We report on a tunable—by magnetic field and gate voltage—conversion of terahertz radiation into a dc current in spatially modulated bilayer graphene. We experimentally demonstrate that the underlying physics is related to the so-called ratchet effect. Our key findings are the direct observation of a sharp cyclotron resonance in the photocurrent and the demonstration of two effects caused by electron-electron interaction: the plasmonic splitting of the resonance due to long-range Coulomb coupling and the partial suppression of its second harmonic due to fast interparticle collisions. We develop a theory, which perfectly fits our data. We argue that the ratchet current is generated in the hydrodynamic regime of nonideal electron liquid.

We proposed a mechanism to generate giant anisotropic second harmonic nonlinear response via double resonance effect, achieved through band nesting via electronic band-structure engineering. The ideal band setup would be a triplet of nested bands separated by the fundamental resonance energy $\hslash \omega$. We demonstrate theoretically that the proposed phenomenon can be realized in bilayer SnS by band tuning with perpendicular electrical bias, which maximizes the second harmonic susceptibility by several orders of magnitude. Moreover, the tunability of the polarization anisotropy can be useful for realizing novel polarization-sensitive devices.

Silicon and germanium are well-known materials used to manufacture electronic devices for integrated circuits, but they themselves are not considered as promising options for interconnecting the devices due to their semiconducting nature. We have discovered that both Si and Ge atoms can form unexpected metallic monolayer structures with a square lattice which are more stable than the semimetallic silicene and germanene, respectively, in line with the energetically more favored dumbbell and wavy-bilayer structures. More importantly, these two-dimensional allotropes of Si and Ge host Dirac fermions with Fermi velocities superior to those in graphene, indicating that the metal wires needed in the silicon-based integrated circuits can be made of the Si atom itself without incompatibility, allowing for all-silicon-based integrated circuits.

We introduce a ${\mathbb{Z}}_N$ stabilizer code that can be defined on any spatial lattice of the form $\mathrm{\Gamma }\times C_{L_z}$, where $\mathrm{\Gamma }$ is a general graph. We also present the low-energy limit of this stabilizer code as a Euclidean lattice action, which we refer to as the anisotropic ${\mathbb{Z}}_N$ Laplacian model. It is gapped, robust (i.e., stable under small deformations), and has lineons. Its ground-state degeneracy (GSD) is expressed in terms of a “mod $N$-reduction” of the Jacobian group of the graph $\mathrm{\Gamma }$. In the special case when space is an $L\times L\times L_z$ cubic lattice, the logarithm of the GSD depends on $L$ in an erratic way and grows no faster than $O\left(L\right)$. We also discuss another gapped model, the ${\mathbb{Z}}_N$ Laplacian model, which can be defined on any graph. It has fractons and a similarly strange GSD.

Bosons in one dimension display a phenomenon called quasicondensation where correlations decay in a power-law fashion. We study the fate of this quasicondensed state in an open-system setup where two macroscopic leads, held at different chemical potentials, drive the system far away from equilibrium. For generic leads, we find that a finite bias destroys the quasicondensed state and the system exhibits exponential-decay correlations and subleading power-law corrections that are bias dependent. Near the equilibrium, we found a diverging correlation length and determine the critical properties, including the critical scaling form of the correlations. Also, we contrast our findings against the fate of quasicondensation in thermal equilibrium and discuss the special case of reflectionless coupling where the quasicondensation survives out of equilibrium. This exactly solvable interacting nonequilibrium system has the remarkable property that the near-equilibrium state cannot be obtained within linear response. These results aid in unraveling the enigmatic properties spawned by strong interactions once liberated from equilibrium constraints.

We investigate the topological properties of the helical atomic chains occurring in elemental selenium and tellurium. We postulate a realistic model that includes spin-orbit interaction and show this to be topologically nontrivial, with a topological invariant protected by a crystalline symmetry. We describe the end-states, which are orbitally polarized, with an orbital density modulation strongly peaked at the edge. Furthermore, we propose a simplified model that decomposes into three orbital chains, allowing us to define a topological invariant protected by a crystalline symmetry. We contrast this result with recent observations made for the orbital Su-Schrieffer-Heeger model containing a $p$-orbital zigzag chain.

