The magnetic order/disorder of the Cr moments in the antiferromagnetic state of the ferrimagnet $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ is reinvestigated by neutron powder diffraction to clarify the observation of reduced Cr moments located on a pyrochlore-like lattice [M. Reehuis et al., Phys. Rev. B 91, 024407 (2015)]. The change of the spin structure in the slightly Cu-doped chromite $\mathrm{N}{\mathrm{i}}_{0.98}\mathrm{C}{\mathrm{u}}_{0.02}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ has been studied as well, where orthorhombic lattice distortions become stronger. For both chromites we observe at 2 K two magnetic phases: one with two propagation vectors $\mathit{k}=$ (0,0,1) and (0,0,$k_z$), and a second one with $\mathit{k}=$ ($1/2,1/2,1/2$). The latter phase disappears at 21 K for $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ and at 24 K for $\mathrm{N}{\mathrm{i}}_{0.98}\mathrm{C}{\mathrm{u}}_{0.02}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$. Data analysis shows that the two phases reside on two disparate Cr sublattices. These are identified by different chain directions of strongly bonded spin pairs along diagonals in the pseudotetragonal $a_1{a}_2$ plane. The less distorted $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ shows the same subdivision of the lattice still with a saturated Cr moment for the (0,0,1)/(0,0,$k_z$) phase, however, with a strongly reduced moment for the ($1/2,1/2,1/2$) phase, whereas an almost saturated moment shows up in the Cu-doped chromite. The reduced moment in $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ indicates disorder on one Cr sublattice down to low temperature due to stronger frustration on the less distorted tetragonal pyrochlore lattice. Magnetoelastic effects and further neighbor spin interactions are mainly considered in the discussion of the observed peculiar ordering processes. Notable is the transition temperature of the ($1/2,1/2,1/2$) phase of $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ around 21 K, where anomalies for the specific heat and for the magnetodielectric behavior have recently been reported [T. D. Sparks et al., Phys. Rev. B 89, 024405 (2014)].

The pulse-noise approach to systems of classical spins weakly interacting with the bath has been applied to study thermally activated escape of magnetic nanoparticles over the uniform and nonuniform energy barriers at intermediate and low damping. The validity of approximating a single-domain particle by a single spin is investigated. Barriers for a nonuniform escape of elongated particles for the uniaxial model with transverse and longitudinal field have been worked out. Pulse-noise computations have been done for finite magnetic chains. The linear stability of the uniform barrier state has been investigated. The crossover between uniform and nonuniform barrier states has been studied with the help of the variational approach.

We study the drag force on objects moving in a Fermi superfluid at velocities on the order of the Landau velocity $v_L$. The expectation has been that $v_L$ is the critical velocity beyond which the drag force starts to increase toward its normal-state value. This expectation is challenged by a recent experiment measuring the heat generated by a uniformly moving wire immersed in superfluid ${}^3\mathrm{He}$. We introduce the basis for the calculation of the drag force on a macroscopic object using the Fermi-liquid theory of superfluidity. As a technical tool in the calculations, we propose a boundary condition that describes diffuse reflection of quasiparticles from a surface on a scale that is larger than the superfluid coherence length. We calculate the drag force on steadily moving objects of different sizes. For an object that is small compared to the coherence length, we find a drag force that is in accordance with the expectation. For a macroscopic object, we need to take into account the spatially varying flow field around the object. At low velocities, this arises from ideal flow of the superfluid. At higher velocities, the flow field is modified by excitations that are created when the flow velocity locally exceeds $v_L$. The flow field causes Andreev reflection of quasiparticles and thus leads to change in the drag force. We calculate multiple limiting cases for a cylinder-shaped object. In the absence of quasiparticle-quasiparticle collisions, we find that the critical velocity is larger than $v_L$ and the drag force (per cross-sectional area) at $2v_L$ is reduced by an order of magnitude compared to the case of a small object. In a collision-dominated limit, the flow shows signs of instability at a velocity below $v_L$.

