We obtain the density-density correlation structure in molecular dynamics (MD) simulations of graphene, and analyze it within the capillary wave theory (CWT), developed for fluid surfaces, to describe the thermal corrugations of the graphene sheet with a wave-vector-dependent surface tension $\gamma \left(q_x\right)$. The density correlation function (from the atomic positions) is compared with the theoretical prediction by Bedeaux and Weeks (BW), within the CWT, in terms of $\gamma \left(q_x\right)$ and the density profile. The agreement is very good, even for relatively large $q_x\approx 0.2{\AA }^{-1}$, and with very little role for the correlation background, which sets an important difficulty for liquid surfaces. We present and test a generic prediction for the structure factor $S(q_x,q_z)$ from $\gamma \left(q_x\right)$, that contains and goes beyond the classical asymptotic expression, developed by Sinha, for the analysis of x-ray surface scattering. We compare our prediction with the formula used in the interpretation of experimental data, that assumes a direct relationship between $\gamma \left(q_x\right)$ and the correlation structure for the same wave vector $q_x$. That relation is exact only for the first (Wertheim's) term of the BW series, and we use our results to test the accuracy of the function $\gamma \left(q_x\right)$ estimated through that method.

We investigate phenomena correlated with multiple degrees of freedom in a geometrically frustrated magnet, ${\mathrm{CuFeO}}_2$, by applying uniaxial pressure $p$ and magnetic field $H$. Ferroelectric polarization is induced by a combination of $p$ and $H$ only in a sinusoidally amplitude-modulated magnetic structure phase, in which the sinusoidal magnetic ordering does not break the inversion symmetry. The crystal structure and magnetic structure in this phase seem to be unchanged, even by the application of $p$ and $H$ within the present neutron diffraction experiments. These results indicate that this phenomenon differs from conventional spin-driven ferroelectricity as observed in an $H$-induced helical magnetic phase of ${\mathrm{CuFeO}}_2$. We propose that the induction of ferroelectric polarization by the combined application of $p$ and $H$ in ${\mathrm{CuFeO}}_2$ can be regarded as a nonlinear piezomagnetoelectric effect.

Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts are lateral and transverse displacements of a wave packet reflecting off a surface. A dramatic real-space manifestation of wave-packet phases, they have traditionally been analyzed in a model-dependent fashion. Here, we argue that GH and IF shifts admit a general geometrical description and arise from a gauge invariant geometric phase. In particular, we show GH/IF shifts can be naturally captured by a shift vector, analogous to the shift vector from shift currents in the bulk photovoltaic effect. Employing Wilson loops to visualize the scattering processes contributing to the shift vector, we separate the shift into an intrinsic (depends solely on the system bulk) and an extrinsic part. This enables us to establish a clear model-independent link between symmetry and the presence/absence of intrinsic and extrinsic GH/IF shifts.

Doped BaCoSO was recently predicted to be a high-temperature superconductor in a new class based on Co and Ni. Using a Co-S self-flux method, we synthesized single crystals of the antiferromagnetic insulator BaCoSO. Our magnetic and specific heat measurements and neutron diffraction provide details of its magnetic anisotropy and order. Its band gap was determined to be about 1.3 eV by our measurements of its photoemission spectrum and infrared optical conductivity. Our results can pave the way to exploring the predicted superconductivity in this Co-based material.

The SU(4)-symmetric spin-orbital model on the honeycomb lattice was recently studied in connection to correlated insulators such as the $e_g$ Mott insulator ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ and the insulating phase of magic-angle twisted bilayer graphene at quarter filling. Here we provide a unified discussion of these systems by investigating an extended model that includes the effects of Hund's coupling and anisotropic, orbital-dependent exchange interactions. Using a combination of mean-field theory, linear flavor-wave theory, and variational Monte Carlo, we show that this model harbors a quantum spin-orbital liquid over a wide parameter regime around the SU(4)-symmetric point. For large Hund's coupling, a ferromagnetic antiferro-orbital ordered state appears, while a valence-bond crystal combined with a vortex orbital state is stabilized by dominant orbital-dependent exchange interactions.