The effects of topological states on the thermoelectric performance of a highly efficient thermoelectric Yb-filled ${\mathrm{CoSb}}_3$ skutterudite are investigated through combined ab initio calculations and electrical transport measurements. The nontrivial topological states are revealed by ab initio calculations and inferred from anomalous Hall conductivity and magnetoresistance. The linear bands associated with the topological states lead to low single-band effective mass and high carrier mobility, and consequently high power factor. Furthermore, the additional band minima due to filling the voids with Yb atoms raise the valley degeneracy, which favors the density-of-states effective mass and thus the Seebeck coefficient but scarcely changing the carrier mobility. These effects together contribute to the high power factor of Yb-filled ${\mathrm{CoSb}}_3$ skutterudite. Our results show that topological states play a crucial role in improving the performance of thermoelectric materials.

Recent experiments have reported a lot of spectacular transport properties in topological materials, such as quantum oscillations and three-dimensional (3D) quantum Hall effect (QHE) in ${\mathrm{ZrTe}}_5$. In this paper, by using a strong topological insulator model to describe ${\mathrm{ZrTe}}_5$, we study the magnetotransport property of the 3D system. With fixed carrier density, we find that there exists a deferring effect in the chemical potential, which favors distinguishing the saddle points of the inverted LLs. On the other hand, with fixed chemical potential, the features of 3D QHE are demonstrated and we attribute the underlying mechanisms to the interplay between Dirac fermions, magnetic field, and impurity scatterings.

Short-time dynamics of many-body systems may exhibit nonanalytical behavior of the systems' properties at particular times, thus dubbed dynamical quantum phase transition. Simulations showed that in the presence of disorder, new critical times appear in the quench evolution of the Ising model. We study the physics behind these new critical times. We discuss the spectral features of the Ising model responsible for the disorder-induced phase transitions. We found the critical value of the disorder sufficient to induce the dynamical phase transition as a function of the number of spins. Most importantly, we argue that this dynamical phase transition while nontopological lacks a local order parameter.

The symmetric (Heisenberg) exchange interaction is fundamental to magnetism and assumes critical importance in designing magnetic materials for novel emergent phenomena and device applications. However, quantifying exchange is extremely challenging for ultrathin magnetic films, as techniques and approximations reliably used for bulk materials are largely inapplicable in the two-dimensional limit. Here we present and contrast the measurement of exchange stiffness $A$ by several methods on a series of five Co/Pt-based ultrathin ($1–2$ nm) films. We compare results from (a) spin-wave spectroscopy by Brillouin light scattering, (b) three analytical models describing the temperature dependence of magnetization obtained by magnetometry, (c) microscopic domain periodicity measurements and simulations, and (d) ab initio density functional theory calculations. While different methods present some qualitatively consistent trends across samples, we note, for any given sample, considerable differences (up to $5\times )$ in the absolute values of $A$ across the techniques, consistent with discrepancies of $A$ reported in literature for nominally similar samples. We analyze possible sources of the discrepancies across various methods, notably including their relationship to the spin-wave dispersion, and the wave-vector ranges probed. We compare the strengths and limitations of the techniques, and outline directions for their future use in characterizing exchange interactions in ultrathin films.

Detection is most often the main impediment to reduce the number of spins in paramagnetic resonance experiments. Here, we propose another route to carry out electrically detected spin resonance of an individual spin, placed at the edge of a quantum spin Hall insulator (QSHI). The edges of a QSHI host a one-dimensional electron gas with perfect spin-momentum locking. Therefore, the spin relaxation induced by emission of an electron-hole pair at the edge state of the QSHI can generate current. Here, we demonstrate that driving the system with an ac signal, a nonequilibrium occupation can be induced in the absence of applied bias voltage, resulting in a dc measurable current. We compute the dc current as a function of the Rabi frequency $\mathrm{\Omega }$, the spin relaxation, and decoherence times $T_1$, and we discuss the feasibility of this experiment with state-of-the-art instrumentation.

In this paper, the rich ground-state structure of orthogonally coupled symmetric all-antiferromagnetic junctions with easy-plane anisotropy is reported. A spin reorientation process rather than the traditional spin flop (SF) occurs, resulting in a phase in which Néel vectors preserve the mirror-reflection symmetry (termed MRS phase). The phase transitions in the ground state between SF and MRS phases can be either first or second order. After disturbed by external stimuli, magnons with different parities emerge. For in-plane dc fields, no couplings between magnons occur. When dc fields become oblique, coherent couplings between magnons with opposite parity emerge, leading to anticrossings in resonance frequencies. However, self-hybridization among magnons with the same parity never happens. More interestingly, spin waves based on MRS phases are linearly polarized and their polarization directions can be fine controlled.