Barlowite ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ shows three-dimensional (3D) long-range antiferromagnetism, which is fully suppressed in ${\mathrm{Cu}}_3\mathrm{Zn}{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ with a kagome quantum spin liquid ground state. Here we report systematic studies on the evolution of magnetism in the ${\mathrm{Cu}}_{4-x}{\mathrm{Zn}}_x{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ system as a function of $x$ to bridge the two limits of ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}(x=0)$ and ${\mathrm{Cu}}_3\mathrm{Zn}{\left(\mathrm{OH}\right)}_6\mathrm{FBr}(x=1)$. Neutron-diffraction measurements reveal a hexagonal-to-orthorhombic structural change with decreasing temperature in the $x=0$ sample. While confirming the 3D antiferromagnetic nature of low-temperature magnetism, the magnetic moments on some ${\mathrm{Cu}}^{2+}$ sites on the kagome planes are found to be vanishingly small, suggesting strong frustration already exists in barlowite. Substitution of interlayer ${\mathrm{Cu}}^{2+}$ with ${\mathrm{Zn}}^{2+}$ with gradually increasing $x$ completely suppresses the bulk magnetic order at around $x=0.4$ but leaves a local secondary magnetic order up to $x\sim 0.8$ with a slight decrease in its transition temperature. The high-temperature magnetic susceptibility and specific-heat measurements further suggest that the intrinsic magnetic properties of kagome spin liquid planes may already appear from $x>0.3$ samples. Our results reveal that the ${\mathrm{Cu}}_{4-x}{\mathrm{Zn}}_x{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ may be the long-thought experimental playground for the systematic investigations of the quantum phase transition from a long-range antiferromagnet to a topologically ordered quantum spin liquid.

The topochemical transformation of single crystals of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ into ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ is reported via fluorine insertion. Characterization of the newly formed ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ phase shows a nearly complete oxidation of ${\mathrm{Ir}}^{4+}$ cations into ${\mathrm{Ir}}^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled $J_{\mathrm{eff}}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent in the Ir metal ions of this compound.

A detailed study of low-temperature properties across the series ${\mathrm{CeRu}}_{2-x}{\mathrm{Co}}_x{\mathrm{Ge}}_2$ ($0\le x\le 2$) using magnetic susceptibility χ($T$), isothermal magnetization $M$($H$), heat capacity $C$($T$), and electrical resistivity ρ($T$) is presented. Using doping as a tuning parameter, a crossover from a Ruderman-Kittel-Kasuya-Yosida (RKKY) dominated region ($0\le x\le 1$) to a Kondo dominated region ($x\ge 1.5$) is evident. χ($T$) and ρ($T$) curves, analyzed in terms of theoretical models proposed by Sales and Freimuth for the Co ($x\ge 1.5$) rich compounds, suggest an intermediate valence state of the Ce ion. The intricate balance between the competing RKKY and Kondo effect is attributed to the volume change upon Co substitution, owing to its smaller ionic size, which enhances the hybridization between $4f$ and conduction electrons. A quantum critical point (QCP) in the (T, x) phase diagram is reached for the critical concentration ($x_c\sim 1.5$) where the competing RKKY and Kondo energy scales tend to zero. Deviation in χ($T$), $C$($T$), and ρ($T$) from Fermi-liquid behavior is observed in the vicinity of the QCP and the former is seen to recover with the application of magnetic field. Further support for the quantum criticality comes from the universal scaling behavior of $M$($H$), χ($T$), and ρ($T$) data for $x_c\sim 1.5$. Also, the valence fluctuations in the vicinity of the compound showing non-Fermi-liquid behavior may suggest the possible role of valence fluctuations in the QCP, which makes the present series an interesting case.

We present a many-body diffusion quantum Monte Carlo (DMC) study on the ground- and excited state properties of crystalline CoO polymorphs. To our knowledge, DMC is the only electronic structure method available to provide correct energetic ordering within experimental error bars between the three CoO polymorphs: rocksalt, wurtzite, and zinc blende. We compare these results to density functional theory (DFT) using state-of-the-art functionals such as SCAN. For the structural properties, such as the lattice parameters and bulk moduli, our results are comparable to HSE and SCAN. Using DMC, we calculated the indirect and direct optical gaps as 3.8(2) and 5.2(2) eV. Our indirect optical gap compares well with the conductivity measurements of 3.6(5) eV and $GW$ calculations with 3.4 eV. Similarly, we obtained the DMC indirect and direct quasiparticle gaps as 3.9(2) and 5.5(2) eV. DMC direct quasiparticle gaps compare well with the direct band gap of 5.53 eV obtained from ellipsometry studies.

