A recent experimental study [Pan et al., arXiv:1902.10262] has shown that fractional quantum Hall effect gaps are essentially consistent with particle-hole symmetry in the lowest Landau level. Motivated by this result, we consider a clean two-dimensional electron system from the viewpoint of composite fermion mean-field theory. In this paper, we show that while the experiment is manifestly consistent with a Dirac composite fermion theory proposed recently by Son, it can equally well be explained within the framework of nonrelativistic composite fermions, first put forward by Halperin, Lee, and Read.

This paper presents the first-principles calculation of the electron-phonon coupling and the temperature dependence of the intrinsic electrical resistivity of the zirconium-hydrogen system with various hydrogen concentrations. The nature of the anomalous decrease in the electrical resistivity of the Zr-H system with the increase of hydrogen concentration (at high concentrations of $\mathrm{H}/\mathrm{Zr}>1.5$) is studied. It is found that the hydrogen concentration where the resistivity starts to decrease is very close to the critical concentration of the $\delta -\varepsilon$ phase transition. It is shown that the tetragonal lattice distortion due to the $\delta -\varepsilon$ phase transition of the Zr-H system eliminates imaginary phonon frequencies and the strong electron-phonon coupling of the $\delta$ phase and, as a result, leads to the reduction of the electrical resistivity of the Zr-H system at a high hydrogen concentration.

The electron is the dominant heat and charge carrier in metal, yet the Monte Carlo method for thermal and electrical transport remains not fully established due to the high density and high degeneracy of electrons. In this work, we develop a deviational Monte Carlo scheme to directly solve the Boltzmann transport equation for electron transport through a simplification of the full scattering term into relaxation time approximation form automatically including the Pauli exclusion principle. It is crucial to track the occupied and unoccupied electron states above and below the Fermi energy level, respectively, for Monte Carlo simulation of electron thermal transport. Our numerical scheme not only provides a clear physical picture, but also displays a good performance in predicting the temperature distribution, electronic thermal conductivity, and electrical conductivity for in-plane and cross-plane electron transport through metallic thin films. This work will promote the fundamental understanding of electron transport at micro- and nanoscale, and also provide a ground for the investigation of electron-phonon coupling transport.

Using state of the art Hybrid Monte Carlo (HMC) simulations we carry out an unbiased study of the competition between spin-density wave (SDW) and charge-density wave (CDW) order in suspended graphene. We determine that the realistic interelectron potential of graphene must be scaled up by a factor of roughly 1.6 to induce a semimetal-SDW phase transition and find no evidence for CDW order. A study of critical properties suggests that the universality class of the three-dimensional chiral Heisenberg Gross-Neveu model with two fermion flavors, predicted by renormalization group studies and strong-coupling expansion, is unlikely to apply to this transition. We propose that our results instead favor an interpretation in terms of a conformal phase transition. In addition, we describe a variant of the HMC algorithm which uses exact fermionic forces during molecular dynamics trajectories and avoids the use of pseudofermions. Compared to standard HMC, this allows for a substantial increase of the integrator step size while achieving comparable metropolis acceptance rates and leads to a sizable performance improvement.

A well-known feature of magnetic field driven dynamics of domain walls in ferromagnets is the existence of a threshold driving force at which the internal magnetization of the domain wall starts to precess—a phenomenon known as the Walker breakdown—resulting in an abrupt drop of the domain-wall propagation velocity. Here, we report on micromagnetic simulations of magnetic field driven domain-wall dynamics in thin ferromagnetic strips with perpendicular magnetic anisotropy which demonstrate that in wide enough strips Walker breakdown is a multistep process: It consists of several distinct velocity drops separated by short linear parts of the velocity vs field curve. These features originate from the repeated nucleation, propagation, and annihilation of an increasing number of Bloch lines within the domain wall as the driving field magnitude is increased. This mechanism arises due to magnetostatic effects breaking the symmetry between the two ends of the domain wall.

Thermodynamic properties of the square-lattice Holstein model of the electron-phonon problem with phonon frequencies small compared to the bare Fermi energy are obtained using Monte Carlo methods, a strong-coupling (bipolaronic) expansion, and a weak-coupling Migdal-Eliashberg approach. Already at elevated temperatures where the charge-density wave (CDW) and superconducting (SC) correlations are very short range, a crossover occurs as a function of increasing electron-phonon coupling, ${\lambda }_0$, from a normal metallic regime to a pseudogap regime. At sufficiently low $T$, a SC phase is found for small ${\lambda }_0$ and a commensurate insulating CDW phase for large ${\lambda }_0$.

