The complex phase diagram of manganites with simultaneously active spin, charge, orbital, and lattice degrees of freedom continues providing surprises. In a recent groundbreaking experiment, membranes of the perovskite manganite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (LCMO) deposited on a flexible polymer layer were strained up to 8% [S. S. Hong et al., Science 368, 71 (2020)], much more than achieved by regular strain induced by a rigid substrate. By increasing this strain, a metal-insulator transition was reported. Here we reproduce the results of the experiments using Monte Carlo simulations of the two-orbital double-exchange model including Jahn-Teller distortions at hole density $x=1/3$. The full phase diagram with varying temperature and Jahn-Teller coupling $\lambda$ is presented. When the bandwidth $W$ of mobile electrons is reduced, thus when the effective Jahn-Teller coupling $\lambda /W$ is increased, a metal-insulator transition is found in our simulations, between a ferromagnetic metallic state with uniform charge distribution and an insulator with diagonal charge stripes that retains its ferromagnetic character. In between the hole-rich diagonals, staggered orbital order occurs. We also report resistivity and magnetization measurements alongside with spin correlations and charge structure factors. Our overall conclusions are in agreement with the recent experimental and density functional theory results by Hong et al., and we confirm much earlier ground state predictions of striped ferromagnetic order using energy optimization techniques by T. Hotta et al. [Phys. Rev. Lett. 86, 4922 (2001)]. The experimental observation of one of the states predicted by theory suggests that diagonal stripes could be achieved at other hole densities as well, such as $x=1/4$, if LCMO membranes with that hole doping were subject to similar strains.

It is well known that the pristine bulk of an interacting one-dimensional system in Tomonaga-Luttinger liquid (TLL) phase shows power law suppression of quasiparticle tunneling amplitude for all values of TLL parameter $g$, in the zero-energy limit. We perform a density matrix renormalization group (DMRG) study of a fully symmetric Y junction of TLL wires and observe an anomalous enhancement of the tunneling density of states (TDOS) in the vicinity of the junction for both (a) interacting bosons case and (b) interacting fermions case, when $g>1$. We also observe suppression of TDOS for $g<1$ for both bosonic and fermionic cases. We find that the TDOS enhancements follow different power laws for bosonic and fermionic cases which suggests that these represent distinct fixed points, owing to statistical correlations that play an important role at the Y junction. Analysis of static conductance for the junction indicates that the fixed point for $1<g<3$ resembles the mysterious $M$ fixed point of Y junction predicted by Oshikawa, Chamon, and Affleck [J. Stat. Mech. (2006) P02008]. We also show that the TDOS enhancement spans over a length scale of $\propto {\omega }^{-1}$ from the junction, for $1<g<3$.

Constrained by the fixed mathematical form of most empirical potentials used in classical molecular dynamics (MD) simulations, many properties of materials cannot be captured within experimental accuracy. On the other hand, accurate electronic structure calculations based on quantum theory, most notably density functional theory (DFT), are limited to several hundred atoms within a picosecond, which makes the method inadequate for modeling systems beyond the nanoscale. A combination of speed from classical MD and fidelity from DFT can be achieved through machine learning methods. Herewith, we developed an approach named spatial density neural network force fields (SDNNFFs) by training neural networks to “learn” and predict DFT-level forces. Our model focuses on the usage of a three-dimensional mesh of density functions, which together act as a mapping of the atomic environment and provides a physical representation of the forces acting on the central atom. Several notable advantages arise from the SDNNFF, including (1) the avoidance of the chain rule on the total energy and other variables by direct calculation of the forces from the neural network, (2) the ever large $N\times t$ scaling of the training data, where $N$ is the number of atoms in a supercell and $t$ is the number of evaluated structures by first-principles, and (3) the significant reduction in parameters and human effort needed to successfully train a force- and/or property-converged neural network force field. Overall, we focus on modeling DFT-level forces with minimal computational cost and parametrization for rapid prediction of phonon-based properties and future molecular dynamics of large-scale systems. To demonstrate the SDNNFF, we trained several models on diamond structures, including bulk silicon (Si), diamond, silicon carbide (SiC), and boron arsenide (BAs), and predicted their phonon dispersions and lattice thermal conductivities using the direct solution to the phonon Boltzmann transport equation. For phonon properties, we utilized a fitting method for obtaining the second- and third-order force constants, which outperforms the highly force-sensitive finite displacement method when employing neural network force fields. In comparison to DFT lattice thermal conductivity, we obtained high precision results from our SDNNFF within 0.7% for Si, 6.2% for diamond, 2.76% for SiC, and 7.46% for BAs, with further agreement with experiments. The phonon dispersions from the SDNNFF also matched those from direct DFT and experiments. The developed approach for accurately predicting phonon transport properties of crystalline materials would largely benefit the design of advanced materials with improved performance, such as complex thermoelectric devices and low thermal resistance interfaces for nanoelectronics. Future applications of our SDNNFF model could be extended toward including atomic energy into the algorithm and simulating large-scale heterogeneous systems for quasielectronic representations for various properties.