Equilibration of quantum Hall edges is studied in a high quality dual gated bilayer graphene device in both unipolar and bipolar regimes when all the degeneracies of the zero energy Landau level are completely lifted. We find that in the unipolar regime when the filling factor under the top gate region is higher than the back gate filling factor, the equilibration is partial based on their spin polarization. However, the mixing of the edge states in the bipolar regime is insensitive to the spin configurations of the Landau levels and the values are very close to the full equilibration prediction. This has been explained by Landau level collapsing at the sharp $p-n$ junction in our thin hBN ($\sim 15$ nm) encapsulated device, in consistent with the existing theory.

We present a model to show that heat propagation away from a local source depends strongly on dimensionality, leading to dramatic localization in low-dimensional systems. An example of such a system is a carbon nanotube array. We further show that this localization is amplified due to a runaway mechanism if thermal conductivity declines rapidly with temperature. Extremely high temperatures of thousands of kelvins and gradients of hundreds of kelvins per micrometer may thus be obtained in a conductor using a modest local power source such as a laser pointer. This is of fundamental importance for high-efficiency energy conversion through thermoelectric and thermionic mechanisms, as well as various other applications.

Information processing devices operating in the quantum mechanical regime strongly rely on the quantum coherence of charge carriers. Studies of electronic dephasing in conventional metallic and semiconductor systems have not only paved the way toward high coherence quantum electronics, but also led to fundamental new insights in condensed matter physics. In this paper, we perform a spatially resolved study of electronic dephasing in three-dimensional topological insulators by exploiting an edge- versus surface-contacted measurement scheme. Unlike conventional two-dimensional systems that are characterized by a single dephasing mechanism, we find that dephasing in our samples evolves from a variable-range-hopping-type mechanism on the sample surface to a Nyquist-type electron-electron interaction mechanism in the subsurface layers. This is confirmed independently by the temperature and chemical potential dependence of the dephasing length, and gate-dependent suppression/enhancement of the weak antilocalization effect. Our devices are fabricated using bulk insulating topological insulator ${\mathrm{BiSbTe}}_{1.25}{\mathrm{Se}}_{1.75}$ capped with hexagonal-boron nitride in an inert environment, ruling out any extrinsic effects and confirming the topological surface state origin of our results. Our work addresses the idea of spatially resolved electronic dephasing and coherent transport in perhaps the most important topological insulator discovered so far. Our edge-versus-surface scheme may be applied to dephasing studies in a wide class of 2D materials.

Toroidal multipole moments are usually underestimated as they are quite weak in most cases of light-matter interaction. Herein, we reveal a strong link between the toroidal dipole resonance and the bound state in the continuum in the context of all-dielectric metasurfaces. We introduce the concept of toroidal dipole bound states in the continuum, in which two eigenmodes of the silicon metasurface exhibit an intrinsic toroidal dipolar character and have an infinite lifetime. They can be classified as transverse (trivial) and longitudinal (nontrivial) toroidal dipole modes, which correspond to symmetry unprotected and protected bound states in the continuum, respectively. We demonstrate that such toroidal bound states in the continuum supported by the symmetry metasurface can be turned into ultrahigh-$Q$ resonances with a dominant toroidal dipole excitation, which validates their physical origin associated with the ultrahigh-$Q$ toroidal dipole leaky resonances. A full multipole decomposition with dispersive, lossy, and substrate effects further validates that the proposed concept is general and can also be generalized to other structures.

Indium oxide is a wide band gap semiconductor that provides the platform for most $n$-type transparent conductors. Optical absorption is dominated by a strong edge starting at the optical gap around 3.7 eV, but the material also exhibits a prominent absorption tail starting around 2.7 eV. We use first principles methods to show that this tail arises from interband transitions that are dipole forbidden at the static lattice level, but become dipole allowed via a dynamical symmetry breaking induced by nuclear motion. We also report the temperature dependence of the absorption onset, which exhibits a redshift with heating, driven by a combination of electron-phonon coupling and thermal expansion. We argue that the role of dynamical symmetry breaking in optical absorption is a general feature of semiconductors and that the computational design of novel materials for optical applications, ranging from transparent conductors to solar cells, should incorporate the lattice dynamics of the crystal.