Local gauge symmetry is intriguing for the study of quantum thermalization breaking. For example, in the high-spin lattice Schwinger model (LSM), the local U(1) gauge symmetry underlies the disorder-free many-body localization (MBL) dynamics of matter fields. This mechanism, however, would not work in a spin-$\frac{1}{2}$ LSM due to the absence of electric energy in the Hamiltonian. In this paper, we show that the spin-$\frac{1}{2}$ LSM can also exhibit disorder-free MBL dynamics, as well as entropy prethermalization, by introducing a four-fermion interaction into the system. The interplay between the fermion interaction and U(1) gauge symmetry endows the gauge fields with an effectively disordered potential which is responsible for the thermalization breaking. It induces anomalous (i.e., nonthermal) behaviors in the long-time evolution of such quantities as local observables, entanglement entropy, and correlation functions. Our work offers a different platform to explore emergent nonthermal dynamics in state-of-the-art quantum simulators with gauge symmetries.

In atomic physics, the Hund's rule states that the largest spin and orbital state is realized due to the interplay of spin-orbit coupling (SOC) and Coulomb interactions. Here, we show that in ferromagnetic solids the effective SOC and the orbital magnetic moment can be dramatically enhanced by a factor of $1/[1-(2U^{\prime }-U-J_H){\rho }_0]$, where $U$ and $U^{\prime }$ are the on-site Coulomb interaction within the same orbitals and between different orbitals, respectively, $J_H$ is the Hund's coupling, and ${\rho }_0$ is the average density of states. This factor is obtained by using the two-orbital as well as five-orbital Hubbard models with SOC. We also find that the spin polarization is more favorable than the orbital polarization, being consistent with experimental observations. The theory is also extended to study the spin fluctuations and long-range Coulomb interactions, and can be applied to understand the enhanced orbital magnetic moment and giant Faraday effect in ferromagnetic nanogranules in recent experiments. This present paper provides a fundamental basis for understanding the enhancements of SOC and orbital moment by Coulomb interactions in ferromagnets, which would have wide applications in spintronics.

We investigate a second-order topological quantum transition of a modified Kane-Mele model driven by an electron-phonon interaction. The results show that the system parameters of the bare modified Kane-Mele model are renormalized by the electron-phonon interaction. Starting from the second-order topological phase for the bare model, the increasing electron-phonon coupling strength can drive the second-order topological insulator into a semimetal phase. Such a second-order topological phase transition is characterized by the band-gap closing, discontinuity of the averaged ferminoic number, and the topological invariant.

Supersymmetry and supergravity were invented in the 1970s to solve fundamental problems in high-energy physics. Even though neither of these ideas has yet been confirmed in high-energy and cosmology experiments, they have been beneficial in constructing numerous theoretical models including superstring theory. Despite the absence of supersymmetry in particle physics, it can potentially emerge in exotic phases of strongly correlated condensed matter systems. In this paper, we propose a supergravity model that describes the low-energy physics of the Haldane-Rezayi state, a gapless quantum Hall state that occurs in a half-filled Landau level. We show that the corresponding edge modes of the Haldane-Rezayi state and the Girvin-MacDonald-Platzman algebra appear naturally in the supergravity model. Finally, we substantiate our theoretical findings with numerical exact diagonalization calculations that support the appearance of the emergent graviton and gravitino excitations in the Haldane-Rezayi state.

The chiral and superconducting gaps are studied in the context of a planar fermion model with four-fermion interactions. The effect of the tilt of the Dirac cone on both gaps is shown and discussed. Our results point to two different behaviors exhibited by planar fermionic systems. We show that there is a threshold value ${\tilde{t}}^{*}$ for the effective tilt parameter such that when $|\tilde{\mathbf{t}}|<{\tilde{t}}^{*}$, the superconducting phase persists for negative values of the superconducting coupling constant. For positive values of the superconducting coupling constant, the induction of a superconducting gap by a chemical potential exists, which is similar to the one seen in graphenelike systems. For $|\tilde{\mathbf{t}}|>{\tilde{t}}^{*}$ and a negative superconducting coupling constant, the superconducting phase can be present, but it is restricted to a smaller area in the phase portrait. Our analysis also shows that when $|\tilde{\mathbf{t}}|>{\tilde{t}}^{*}$ and for positive values for the superconducting coupling constant, the induction of a superconducting gap in the presence of a chemical potential is ruled out. In this case, the increase of the chemical potential works in favor of the manifestation of a metallic phase.