A theory of parametric interaction between spin waves localized in a waveguide and traveling elastic waves is developed for ferromagnetic thin films. The presented theoretical formalism takes into account an arbitrary spatial distribution of the displacement field in the acoustic waves and an arbitrary magnetization in spin waves. Using the theory, we examine interaction of forward-volume spin waves (FVSW) localized in a narrow waveguide and Rayleigh surface acoustic waves traveling in a substrate underneath the waveguide. We show that, in contrast to classical electromagnetic pumping, the symmetry of the magnetoelastic interaction allows for the generation of first-order parametric instabilities in spin waves with circular precession, such as FVSW. At the same time the localization of spin waves modifies the momentum conservation law for the parametric process to include the transfer of momentum to the waveguide, which allows for a frequency separation of the interacting counterpropagating spin waves. The frequency separation enables amplification of a localized spin wave without generation of a traveling idler wave, which results in a greater amplification efficiency.

The magnetism in the sawtooth lattice of Mn in the olivine chalcogenides, ${\mathrm{Mn}}_2{\mathrm{SiS}}_{4-x}{\mathrm{Se}}_x$ ($x=1–4$), is studied in detail by analyzing their magnetization, specific heat, and thermal conductivity properties and complemented with density functional theory calculations. The air-stable chalcogenides are antiferromagnets and show a linear trend in the transition temperature $T_N$ as a function of Se content ($x$), which shows a decrease from $T_N\approx 86\mathrm{K}$ for ${\mathrm{Mn}}_2{\mathrm{SiS}}_4$ to 66 K for ${\mathrm{Mn}}_2{\mathrm{SiSe}}_4$. Additional magnetic anomalies are revealed at low temperatures for all the compositions. Magnetization irreversibilities are observed as a function of $x$. The specific heat and the magnetic entropy indicate the presence of short-range spin fluctuations in ${\mathrm{Mn}}_2{\mathrm{SiS}}_{4-x}{\mathrm{Se}}_x$. A spin-flop antiferromagnetic phase transition in the presence of applied magnetic field is present in ${\mathrm{Mn}}_2{\mathrm{SiS}}_{4-x}{\mathrm{Se}}_x$, where the critical field for the spin flop increases from $x=0$ towards 4 in a nonlinear fashion. Density functional theory calculations show that an overall antiferromagnetic structure with ferromagnetic coupling of the spins in the $ab$ plane minimizes the total energy. The band structures calculated for ${\mathrm{Mn}}_2{\mathrm{SiS}}_4$ and ${\mathrm{Mn}}_2{\mathrm{SiSe}}_4$ reveal features near the band edges similar to those reported for Fe-based olivines suggested as thermoelectrics; however the experimentally determined thermal transport data do not support superior thermoelectric features. The transition from long-range magnetic order in ${\mathrm{Mn}}_2{\mathrm{SiS}}_4$ to short-range order and spin fluctuations in ${\mathrm{Mn}}_2{\mathrm{SiSe}}_4$ is explained using the variation of the Mn-Mn distances in the triangle units that constitutes the sawtooth lattice upon progressive replacement of sulfur with selenium. Overall, the results presented here point towards the role played by magnetic anisotropy and geometric frustration in the antiferromagnetic state of the sawtooth olivines.

The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on ${\mathrm{LiNbO}}_3$ substrates are patterned into Hall bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.

Exact diagonalization of small model systems gives the thermodynamics of spin chains or quantum cell models at high temperatures $T$. Density matrix renormalization group calculations of progressively larger systems are used to obtain excitations up to a cutoff $W_C$ and the low-$T$ thermodynamics. The hybrid approach is applied to the magnetic susceptibility $\chi \left(T\right)$ and specific heat $C\left(T\right)$ of spin-$1/2$ chains with isotropic exchange such as the linear Heisenberg antiferromagnet and the frustrated $J_1-J_2$ model with ferromagnetic $J_1<0$ and antiferromagnetic $J_2>0$. The hybrid approach is fully validated by comparison with Heisenberg antiferromagnet results. It extends $J_1-J_2$ thermodynamics down to $T\sim 0.01|J_1|$ for $J_2/\left|J_1\right|\ge {\alpha }_c=1/4$ and is consistent with other methods. The criterion for the cutoff $W_C\left(N\right)$ in systems of $N$ spins is discussed. The cutoff leads to bounds for the thermodynamic limit that are best satisfied at a specific $T\left(N\right)$ at system size $N$.