Orbital-free density functional theory (OF-DFT) holds great promise for large-scale simulations since there is a linear scaling for the computational cost. However, OF-DFT faces an essential challenge on construction of accurate kinetic energy density functional (KEDF) for evaluating the noninteracting kinetic energy. In contrast to most of previous KEDFs constructed by satisfaction of known constraint conditions with pregiven forms, a nonlocal KEDF named Xu-Wang-Ma (XWM) is constructed from “scratch” by the line integrals, allowing us to ensure the inclusion of the corrected response behavior deviation from the uniform electron gas. XWM has been benchmarked on a range of model systems with different chemical environments. Numerical tests show that, in general, XWM can quantitatively reproduce the Kohn-Sham predictions of the basic bulk properties, electron density and vacancy formation energies. Particularly, the XWM functional is found to be numerically stable for random structures of both simple metals and several phases of silicon. The high accuracy and numerical stability of XWM functional yield improvements over most of KEDFs currently in use for applications, providing more insight into the development of new KEDFs.

We investigated two-dimensional (2D) exciton diffusion in monolayer $\mathrm{W}{\mathrm{S}}_2$ on both $\mathrm{Si}{\mathrm{O}}_2$ and hexagonal boron nitride ($h-\mathrm{BN}$) layers to identify the exciton diffusion enhanced by the $h-\mathrm{BN}$ bottom layer using spatially resolved photoluminescence imaging combined with time-resolved spectroscopy. The $\mathrm{W}{\mathrm{S}}_2$ on the $h-\mathrm{BN}$ bottom layer shows an exciton diffusion coefficient of $38\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$, which is almost 1.7 times that of $\mathrm{W}{\mathrm{S}}_2$ on the $\mathrm{Si}{\mathrm{O}}_2$ layer. Electrostatic force microscopy confirms that the increase in the 2D exciton diffusion is mainly due to the reduction in the charge impurities and traps on the $h-\mathrm{BN}$ surface compared to the $\mathrm{Si}{\mathrm{O}}_2$ surface.

In this work, we study the full set of eigenstates of a $p_x+i{p}_y$ topological superconductor coupled to a particle bath, which can be described in terms of an integrable Hamiltonian of the Richardson-Gaudin class. The results derived in this work also characterize the behavior of an anisotropic XXZ central spin model in an external magnetic field since both types of Hamiltonian are known to share the exact same conserved quantities, making them formally equivalent. We show how by ramping the coupling strength (or equivalently the magnetic field acting in the $z$-direction on the central spin), each individual eigenstate undergoes a sequence of gain/loss of excitations when crossing the specific values known as Read-Green points. These features are shown to be completely predictable for every one of the $2^N$ eigenstates, using only two integers easily obtainable from the zero-coupling configuration, which defines the eigenstate in question. These results provide a complete map of the particle-number sectors (superconductor) or magnetization sectors (central spin) involved in the large number of level crossings that occur in these systems at the Read-Green points. It further allows us to define quenching protocols that could create states with remarkably large excitation-number fluctuations.

For many solid targets, like semiconductors, that become amorphous under irradiation by energetic ions, the outermost surface layer displays formation of asymmetric nanoscale ripples in macroscopic timescales. In contrast to the well-known macroscopic case of an incompressible thin fluid film spreading down an incline, in which the morphological instability is controlled by gravity, here we prove that residual stress induced by the ions is responsible for pattern formation and accounts for its long-time dynamics, even in absence of sputtering effects. Using a continuum framework, we derive closed nonlinear evolution equations for the depth of the irradiated layer. This description includes novel terms associated with the spatial distribution of damage that builds up through sustained bombardment, thus extending to the nanoscale classic models of macroscopic fluid-flow systems, and providing detailed information on the pressure and velocity fields within the irradiated layer. Numerical simulations reproduce the main dynamical features of surface nanopatterning under the assumed conditions, elucidating the ensuing nonlinear properties on ripple amplification and transport.

Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwell's equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we demonstrate the equivalence of different theories for these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat-transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and $\mathrm{Si}{\mathrm{O}}_2$ nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems, because packed nanoparticles have low phonon thermal conductivity. We demonstrate that the radiative heat transfer strongly depends on the materials and geometrical arrangement of the nanoparticle array, and we verify our calculations by comparison to the exact solution for a one-dimensional particle chain.

The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, $\mathrm{C}{\mathrm{u}}_{1/3}\mathrm{A}{\mathrm{g}}_{1/3}\mathrm{A}{\mathrm{u}}_{1/3}\mathrm{CN}$, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN, and bimetallic cyanides $M_x{M}_{1-x}^{\prime }\mathrm{CN}$ ($M$, $M^{\prime }=\mathrm{Cu}$, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. $\mathrm{C}{\mathrm{u}}_{1/3}\mathrm{A}{\mathrm{g}}_{1/3}\mathrm{A}{\mathrm{u}}_{1/3}\mathrm{CN}$ exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function analysis and ${}^{13}\mathrm{C}$ solid-state nuclear-magnetic-resonance measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However, NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in $\mathrm{C}{\mathrm{u}}_{1/3}\mathrm{A}{\mathrm{g}}_{1/3}\mathrm{A}{\mathrm{u}}_{1/3}\mathrm{CN}$, the metals in neighboring chains cannot all be truly coplanar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature.

We study the critical behavior of the nonequilibrium dynamics and of the steady states emerging from the competition between coherent and dissipative dynamics close to quantum phase transitions. The latter is induced by the coupling of the system with a Markovian bath, such that the evolution of the system's density matrix can be effectively described by a Lindblad master equation. We devise general scaling behaviors for the out-of-equilibrium evolution and the stationary states emerging in the large-time limit for generic initial conditions in terms of the parameters of the Hamiltonian providing the coherent driving and those associated with the dissipative interactions with the environment. Our framework is supported by numerical results for the dynamics of a one-dimensional lattice fermion gas undergoing a quantum Ising transition in the presence of dissipative mechanisms which include local pumping and decay of particles.

The tetragonal phase of chromium (III) oxide, although unstable in the bulk, can be synthesized in epitaxial heterostructures. Theoretical investigation by density-functional theory predicts an antiferromagnetic ground state for this compound. We demonstrate experimentally antiferromagnetism up to 40 K in ultrathin films of $t\text{−}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$ by electrical measurements exploiting interface effect within a neighboring ultrathin Pt layer. We show that magnetotransport in Pt is affected by both spin-Hall magnetoresistance and magnetic proximity effect while we exclude any role of magnetism for the low-temperature resistance anomaly observed in Pt.

We use the spin functional renormalization group recently developed by two of us [Krieg and Kopietz, Phys. Rev. B 99, 060403(R) (2019)] to calculate the magnetization $M(H,T)$ and the damping of magnons due to classical longitudinal fluctuations of quantum Heisenberg ferromagnets. In order to guarantee that for vanishing magnetic field $H\to 0$, the magnon spectrum is gapless when the spin rotational invariance is spontaneously broken, we use a Ward identity to express the magnon self-energy in terms of the magnetization. In two dimensions, our approach correctly predicts the absence of long-range magnetic order for $H=0$ at finite temperature $T$. The magnon spectrum then exhibits a gap from which we obtain the transverse correlation length. We also calculate the wave-function renormalization factor of the magnons. As a mathematical byproduct, we derive a recursive form of the generalized Wick theorem for spin operators in frequency space which facilitates the calculation of arbitrary time-ordered connected correlation functions of an isolated spin in a magnetic field.

