The electronic structure of a ${\mathrm{LaCoO}}_3$(001) perovskite is analyzed using simulated and measured angle-resolved valence-band photoemission spectroscopy data with emphasis on the effects of the surface truncation, the final state effects, and the spin degeneracy removal. Supercell simulations reveal instability towards the formation of nonuniform spin polarization in the Co-containing layers under tensile stress. Lattice expansion reduces antiferrodistortive octahedron tilting and tips the ratio between the crystal field parameter and the exchange energy in favor of local magnetization. While the magnitude of the local moment scales with lateral strain, the Curie temperature saturates for straight Co-O-Co chains below 75 K. The comparison of simulated photoemission spectra with experiments suggests Co-${\mathrm{O}}_2$ to be the most stable surface stoichiometry, as proposed from transport and catalytic studies.

By using variational wave functions and quantum Monte Carlo techniques, we investigate the interplay between electron-electron and electron-phonon interactions in the two-dimensional Hubbard-Holstein model. Here, the ground-state phase diagram is triggered by several energy scales, i.e., the electron hopping $t$, the on-site electron-electron interaction $U$, the phonon energy ${\omega }_0$, and the electron-phonon coupling $g$. At half filling, the ground state is an antiferromagnetic insulator for $U\gtrsim 2g^2/{\omega }_0$, while it is a charge-density-wave (or bipolaronic) insulator for $U\lesssim 2g^2/{\omega }_0$. In addition to these phases, we find a superconducting phase that intrudes between them. For ${\omega }_0/t=1$, superconductivity emerges when both $U/t$ and $2g^2/t{\omega }_0$ are small; then, by increasing the value of the phonon energy ${\omega }_0$, it extends along the transition line between antiferromagnetic and charge-density-wave insulators. Away from half filling, phase separation occurs when doping the charge-density-wave insulator, while a uniform (superconducting) ground state is found when doping the superconducting phase. In the analysis of finite-size effects, it is extremely important to average over twisted boundary conditions, especially in the weak-coupling limit and in the doped case.

Machine learning is capable of discriminating phases of matter, and finding associated phase transitions, directly from large data sets of raw state configurations. In the context of condensed matter physics, most progress in the field of supervised learning has come from employing neural networks as classifiers. Although very powerful, such algorithms suffer from a lack of interpretability, which is usually desired in scientific applications in order to associate learned features with physical phenomena. In this paper, we explore support vector machines (SVMs), which are a class of supervised kernel methods that provide interpretable decision functions. We find that SVMs can learn the mathematical form of physical discriminators, such as order parameters and Hamiltonian constraints, for a set of two-dimensional spin models: the ferromagnetic Ising model, a conserved-order-parameter Ising model, and the Ising gauge theory. The ability of SVMs to provide interpretable classification highlights their potential for automating feature detection in both synthetic and experimental data sets for condensed matter and other many-body systems.

We report high-pressure magnetization and ${}^{35}\mathrm{Cl}$ NMR studies on $\alpha \text{−}{\mathrm{RuCl}}_3$ with pressure up to 1.5 GPa. At low pressures, the magnetic ordering is identified by both the magnetization data and the NMR data, where the $T_N$ shows a concave shape dependence with pressure. These data suggest stacking rearrangement along the $c$ axis. With increasing pressure, phase separation appears prominently at $P\ge$ 0.45 GPa, and the magnetic volume fraction is completely suppressed at $P\ge$ 1.05 GPa. Meanwhile, a phase-transition-like behavior emerges at high pressures in the remaining volume by a sharp drop of magnetization $M\left(T\right)$ upon cooling, with the transition temperature $T_x$ increased to 250 K at 1 GPa. The $1{/}^{35}{T}_1$ is reduced by over three orders of magnitude when cooled below 100 K. This characterizes a high-pressure, low-temperature phase with nearly absent static susceptibility and low-energy spin fluctuations. The nature of the high-pressure ground state is discussed, where a magnetically disordered state is proposed as a candidate state.