Despite intense experimental effort, theoretical proposals, and modeling approaches, a lack of consensus exists about the intrinsic mechanisms driving ultrafast magnetization dynamics in $3d$ ferromagnets. In this Letter, we find evidence of an electron-phonon mediated spin flip as the driving mechanism for the ultrafast magnetization dynamics in all three $3d$ ferromagnets: nickel, iron, and cobalt. We use a microscopic three-temperature model with parameters calculated from first principles, which has been validated by a direct comparison to the electron and lattice dynamics extracted from previous experiments. By a direct comparison to the experimentally measured magnetization dynamics for different laser fluences, we determine the spin-flip probability of each material. We find robust evidence that relatively small values of the spin-flip probability enable ultrafast demagnetization in all three $3d$ ferromagnets.

${\mathrm{Co}}_{2.5}{\mathrm{Fe}}_{0.5}{\mathrm{Sn}}_2{\mathrm{S}}_2$ single crystal was grown by a flux method, and its magnetocaloric effect and critical behavior were systematically investigated. Significant changes in magnetic properties and critical behavior were observed in Fe-doped ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, compared with undoped ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. Specifically, the magnetization of ${\mathrm{Co}}_{2.5}{\mathrm{Fe}}_{0.5}{\mathrm{Sn}}_2{\mathrm{S}}_2$ measured at a low magnetic field ($H<500$ Oe, $H\parallel c$ axis) does not show the thermal hysteresis phenomenon, whereas it was observed in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ near $T_{\mathrm{c}}$, which is characteristic of a typical second-order phase transition. Previous reports usually attribute the suppression of ferromagnetism by the substitution of Fe for Co to the decrease in the number of $3d$ electrons. Herein, according to the first-principles calculation, the magnetic moments of dopant Fe align antiparallelly to those of Co atoms, which is also the possible reason for the decrease in magnetization. The critical behavior analysis shows that the critical exponents of the Fe-doped sample around magnetic transition are very close to theoretical values of the mean-field model. Furthermore, the exchange interaction distance $J\left(r\right)\approx r^{-4.65}$ and the Rhodes-Wolfarth ratio (RWR) (RWR > 1) prove the long-range itinerant ferromagnetism. The hybridization between the $3d$ orbitals of Fe/Co and the orbitals of $\mathrm{Sn}\text{−}5p/\mathrm{S}\text{−}3p$ may be an important factor for long-range itinerant ferromagnetism. The zero-field cooling magnetization ($H=100$ Oe, $H\parallel ab$ plane) undergoes the spin reorientation around 76 K, maybe because the magnetic moments of Fe, aligning antiparallelly to moments of Co atoms, reorient from out of plane to in plane. The critical behavior analysis shows that the first-order phase transition may occur below $T_{\mathrm{c}}$ in the case of the $H\parallel ab$ plane.

Cubic phonon interactions are now regularly computed from first principles, and the quartic interactions have begun to receive more attention. Given this realistic anharmonic vibrational Hamiltonian, the classical phonon Green's function can be precisely measured using molecular dynamics, which can then be used to rigorously assess the range of validity for self-consistent diagrammatic approaches in the classical limit. Here we use the bundled irreducible derivative approach to efficiently and precisely compute the cubic and quartic phonon interactions of ${\mathrm{CaF}}_2$, systematically obtaining the vibrational Hamiltonian purely in terms of irreducible derivatives. We demonstrate that the 4-phonon sunset diagram has an important contribution to the optical phonon linewidths beyond $T=500$ K. Reasonable results are obtained even at $T=900$ K when performing self-consistency using the 4-phonon loop diagram and evaluating the 3-phonon bubble and 4-phonon sunset diagrams post-self-consistency. Further improvements are obtained by performing quasiparticle perturbation theory, where both the 4-phonon loop and the real part of the 3-phonon bubble are employed during self-consistency. Our irreducible derivative approach to self-consistent perturbation theory is a robust tool for studying anharmonic phonons in both the quantum and classical regimes.