Gated trilayer graphene exhibits an energy gap in its most stable ABC stacking. Here we show that when the stacking order changes from ABC to CBA, three gapless states appear in each valley. The states are topologically protected, and their number is related to the change in the valley Chern number across the stacking boundary. The stacking change is achieved by corrugation or delamination in the top and bottom layers, which simultaneously yields two AB/BA stacking domain walls in the pairs of adjacent layers (in bilayers). This in turn causes, for some gate voltages, two pairs of topological resonances to appear additionally in the conduction and valence band continua.

We investigate the variation of charge dynamics in the course of the filling-control metal-insulator transitions (MITs) for pyrochlore $R_2{\mathrm{Ru}}_2{\mathrm{O}}_7$ ($R$ being rare-earth ions). With widely changing of $R$ ions from Lu to Pr, the antiferromagnetic transition temperature systematically increases while the magnitude of the charge gap decreases toward zero at the hypothetical bandwidth-control Mott transition. It is attributable to the reduction of effective electron correlation in the Mott-Hubbard insulator regime. Doping holes give rise to the filling-control MITs from antiferromagnetic insulators to paramagnetic metals accompanied by the collapse of the Mott-Hubbard gaps. The doping-induced spectral weight transfer of the optical conductivity from the above-gap region to the in-gap region is nearly proportional to the doping level, whose rate is enhanced with the reduction of the electron correlation, in accord with the standard Mott criticality.

We develop a scheme for finite-size corrections of vertical transition energies and single-particle energy levels involving defect states with built-in ionic polarization in supercell calculations. The method accounts on an equal footing for the screening of the electrons and of the ionic polarization charge arising from the lattice distortions. We demonstrate the accuracy of our corrections for various defects in MgO and in water by comparing with the dilute limit achieved through the scaling of the system size. The general validity of our formulation is also confirmed through a sum rule that connects vertical transition energies with the formation energies of structurally relaxed defects.

The growth rate of the out-of-time-ordered correlator in a $N$-flavor Fermi gas is investigated and the Lyapunov exponent ${\lambda }_L$ is calculated to the order of $1/N$. We find that the Lyapunov exponent monotonically increases as the interaction strength increases from the BCS limit to the unitary region. At the unitarity, the Lyapunov exponent increases while the temperature drops and it can reach to the order of ${\lambda }_L\sim T$ around the critical temperature for the $N=1$ case. For $N\to \infty$, the Lyapunov exponent reaches its maximum ${\lambda }_L\approx 4.35T/N$ at the critical temperature. The system scrambles faster for stronger pairing fluctuations. At the BCS limit, the Lyapunov exponent behaviors as ${\lambda }_L\propto e^{\mu /T}{a}_s^2{T}^2/N$.

We experimentally measure the band dispersions of topological Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ using Fourier-transform scanning tunneling spectroscopy to image quasiparticle interference on the (001) surface of molecular-beam epitaxy-grown ${\mathrm{Na}}_3\mathrm{Bi}$ thin films. We find that the velocities for the lowest-lying conduction and valence bands are $1.6\times {10}^6\mathrm{m}{\mathrm{s}}^{-1}$ and $4.2\times {10}^5\mathrm{m}{\mathrm{s}}^{-1}$ respectively, significantly higher than previous theoretical predictions. We compare the experimental band dispersions to the theoretical band structures calculated using an increasing hierarchy of approximations of self-energy corrections due to interactions: generalized gradient approximation (GGA), meta-GGA, Heyd-Scuseria-Ernzerhof exchange-correlation functional (HSE06), and $GW$ methods. We find that density functional theory methods generally underestimate the electron velocities. However, we find significantly improved agreement with an increasingly sophisticated description of the exchange and interaction potential, culminating in reasonable agreement with experiments obtained by the $GW$ method. The results indicate that exchange-correlation effects are important in determining the electronic structure of this ${\mathrm{Na}}_3\mathrm{Bi}$, and are likely the origin of the high velocity. The electron velocity is consistent with recent experiments on ultrathin ${\mathrm{Na}}_3\mathrm{Bi}$ and also may explain the ultrahigh carrier mobility observed in heavily electron-doped ${\mathrm{Na}}_3\mathrm{Bi}$.