Grazing incidence fast molecule diffraction (GIFMD) has been recently used to study a number of surfaces, but this experimental effort has not been followed, to present, by a subsequent theoretical endeavor. Aiming at filling this gap, in this work, we have carried out GIFMD simulations for the benchmark system ${\mathrm{H}}_2/\mathrm{LiF}\left(001\right)$. To perform our study, we have built a six-dimensional potential energy surface (6D-PES) by applying a modified version of the corrugation reducing procedure (CRP) to a set of density functional theory (DFT) energies. Based on this CRP interpolated PES, we have conducted quantum dynamics calculations using both the multiconfiguration time-dependent Hartree and the time-dependent wave packet propagation methods. We have compared the results of our GIFMD simulations with available experimental spectra. From this comparison, we have uncovered a prominent role of the interaction between the quadrupole moment of ${\mathrm{H}}_2$ and the electric field associated with LiF(001) for specific incidence crystallographic directions. We show that, on the one hand, the molecule's initial rotation strongly affects its diffractive scattering and, on the other hand, the scattering is predominantly rotationally elastic over a wide range of incidence conditions typical for GIFMD experiments.

The quantum anomalous Hall (QAH) effect has attracted extensive attention due to time-reversal symmetry broken by a staggered magnetic flux emerging from ferromagnetic ordering and spin-orbit coupling. However, the experimental observations of the QAH effect are still challenging due to its small nontrivial bulk gap. Here, based on density functional theory and Berry curvature calculations, we propose the realization of intrinsic QAH effect in two-dimensional hexagonal metal-oxide lattice, $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_3$, which is characterized by the nonzero Chern number $(C=1)$ and chiral edge states. Spin-polarized calculations indicate that it exhibits a Dirac half-metal feature with temperature as large as $T_C=392\mathrm{K}$ using spin-wave theory. When the spin-orbit coupling is switched on, $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_3$ becomes a QAH insulator. Notably, the nontrivial topology is robust against biaxial strain with its band gap reaching up to $E_g=75\mathrm{meV}$, which is far beyond room temperature. A tight-binding model is further constructed to understand the origin of nontrivially electronic properties. Our findings on the Dirac half-metal and room-temperature QAH effect in the $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_3$ lattice can serve as an ideal platform for developing future topotronics devices.

We report the observation of a new allotrope of two-dimensional bismuth. Our scanning tunneling microscopy experiments show that the structure is clearly different than the previously synthesized allotropes $\beta$- and $\alpha$-bismuthene. It has a rectangular symmetry similar to that of $\alpha$-bismuthene, but is composed of a puckered single monolayer of Bi atoms ($\alpha$-bismuthene is intrisincally a paired layer material similar to black phosphorous). Atomic resolution images and an observed moiré pattern show that the new allotrope has a significantly contracted surface unit cell. The electronic structure is dominated by high density of states at the Fermi level as measured using scanning tunneling spectroscopy (STS) and confirmed by calculations based on density functional theory (DFT) which reveal Dirac cones at three different points in the Brillouin zone.

The twofold twist defects in the $D\left({\mathbb{Z}}_k\right)$ quantum double model (Abelian topological phase) carry non-Abelian fractional Majorana-like characteristics. We align these twist defects in a line and construct a one-dimensional Hamiltonian which only includes the pairwise interaction. For the defect chain with an even number of twist defects, it is equivalent to the ${\mathbb{Z}}_k$ clock model with a periodic boundary condition (up to some phase factor for the boundary term), while for the odd number case, it maps to the ${\mathbb{Z}}_k$ clock model with a duality twisted boundary condition. At the critical point, for both cases, the twist defect chain enjoys an additional translation symmetry, which corresponds to the Kramers-Wannier duality symmetry in the ${\mathbb{Z}}_k$ clock model and can be generated by a series of braiding operators for twist defects. We further numerically investigate the low energy excitation spectrum for $k=3,4,5$, and 6. For the even-defect chain, the critical points are the same as the ${\mathbb{Z}}_k$ clock conformal field theories (CFTs), while for the odd-defect chain, when $k\ne 4$, the critical points correspond to orbifolding a ${\mathbb{Z}}_2$ symmetry of CFTs of the even-defect chain. For $k=4$ case, we numerically observe some similarity to the ${\mathbb{Z}}_4$ twist fields in the $SU{\left(2\right)}_1/D_4$ orbifold CFT.

We present a detailed spectroscopic study of photoluminescence quenching in epitaxial structures containing CdSe/ZnSe quantum dots doped with low concentration of ${\mathrm{Mn}}^{2+}$ ions. Our time-resolved and time-integrated experiments reveal the origin of the quenching observed in macro-photoluminescence studies of ensembles of such dots. We show that incorporation of even a few ions to an individual dot does not quench its luminescence effectively, although some fingerprints of expected spin-dependent quenching are visible. At the same time, the presence of ${\mathrm{Mn}}^{2+}$ ions in the sample significantly affects the luminescence intensity of the wetting layer, resulting in a quenching of the global luminescence from studied structure. On the other hand, the luminescence decay dynamics are found to be independent of the presence of ${\mathrm{Mn}}^{2+}$ ions, which suggests that the observed quenching occurs for the excited excitonic states.

