrium free energy landscape reveals important information about the impact of both thermal and nonthermal fluctuations on the dynamics of the metastable state. Our results provide an interpretation of previously unexplained ultrafast experiments on superconductors with competing charge density wave order. Our formalism is relevant across broad classes of out-of-equilibrium systems beyond the condensed matter context, such as the Kibble-Zurek cosmological model.

cond-mat.str-el↗

arXiv2024

Magnetic Order and Strain in Hexagonal Manganese Pnictide CaMn$_2$Bi$_2$

The manganese pnictide CaMn$_2$Bi$_2$, with Mn atoms arranged in a puckered honeycomb structure, exhibits narrow-gap antiferromagnetism, and it is currently a promising candidate for the study of complex electronic and magnetic phenomena, such as magnetotransport effects and potential spin spirals under high pressure. In this paper, we perform a detailed research of the magnetic properties of CaMn$_2$Bi$_2$ using density functional theory (DFT) combined with the Hubbard U correction and spin-orbit coupling, which accurately describe the magnetic interactions. Our results obtained for a large number of magnetic configurations are accurately captured by a modified Heisenberg model that includes on-site magnetization terms to describe magnetic energy excitations. We further investigate the role of the spin-orbit coupling, and find that the magnetic anisotropy of CaMn$_2$Bi$_2$ shows an easy plane, with the preferred magnetization direction being exchanged between axes in the plane by applying small strain values. This strain-tunable magnetization, driven by the interplay between spin-orbit interactions and lattice distortions, highlights the potential for controlling magnetic states in Mn-pnictides for future applications in spintronic and magnetoelectric devices.

cond-mat.mtrl-sci↗

arXiv2024

Nonlinear optical responses and quantum geometry in rhombohedral trilayer graphene

We theoretically study the nonlinear optical response of ABC trilayer graphene with inversion symmetry broken by the application of a displacement field perpendicular to the trilayer. We show that rhombohedral trilayer graphene exhibits a large bulk photovoltaic effect arising from a DC shift current response. The conductivity of the trilayer contains features similar to AB bilayer graphene as well as features distinct from AB bilayer graphene. The new features of ABC trilayer graphene relative to AB bilayer graphene arise from the quantum geometric features of the electronic band structure and can be tuned by varying the displacement field. We focus on a regime of displacement field where certain band gaps close and reopen away from the charge neutrality point leading to drastic changes in the quantum geometric structure in momentum space, a feature characteristic of the trilayer graphene band structure. These features manifest as a sign change in shift current conductivity in a certain frequency window and can thus serve as a probe of quantum geometry.

cond-mat.mes-hall↗

arXiv2024

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive SN junctions

We consider a planar superconducting-normal-metal (SN) junction with both inelastic and spin-flip scattering processes present. In the diffusive limit, we use a one-dimensional formulation of the Usadel equation to compute the self-consistent energy dependence of the single-particle density of states as a function of distance from the interface on both the superconducting and metallic sides for various spatial profiles of a pair-breaking spin-flip term. The pair-breaking processes fill in the superconducting gap at zero energy, which is reflected in the zero-bias tunneling conductance in scanning tunneling microscopy/spectroscopy experiments, in the vicinity of the junction. We also investigate the impact of having a partially transparent interface at the junction. We compare our findings with the observed exponential rise in the zero-bias conductance at the 1H step edge in recent experiments on 4Hb-TaS$_2$ [A. K. Nayak et al., Nat. Phys. 17, 1413 (2021)].

cond-mat.supr-con↗

arXiv2024

Spin-Resolved Topology and Partial Axion Angles in Three-Dimensional Insulators

Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- ($\mathcal{T}$-) invariant (helical) 3D TCI$\unicode{x2014}$termed higher-order TCIs (HOTIs)$\unicode{x2014}$the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetals, and $\mathcal{T}$-doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that $β$-MoTe$_2$ realizes a spin-Weyl state and that $α$-BiBr hosts both 3D QSHI and T-DAXI regimes.

cond-mat.mes-hall↗

arXiv2024

Topological transverse spin transport in a canted antiferromagnet/heavy metal heterostructure

We theoretically study the conditions under which a spin Nernst effect - a transverse spin current induced by an applied temperature gradient - can occur in a canted antiferromagnetic insulator, such as ${\rm LaFeO_3}$ and other materials of the same family. The spin Nernst effect may provide a microscopic mechanism for an experimentally observed anomalous thermovoltage in ${\rm LaFeO_3}$/Pt heterostructures, where spin is transferred across the insulator/metal interface when a temperature gradient is applied to ${\rm LaFeO_3}$ parallel to the interface [W. Lin ${\it et \; al}$, Nat. Phys. ${\bf 18}$, 800 (2022)]. We find that ${\rm LaFeO_3}$ exhibits a topological spin Nernst effect when inversion symmetry is broken on the axes parallel to both the applied temperature gradient and the direction of spin transport, which can result in a spin injection across the insulator/metal interface. Our work provides a general derivation of a symmetry-breaking-induced spin Nernst effect, which may open a path to engineering a finite spin Nernst effect in systems where it would otherwise not arise.