We show that in superconductors that break time-reversal symmetry and have anisotropy, such as $p+ip$ materials, all order parameters and magnetic modes are mixed. Excitation of the gap fields produces an excitation of the magnetic field and vice versa. Correspondingly, the long-range decay of the magnetic field and order parameter is in general given by the same exponent. Thus, one cannot characterize $p+ip$ superconductors by the usual coherence and magnetic field penetration lengths. Instead, the system has normal modes that are associated with linear combinations of magnetic fields, moduli of and phases of the order-parameter components. Each such normal mode has its own decay length that plays the role of a hybridized coherence/magnetic field penetration length. On a large part of the parameter space, these exponents are complex. Therefore, the system in general has damped oscillatory decay of the magnetic field accompanied by damped oscillatory variation of the order-parameter fields.

Extensive experimental and theoretical studies have been devoted to determining the high-pressure phase diagram of hydrogen. We present evidence of a phase at a higher pressure than phase III and below the pressure of the recently observed phase of metallic hydrogen (495 GPa). This phase was determined from infrared (IR) spectroscopy of hydrogen samples at static pressures above 360 ± 15 GPa in a diamond anvil cell, and has been observed in three separate experiments. Whereas earlier studies found new high-pressure phases that only occurred at elevated temperatures, this phase transition occurs at the lowest temperatures investigated, ∼5 K, and the steep phase line indicates that it is a quantum phase transition. This phase is characterized by two distinct IR absorption bands (2950 and $3335\mathrm{c}{\mathrm{m}}^{-1}$ at 365 GPa). Above the transition pressure we observe strong darkening of the sample in the visible spectrum as pressure is increased. Observations are compatible with the cmca-12 crystal structure.

We study the out-of-equilibrium dynamics of $p$-wave superconducting quantum wires with long-range interactions when the chemical potential is linearly ramped across the topological phase transition. We show that the heat produced after the quench scales with the quench rate $\delta$ according to the scaling law ${\delta }^{\theta }$ where the exponent $\theta$ depends on the power-law exponent of the long-range interactions. The presence of the long-range pairing term increases the exponent $\theta$ and thus improves the adiabatic preparation of topological states. Moreover, we identify the parameter regimes where the heat scaling can be cast in terms of the universal equilibrium critical exponents and can thus be understood within the Kibble-Zurek framework. When the electron hopping decays more slowly in space than pairing, it dominates the equilibrium scaling. Surprisingly, in this regime the dynamical critical behavior arises only from pairing and thus exhibits a dynamical universality unrelated to equilibrium scaling. The discrepancy from the expected Kibble-Zurek scenario can be traced back to the presence of multiple universal terms in the equilibrium scaling functions of long-range interacting systems close to a second order critical point.

The dynamics of two-dimensional (2D) topological quadrupole insulator (TQI) and Chern insulator (CI) after the real-space configuration is transformed from a cylinder or Mobius strip to open boundary condition (OBC) and vice versa is analyzed. Similar dynamics of both models are observed, but the quadrupole corner states of the TQI makes the signatures more prominent. After the systems transform from a cylinder or Mobius strip to OBC, the occupation of the corner state of the TQI and the edge state of the CI exhibits steady-state behavior. The steady-state values depend on the ramping rate of the configuration transformation, manifesting a type of quantum memory effect. On the other hand, oscillatory density ripples from the merging of edge states persist after the systems transform from OBC to a cylinder or Mobius strip. If the final configuration is a cylinder, the density ripples are along the edges of the cylinder. In contrast, the density ripples can traverse the bulk after the systems transform from OBC to a Mobius strip. The transformation of real-space topology thus can be inferred from the dynamical signatures of the topological edge states.

We apply the compressive sensing lattice dynamics method to calculate phonon dispersion for crystalline solids. While existing methods such as frozen phonon, small displacement, and linear response are routinely applied for phonon calculations, they are considerably more expensive or cumbersome to apply to certain solids, including structures with large unit cells or low symmetry, systems that require more expensive electronic structure treatment, and polar semiconductors/insulators. In the latter case, we propose an approach based on a corrected long-range force constant model with proper treatment of the acoustic sum rule and the symmetric on-site force constant matrix. Our approach is demonstrated to be accurate and efficient for these systems through case studies of NaCl, ${\mathrm{CeO}}_2$, ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, and ${\mathrm{La}}_2{\mathrm{Fe}}_{14}\mathrm{B}$.