Spectroscopic line-shape analyses for single-${\mathrm{C}}_{60}$ vibrational modes are presented for two kinds of scanning tunneling microscope experiments. Inelastic electron tunneling spectroscopy is performed for ${\mathrm{C}}_{60}$ molecules with different adsorption geometries on Pb(111). Depending on the ${\mathrm{C}}_{60}$ adsorption site and rotational orientation, the lowest unoccupied molecular orbital exhibits varying degrees of overlap with ${\mathrm{C}}_{60}$ vibrational energies. Concomitantly, the line shapes of vibrational modes are affected according to expectations for on-resonance inelastic electron tunneling. Inelastic electron transport is further studied for decreasing tip-${\mathrm{C}}_{60}$ distances covering tunneling and contact ranges. Line-shape changes signaling a conductance increase rather than the expected decrease upon exciting vibrational quanta are observed. A phenomenological approach is suggested to understand the dissimilar behavior of the junction conductance in the different electron transport ranges.

We investigate low-frequency fluctuations in the transport characteristics of a quasi-1D Wigner solid (WS) moving along a liquid helium substrate in response to a sinusoidal driving voltage. The fluctuations occur between distinct transport modes in which the decoupling of the WS from ripplonic polarons (or “dimple lattice,” DL) formed on the helium surface does, or does not, occur during each ac cycle. We demonstrate that a Gaussian-like distribution in the decoupling threshold force gives rise to this bistability, as the low-frequency switching occurs when the probability of decoupling during each ac cycle is small but finite. We attribute the distribution in the decoupling threshold force to the range of structural configurations allowed for the quasi-1D electron lattice, which influences the strength of the WS-DL coupling. Hence, the switching rate between the ac transport modes is extremely sensitive to the microscopic properties of the electron solid.

We study quantum Brownian motion (QBM) models for a particle in a dissipative environment coupled to a periodic potential. We review QBM for a particle in a one-dimensional periodic potential and extend the study to that for a particle in two-dimensional (2D) periodic potentials of four Bravais lattice types: square, rectangular, triangular (hexagonal), and centered rectangular. We perform perturbative renormalization group analyses to derive the zero temperature flow diagrams and phase boundaries for a particle in these potentials, and observe localization behavior dependent on the anisotropy of the lattice parameters.

Lead-halide organic-inorganic perovskites consist of an inorganic host framework with an organic molecule occupying the interstitial space. The structure and dynamics of these materials have been heavily studied recently due to interest in their exceptional photovoltaic properties. We combine inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron scattering to study the temperature dependent dynamics of the molecular cation in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$. By applying high resolution quasielastic neutron scattering, we confirm the ${\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]}^{+}$ ions are static in the low temperature orthorhombic phase yet become dynamic above 150 K where a series of structural transitions occur. This molecular melting is accompanied by a temporal broadening in the intramolecular modes probed through high energy inelastic spectroscopy. Simultaneous Raman measurements, a strictly $\left|Q\right|=0$ probe, are suggestive that this broadening is due to local variations in the crystal field environment around the hydrogen atoms. These results confirm the strong role of hydrogen bonding and also a coupling between molecular and framework dynamics.

The surprising discovery of tripling the superconducting critical temperature of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ at high pressures has led to an intriguing question of how the superconductivity in the collapsed tetragonal phase differs from that in the noncollapsed phases of Fe-based superconductors. Here we report a ${}^{89}\mathrm{Y}$ nuclear magnetic resonance study of ${\mathrm{YFe}}_2{\mathrm{Ge}}_x{\mathrm{Si}}_{2-x}$ compounds whose electronic structure is similar to that of iron-pnictide collapsed tetragonal phases already at ambient pressure. We find that Fe(Ge,Si) layers show ferromagnetic spin fluctuations, whereas the layers are coupled antiferromagnetically. Furthermore, localized moments attributed either to Fe interstitial or antisite defects may account for magnetic impurity pair-breaking effects, thus explaining the substantial variation of superconductivity among different ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ samples.