cond-mat.mes-hall↗

arXiv2023

Nonlinear optical responses in multi-orbital topological superconductors

We theoretically study first and second-order optical responses in a transition metal dichalcogenide monolayer with distinct trivial, nodal, and time-reversal invariant topological superconducting (TRITOPS) phases. We show that the second-order DC response, also known as the photogalvanic response, contains signatures for differentiating these phases while the first-order optical response does not. We find that the high-frequency photogalvanic response is insensitive to the phase of the system, while the low-frequency response exhibits features distinguishing the three phases. At zero doping, corresponding to an electron filling in which the Fermi level lies at nodal points, there are opposite sign zero-frequency divergences in the response when approaching the nodal phase boundaries from the trivial and the TRITOPS phases. In the trivial phase, both the high-frequency and low-frequency response of the system are negative, but in the TRITOPS phase, the low-frequency response becomes positive while the high-frequency response remains negative. Furthermore, since phase transitions are controlled by the Rashba spin-orbit coupling and the ratio of intra-orbital and inter-orbital paring amplitudes, our results not only help distinguish the phases but can also provide an estimate of the pairing amplitudes based on the photogalvanic response of the system.

cond-mat.supr-con↗

arXiv2023

Site-selective doublon-holon dynamics in a pumped one-dimensional Hubbard superlattice with staggered Coulomb interactions

Doublon-holon dynamics is investigated in a pumped one-dimensional Hubbard model with a staggered on?site Coulomb interaction at half-filling. When the system parameters are set to be in the Mott insulating regime the equilibrium sublattice density of states exhibits several characteristic peaks, corresponding to the lower and upper Hubbard bands as well as hybridization bands. We study the linear absorption spectrum and find two main peaks characterizing the photon frequencies which excite the ground state to an excited state. For a system driven by a laser pulse with general intensity and frequency, both the energy absorption and the doublon-holon dynamics exhibit distinct behaviors as a function of laser amplitude and frequency. Single-photon processes are observed at low laser intensity where the energy is absorbed for resonance laser frequencies. For strong laser intensity multi-photon induced dynamics are observed in the system, which are confirmed by an evaluation of the Loschmidt amplitude. The contribution of multi-photon processes to site-specific double occupancy is also characterized by the generalized Loschmidt amplitude. The site-selective doublon-holon dynamics are observed in both the one and multi-photon processes and the site-selective behavior is explained within a quasiparticle picture. Our study suggests strategies to optically engineer the doublon-holon dynamics in one dimensional strongly correlated many-body systems.

cond-mat.str-el↗

arXiv2023

Giant effective magnetic moments of chiral phonons from orbit-lattice coupling

Circularly polarized lattice vibrations carry angular momentum and lead to magnetic responses in applied magnetic fields or when resonantly driven with ultrashort laser pulses. Recent measurements have found responses that are orders of magnitude larger than those calculated in prior theoretical studies. Here, we present a microscopic model for the effective magnetic moments of chiral phonons in magnetic materials that is able to reproduce the experimentally measured magnitudes and that allows us to make quantitative predictions for materials with giant magnetic responses using microscopic parameters. Our model is based on orbit-lattice couplings that hybridize optical phonons with orbital electronic transitions. We apply our model to two types of materials: $4f$ rare-earth halide paramagnets and $3d$ transition-metal oxide magnets. In both cases, we find that chiral phonons can carry giant effective magnetic moments of the order of a Bohr magneton, orders of magnitude larger than previous predictions.

cond-mat.mes-hall↗

arXiv2023

Photogalvanic response in multi-Weyl semimetals

We investigate the dependence of the photogalvanic response of a multi-Weyl semimetal on its topological charge, tilt, and chemical potential. We derive analytical expressions for the shift and injection conductivities for tilted charge-$n$ Weyl points $(n=1,2,3)$ using a low energy two-band effective Hamiltonian. For double-Weyl semimetals, we also compute the response from two-band and four-band tight-binding models with broken time-reversal symmetry to study the effect of band bending and the contributions from higher bands. We find a significant deviation in the responses obtained from the effective low-energy continuum model and more realistic four-band continuum and tight-binding models. We analyze several different limits of these models. We describe the nature of the deviations and provide estimates of their dependence on the frequency and other model parameters. Our analysis provides a simple explanation for the first-principle calculation based frequency dependence of the injection current in SrSi$_2$. Additionally, we find interesting parameter regimes where the frequency dependence of the non-linear optical response can be directly used to probe the type-I/type-II nature of the Weyl cone. We obtain analytical results for the charge-4 Weyl semimetal by reducing the original problem involving a triple $k$-space integral to one with only a double integral. This simplification allows us to extract all relevant information about the nature of its second-order dc response and the precise condition for observing circular photogalvanic effect quantization. The semi-analytical approach presented here can also be extended to a systematic study of second harmonic generation and first-order optical conductivity in charge-4 Weyl semimetals.

cond-mat.mes-hall↗