When pushed out of equilibrium, generic interacting quantum systems equilibrate locally and are expected to evolve towards a locally thermal description despite their unitary time evolution. Systems in which disorder competes with interactions and transport can violate this expectation by exhibiting many-body localization. The strength of the disorder with respect to the other two parameters drives a transition from a thermalizing system towards a nonthermalizing one. The existence of this transition is well established both in experimental and numerical studies for finite systems. However, the stability of many-body localization in the thermodynamic limit is largely unclear. With increasing system size, a generic disordered system will contain with high probability areas of low disorder variation. If large and frequent enough, those areas constitute ergodic grains which can hybridize and thus compete with localization. While the details of this process are not yet settled, it is conceivable that if such regions appear sufficiently often, they might be powerful enough to restore thermalization. We set out to shed light on this problem by constructing potential landscapes with low disorder regions and numerically investigating their localization behavior in the Heisenberg model. Our findings suggest that many-body localization may be more stable than anticipated in other recent theoretical works.

We show that three-dimensional Lifshitz fermions arising as the critical theory at the Weyl-semimetal–insulator transition naturally develop an anomalous Hall viscosity at finite temperature. We discuss how to couple the system to nonrelativistic background sources for stress-tensor and momentum currents via a form of Newton-Cartan geometry with torsion and derive the Kubo formulas for the Hall viscosities. While the Lifshitz system that arises most naturally has scaling exponent $z=\frac{1}{2}$, we also generalize the theory for arbitrary Lifshitz scaling $z$ and show that, in the limit $z\to 0$, it may be given a Chern-Simons interpretation by dimensionally reducing along the anisotropic direction. The Hall viscosities are expressed in terms of Dirichlet eta functions and their temperature dependence is dictated by the scaling exponent.

We study the ground state of the one-dimensional extended Hubbard model at half filling using the entanglement entropy calculated by density matrix renormalization-group techniques. We apply curve fitting and scaling methods to accurately identify a second-order critical point as well as a Berezinskii-Kosterlitz-Thouless critical point. Using open boundary conditions and medium-sized lattices with very small truncation errors, we are able to achieve similar accuracy to that of previous authors. We also report observations of finite-size and boundary effects that can be remedied with careful pinning.

We report on the observation of very high and narrow magnetoconductance peaks which we attribute to the transition to the quantum Hall ferromagnetic (QHFM) state occurring at the edges of the sample. We show that the expected spatial degeneracy of chiral edge channels is spontaneously lifted in agreement with theoretical studies performed within the Hartree-Fock approximation. We indicate also that separated edge currents which flow in parallel may nevertheless cross at certain points, giving rise to the formation of topological defects or one-dimensional magnetic domains. Furthermore, we find that such local crossing of chiral channels can be induced on demand by coupling spin states of a Landau level to different current terminals and applying a dc source-drain voltage.

Recent experiments on twisted bilayer graphene (TBLG) have observed insulating states for two and three unit charges per moiré supercell, whereas the quarter-filling state (QFS) remained metallic. Subsequent experiments show that under hydrostatic pressure the QFS turns insulating for a certain window of pressure. In fact, the resistivity of the 1/2-filling and 3/4-filling states are also enhanced in the same pressure-window. Using pressure-dependent band structure calculations we compute the ratio of the potential to the kinetic energy, $r_s$. We find a window of pressure for which $r_s$ crosses the threshold for a triangular Wigner crystal, thereby corroborating our previous work that the insulating states in TBLG are driven by Wigner physics, A key prediction of this work is that the window for the onset of the hierarchy of Wigner states that obtains at commensurate fillings conforms to a dome shape under pressure. We also predict the optimal condition for Wigner crystallization to be around $1.5\mathrm{GPa}$. Consequently, TBLG provides a new platform for the exploration of Wigner physics and its relationship with superconductivity.

We present the real-time renormalization group (RTRG) method as a method to describe the stationary state current through generic multilevel quantum dots in nonequilibrium. The employed approach consists of a very rudimentary approximation for the renormalization group (RG) equations which neglects all vertex corrections while it provides a means to compute the effective dot Liouvillian self-consistently. Being based on a weak-coupling expansion in the tunneling between dot and reservoirs, the RTRG approach turns out to reliably describe charge fluctuations in and out of equilibrium for arbitrary coupling strength, even at zero temperature. We confirm this in the linear response regime with a benchmark against highly accurate numerical renormalization group data in the exemplary case of three-level quantum dots. For small to intermediate bias voltages and weak Coulomb interactions, we find an excellent agreement between RTRG and functional renormalization group data, which can be expected to be accurate in this regime. As a consequence, we advertise the presented RTRG approach as an efficient and versatile tool to describe charge fluctuations in quantum dot systems.