Recent experiments in unconventional superconductors, and in particular iron-based materials, have reported evidence of an antiferromagnetic quantum critical point (AFM-QCP) emerging inside the superconducting dome of the phase diagram. Fluctuations associated with such an AFM-QCP are expected to promote unusual temperature dependencies of thermodynamic quantities. Here, we compute the $T$ dependence of the specific heat $C\left(T\right)$ deep inside a fully gapped $s^{+-}$ superconducting state as the AFM-QCP is approached. We find that at the AFM-QCP, the specific heat $C\left(T\right)$ vanishes quadratically with temperature, as opposed to the typical exponential suppression seen in fully gapped BCS superconductors. This robust result is due to a nonanalytic contribution to the free energy arising from the general form of the bosonic (AFM) propagator in the SC state. Away from the AFM-QCP, as temperature is lowered, $C\left(T\right)$ shows a crossover from a $T^2$ behavior to an exponential behavior, with the crossover temperature scale set by the value of the superconducting gap and the distance to the QCP. We argue that these features in the specific heat can be used to unambiguously determine the existence of AFM-QCPs inside the superconducting domes of iron-based and other fully gapped unconventional superconductors.

We propose a hypothesis that the potential energy surface (PES) of interlayer interaction in diverse 2D materials can be universally described by the first spatial Fourier harmonics. This statement (checked previously for the interactions between graphene and hexagonal boron nitride layers in different combinations) is verified in the present paper for the case of hydrofluorinated graphene (HFG) bilayer with hydrogen bonding between fluorine and hydrogen at the interlayer interface. The PES for HFG bilayer is obtained through density functional theory calculations with van der Waals corrections. An analytical expression based on the first Fourier harmonics describing the PES which corresponds to the symmetry of HFG layers is derived. It is found that the calculated PES can be described by the first Fourier harmonics with the accuracy of 3% relative to the PES corrugation. The shear mode frequency, shear modulus and barrier for relative rotation of the layers to incommensurate states of HFG bilayer are estimated. Additionally it is shown that HFG bilayer is stable relative to the formation of HF molecules as a result of chemical reactions between the layers.

A group theoretical model is proposed for linear/quadratic coupling between order parameters which arise from electronic and soft-mode instabilities in doped shape memory alloys, together with coupling to symmetry breaking shear strains. This model is tested by using resonant ultrasound spectroscopy (RUS) to follow the elastic and anelastic anomalies which accompany transitions from B2 to B19, 9R, and incommensurate structures in ${\mathrm{Ti}}_{50}{\mathrm{Pd}}_{50-x}{\mathrm{Cr}}_x$ alloy samples ($0\le x\le 12$). The pure soft-mode transition gives rise to an incommensurate structure but without any associated changes in the shear modulus, implying that coupling with shear strains is weak. By way of contrast, the observed pattern of softening ahead of and stiffening below the martensitic transition is typical of pseudoproper ferroelastic behavior and confirms that there is strong bilinear coupling of the tetragonal shear strain to the order parameter associated with irrep ${\mathrm{\Gamma }}_3^{+}$ of the parent space group. The second order parameter has the symmetry properties of ${\mathrm{M}}_5^{-}$ in TiPd or of a point along the Σ line of the Brillouin zone for the 9R and incommensurate structures with high Cr contents. Comparison of shear modulus data for Cr-rich samples obtained by RUS at ${10}^5-{10}^6\mathrm{Hz}$ with previously reported Young's modulus data obtained for different samples by dynamical mechanical analysis at ∼0.1–10 Hz has not revealed the dispersion with frequency that would be expected for a glass transition governed by Vogel-Fulcher dynamics. The two techniques differ in the magnitude of effective applied stress, however, and differences in chemical homogeneity between samples or decomposition during high-temperature measurements might also be a factor.

Weakly coupled Ising chains provide a condensed-matter realization of confinement. In these systems, kinks and antikinks bind into mesons due to an attractive interaction potential that increases linearly with the distance between the particles. While single mesons have been directly observed in experiments, the role of the multiparticle continuum and bound states of mesons in the excitation spectrum is far less clear. Using time-dependent density-matrix renormalization group methods, we study the dynamical structure factors of one- and two-spin operators in a transverse-field two-leg Ising ladder in the ferromagnetic phase. The propagation of time-dependent correlations and the two-spin excitation spectrum reveal the existence of interchain bound states, which are absent in the one-spin dynamical structure factor. We also identify two-meson bound states that appear at higher energies, above the thresholds of several two-meson continua.

An ideal layered $\widehat{x}$-polarized antiferromagnet (AFM) between two antialigned $\pm \widehat{z}$ polarized ferromagnetic (FM) contacts transmits no current due to a $\pi$ phase difference of the matrix elements coupling the spin degenerate states to the two FM contacts. Inserting a normal-metal layer or tunnel barrier layer between one FM contact and the AFM alters this phase difference, and due to the unequal weighting of the two spins at the interface, it also breaks the spin degeneracy of the two AFM states. The broken symmetry of the matrix elements combined with the broken degeneracy of the AFM states results in a Fano resonance in the transmission and a turn-on of the $T_{\uparrow ,\downarrow }$ transmission channel. Such a magnetic tunnel junction geometry with two antialigned $\pm \widehat{z}$ FM contacts can electrically detect an AFM skyrmion. The AFM skyrmion serves as an analog of the oblique polarizer in the triple-polarizer experiment. Resistances and resistance ratios are calculated and compared for FM and AFM skyrmions in a magnetic tunnel junction.