Magnetic and lattice excitations in the quantum antiferromagnet $({\mathrm{C}}_4{\mathrm{H}}_{12}{\mathrm{N}}_2){\mathrm{Cu}}_2{\mathrm{Cl}}_6$ (PHCC) are studied across two pressure-induced phase transition at $P_c=4.3$ kbar and $P_1=13.4$ kbar using Raman spectroscopy. It is confirmed that neither transition is a result of a structural transformation. Magnetic scattering is detected. It shows a pronounced pressure dependence and undergoes substantial changes at both transitions. The results are in clear contradiction with previous neutron studies, which detected only minor changes of the magnon spectrum at $P_1$. A number of phonons show anomalous frequency shifts at low temperatures. This effect is pressure dependent and for two of the observed phonons dramatically reverses sign at around $P_1$. The anomalous behavior is attributed to strong magnetoelastic coupling in PHCC.

There exists considerable confusion in estimating the spin diffusion length of materials with high spin-orbit coupling from spin pumping experiments. For designing functional devices, it is important to determine the spin diffusion length with sufficient accuracy from experimental results. An inaccurate estimation of spin diffusion length also affects the estimation of other parameters (e.g., spin mixing conductance, spin Hall angle) concomitantly. The spin diffusion length for platinum (Pt) has been reported in the literature in a wide range of 0.5–14 nm, and in particular it is a constant value independent of Pt's thickness. Here, the key reasonings behind such a wide range of reported values of spin diffusion length have been identified comprehensively. In particular, it is shown here that a thickness-dependent conductivity and spin diffusion length is necessary to simultaneously match the experimental results of effective spin mixing conductance and inverse spin Hall voltage due to spin pumping. Such a thickness-dependent spin diffusion length is tantamount to the Elliott-Yafet spin relaxation mechanism, which bodes well for transitional metals. This conclusion is not altered even when there is significant interfacial spin memory loss. Furthermore, the variations in the estimated parameters are also studied, which is important for technological applications.

The phase diagram of the quasi-one-dimensional magnet ${\mathrm{NaCuMoO}}_4\left(\mathrm{OH}\right)$ is established through single-crystal NMR and heat-capacity measurements. The ${}^{23}\mathrm{Na}$ and ${}^1\mathrm{H}$ NMR experiments indicate a spiral and a collinear spin density wave (SDW) order below and above $B_c$ = 1.5–1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate $1/T_1$ indicates anisotropic spin fluctuations that have gapped excitations in the transverse spectrum but gapless ones in the longitudinal spectrum. These static and dynamic properties are well described by a theoretical model assuming quasi-one-dimensional chains with competing ferromagnetic nearest-neighbor interactions $J_1$ and antiferromagnetic next-nearest-neighbor interactions $J_2$ ($J_1-J_2$ chains). Because of the excellent crystal quality and good one-dimensionality, ${\mathrm{NaCuMoO}}_4\left(\mathrm{OH}\right)$ is a promising compound to elucidate the unique physics of the frustrated $J_1-J_2$ chain.

We study the superconducting current of a Josephson junction (JJ) coupled to an external nanomagnet driven by a time-dependent magnetic field both without and in the presence of an external ac drive. We provide an analytic, albeit perturbative, solution for the Landau-Lifshitz (LL) equations governing the coupled JJ-nanomagnet system in the presence of a magnetic field with arbitrary time dependence oriented along the easy axis of the nanomagnet's magnetization and in the limit of weak dimensionless coupling ${\epsilon }_0$ between the JJ and the nanomagnet. We show the existence of Shapiro-type steps in the $I-V$ characteristics of the JJ subjected to a voltage bias for a constant or periodically varying magnetic field and explore the effect of rotation of the magnetic field and the presence of an external ac drive on these steps. We support our analytic results with exact numerical solution of the LL equations. We also extend our results to dissipative nanomagnets by providing a perturbative solution to the Landau-Lifshitz-Gilbert (LLG) equations for weak dissipation. We study the fate of magnetization-induced Shapiro steps in the presence of dissipation both from our analytical results and via numerical solution of the coupled LLG equations. We discuss experiments which can test our theory.

We combined magnetoinfrared spectroscopy, magnetization, and lattice dynamics calculations to explore the phase transitions in multiferroic $\left[{\left({\mathrm{CH}}_3\right)}_2{\mathrm{NH}}_2\right]\mathrm{Mn}{\left(\mathrm{HCOO}\right)}_3$. Both the 185 K ferroelectric transition and the magnetically driven transition to the fully saturated state at 15.3 T involve the formate bending mode, direct evidence of a common connection between the two types of ferroicities and for how lattice distortions promote high-temperature magnetoelectric coupling in this system.