Motivated by the recent experiment on ${\mathrm{K}}_2{\mathrm{Cu}}_3\mathrm{O}{\left({\mathrm{SO}}_4\right)}_3$, an edge-shared tetrahedral spin-cluster compound [M. Fujihala, T. Sugimoto, T. Tohyama, S. Mitsuda, R. A. Mole, D. H. Yu, S. Yano, Y. Inagaki, H. Morodomi, T. Kawae, H. Sagayama, R. Kumai, Y. Murakami, K. Tomiyasu, A. Matsuo, and K. Kindo, Phys. Rev. Lett. 120, 077201 (2018)], we investigate two-leg spin-cluster ladders with the plaquette number $n_p$ in each cluster up to 6 by the density-matrix renormalization-group method. We find that the phase diagrams of such ladders strongly depend on the parity of $n_p$. For even $n_p$, the phase diagrams have two phases; one is the Haldane phase, and the other is the cluster rung-singlet phase. For odd $n_p$, there are four phases, which are a cluster-singlet phase, a cluster rung-singlet phase, a Haldane phase, and an even Haldane phase. Moreover, in the latter case the region of the Haldane phase increases while that of the cluster-singlet phase and the even Haldane phase shrinks as $n_p$ increases. We thus conjecture that in the large $n_p$ limit the phase diagrams will become independent of $n_p$. By analyzing the ground-state energy and entanglement entropy we obtain the order of the phase transitions. In particular, for $n_p=1$ there is no phase transition between the even Haldane phase and the cluster rung-singlet phase while for other odd $n_p$ there is a first-order phase transition. Our paper provides comprehensive phase diagrams for these cluster-based models and may be helpful to understand experiments on related materials.

We present a position operator that is compatible with periodic boundary conditions (PBCs). It is a one-body operator that can be applied in calculations of correlated materials by simply replacing the traditional position vector by the new definition. We show that it satisfies important fundamental as well as practical constraints. To illustrate the usefulness of the PBC position operator, we apply it to the localization tensor, a key quantity that is able to differentiate metallic from insulating states. In particular, we show that the localization tensor given in terms of the PBC position operator yields the correct expression in the thermodynamic limit. Moreover, we show that it correctly distinguishes between finite precursors of metals and insulators.

Recent studies of pairing and charge order in materials such as FeSe, ${\mathrm{SrTiO}}_3$, and 2H-${\mathrm{NbSe}}_2$ have suggested that momentum dependence of the electron-phonon coupling plays an important role in their properties. Initial attempts to study Hamiltonians which either do not include or else truncate the range of Coulomb repulsion have noted that the resulting spatial nonlocality of the electron-phonon interaction leads to a dominant tendency to phase separation. Here we present quantum Monte Carlo results for such models in which we incorporate both on-site and intersite electron-electron interactions. We show that these can stabilize phases in which the density is homogeneous and determine the associated phase boundaries. As a consequence, the physics of momentum dependent electron-phonon coupling can be determined outside of the trivial phase separated regime.

Acoustic analogs of electronic or photonic topological insulators provide unique approaches to manipulate sound wave propagation. Inspired by twist-induced topological photonic insulators, here we propose a type of two-dimensional acoustic topological insulator (TI) via projecting a section of a three-dimensional twisting structure to a plane, assembling the projected meta-atoms into metamolecules, and arranging the metamolecules into unit cells to form a honeycomb lattice. It follows that in this acoustic TI, topological phases mimic pseudospin-up and pseudospin-down states, and the pseudospin-orbital couplings are tuned via changing the rotation angles of the meta-atoms, which eventually leads to band inversion. By calculating acoustic band structures, pressure field distributions, and spin Chern numbers of bands, we verify that the topological phase transition occurs around the double Dirac cone and present the topological phase diagram as a function of the rotation angle of the meta-atoms. Once the coupling between adjacent metamolecules is sufficiently strong, mode inversion of topological states emerges. Furthermore, we numerically demonstrate the existence of topologically protected edge states. It is shown that robust pseudospin-dependent one-way transmission is immune to defects at the edge of topological distinct regions, which can be applied to acoustic wave transmissions and communications. Our approach in acoustic systems provides a strategy to explore abundant topological states in two-dimensional systems.