The magnetic-order induced effects in nanocrystalline NiO are investigated through the phonons and magnons observed in the Raman spectra. The key observations are (i) an anisotropy of the first-order transverse and longitudinal optical phonons, with a splitting on the order of 5 meV and (ii) a marked size and excitation wavelength variation of the two-magnon peak, which varies linearly with a redshift of $\sim 50\mathrm{c}{\mathrm{m}}^{-1}$ with a size reduction from 105 to 30 nm. The magnon-related peaks, in contrast to the phonons, are suppressed for near-resonance laser excitations. The experimental results are interpreted in terms of the exchange interactions and strong electron-electron correlations. The magnetization measurements shows a crossover to ferromagnetism with large coercivities and magnetization with decreasing size, which is shown to be due to the thermoinduced contribution.

The delafossite family of compounds with a triangular lattice of rare earth ions has been recently proposed as a candidate host for quantum spin liquid (QSL) states. To realize QSLs, the crystal electric field (CEF) ground state of the rare earth ions should be composed of a doublet that allows sizable quantum tunneling, but until now the knowledge on CEF states in the delafossite compounds is still limited. Here we employ inelastic neutron scattering (INS) to study the CEF transitions in a powder sample of the delafossite ${\mathrm{NaErS}}_2$, where the large total angular momentum $J=15/2$ of the ${\mathrm{Er}}^{3+}$ ions and the resulting plethora of CEF transitions enable an accurate fit of the CEF parameters. Our study reveals nearly isotropic spins with large $J_z=\pm 1/2$ components for the ${\mathrm{Er}}^{3+}$ CEF ground states, which might facilitate the development of a QSL state. The scaling of the obtained CEF Hamiltonian to different rare earth ions suggests that sizable $J_z=\pm 1/2$ components are generally present in the CEF ground states, supporting the ternary sulfide delafossites as potential QSL hosts.

The Berry curvature in magnetic systems is attracting interest due to the potential tunability of topological features via the magnetic structure. $f$ electrons, with their large spin-orbit coupling, abundance of noncollinear magnetic structures, and high electronic tunability, are attractive candidates to search for tunable topological properties. In this study, we measure anomalous Hall effect (AHE) in the distorted kagome heavy fermion antiferromagnet ${\mathrm{U}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$. A large intrinsic AHE in high fields reveals the presence of a large Berry curvature. Moreover, the fields required to obtain the large Berry curvature are significantly different between $B\parallel a$ and $B\parallel a^{*}$, providing a mechanism to control the topological response in this system. Theoretical calculations illustrate that this sensitivity may be due to the heavy fermion character of the electronic structure. These results shed light on the Berry curvature of a strongly correlated band structure in magnetically frustrated heavy fermion materials but also emphasize $5f$ electrons as an ideal playground for studying field-tuned topological states.

Copper metaborate ${\mathrm{CuB}}_2{\mathrm{O}}_4$ crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below $T_{\text{N1}}=20$ K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic ${\mathrm{Cu}}^{2+}$ ($3d^9$, $S=1/2$) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the ${\mathrm{Cu}}^{2+}$ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split $d\text{−}d$ electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range $1.7–40$ K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets.

We develop and demonstrate the performance of a nonseparable form of the generalized gradient approximation (GGA) for exchange and correlation that includes locally varying parameters that match second-order gradient expansion behavior. Specifically, the high- and low-density limits are included to recover locally the linear response through inclusion of their dependence on the electron density. This local parametrization allows the GGA form to provide varying behavior depending on the density regime. On the basis of an extensive series of property calculations involving both molecules and solids, we show that this nonempirical methodology can lead to a balanced GGA description of both finite and extended systems.

Topological protection of edge state in quantum spin Hall systems relies only on time-reversal symmetry. Hence, $S_z$ conservation on the edge can be relaxed which can have an interferometric manifestation in terms of spin Berry phase. Primarily it could lead to the generation of spin Berry phase arising from a closed loop dynamics of electrons. Our work provides a minimal framework to generate and detect these effects by employing both spin-unpolarized and spin-polarized leads. We show that spin-polarized leads could lead to resonances or antiresonances in the two-terminal conductance of the interferometer. We further show that the positions of these antiresonances (as a function of energy of the incident electron) get shifted owing to the presence of spin Berry phase. Finally, we present simulations of a device setup using kwant package which put our theoretical predictions on a firm footing.