We study the universal critical properties of the ${\mathrm{QED}}_3$-Gross-Neveu-Yukawa model with $N$ flavors of four-component Dirac fermions coupled to a real scalar order parameter at four-loop order in the $\epsilon$ expansion below four dimensions. For $N=1$, the model is conjectured to be infrared dual to the $\text{SU}\left(2\right)$-symmetric noncompact $\mathbb{C}{\mathrm{P}}^1$ model, which describes the deconfined quantum critical point of the Néel-valence-bond-solid transition of spin-1/2 quantum antiferromagnets on the two-dimensional square lattice. For $N=2$, the model describes a quantum phase transition between an algebraic spin liquid and a chiral spin liquid in the spin-1/2 kagome antiferromagnet. For general $N$ we determine the order-parameter anomalous dimension, the correlation length exponent, the stability critical exponent, as well as the scaling dimensions of $\text{SU}\left(N\right)$ singlet and adjoint fermion mass bilinears at the critical point. We use Padé approximants to obtain estimates of critical properties in 2+1 dimensions.

We calculate the two-photon absorption in bulk and single-layer hexagonal boron nitride ($h$-BN) both by an ab initio real-time Bethe-Salpeter approach and by a real-space solution of the excitonic problem in tight-binding formalism. The two-photon absorption obeys different selection rules from those governing linear optics and therefore provides complementary information on the electronic excitations of $h$-BN. Combining the results from the simulations with an analysis of the crystal symmetries, we show that two-photon absorption is able to probe the lowest-energy $1s$ state in the single-layer $h$-BN and the lowest degenerate exciton of bulk $h$-BN. This result indicates that in $h$-BN multilayer stackings with inversion symmetry one can measure the Davydov splitting by means of a combination of one and two-photons excitations. The same analysis can be applied to other two-dimensional materials with the same point-group symmetry—such as the transition metal chalcogenides.

We investigate the hydrostatic pressure dependence of the zone center optical phonons of $c$-plane and $a$-plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the $A_1$ and $E_1$ modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes as $e_T^{*}\left(A_1\right)=2.93–9.9\times {10}^{-3}P$ and $e_T^{*}\left(E_1\right)=2.80–10.6\times {10}^{-3}P$ (in units of elementary charge and $P$ in GPa). These observations are well in line with results for other II–VI, III–V, and group-IV semiconductor compounds as far as the relation between the magnitude and sign of the pressure derivative of $e_T^{*}$ and the bond ionicity is concerned. As the latter increases so does $|\partial e_T^{*}/\partial P|$ with a sign change from positive to negative for bond ionicities around $f_i=0.46$ for compounds with anions belonging to the first row of the Periodic Table. A comparison of the results for InN and other nine tetrahedrally bonded compounds indicate that the pressure behavior of the transverse effective charge is mainly determined by the strength of the Pauli repulsion between cation valence electrons and those of the anion core. We also perform ab initio calculations in order to address the origin of the observed increase in linewidth of the $E_2^{\text{high}}$ mode which is found to arise from a pressure-induced increase in the rate of two-phonon decay processes. This broadening is associated with tuning into resonance of a steep edge in the two-phonon density of states around 460 ${\mathrm{cm}}^{-1}$ with the frequency of the $E_2^{\text{high}}$ mode.

Helical edge states appear at the surface of two-dimensional topological insulators and are characterized by spin up traveling in one direction and the spin down traveling in the opposite direction. Such states are protected by time-reversal symmetry and no backscattering due to scalar impurities can occur. However, magnetic impurities break time-reversal symmetry and lead to backscattering. Often their presence is unintentional, but in some cases they are introduced into the sample to open up gaps in the spectrum. We investigate the influence of random impurities on helical edge states, specifically how the gap behaves in the realistic case of impurities having both a magnetic and a scalar component. It turns out that for a fixed magnetic contribution the gap closes when either the scalar component is increased, or Fermi velocity is decreased. We compare diagrammatic techniques in the self-consistent Born approximation to numerical calculations, which yields good agreement. For experimentally relevant parameters we find that even moderate scalar components can be quite detrimental for the gap formation.

Electric depolarization fields have a great impact on the polarization-switching kinetics in ferroelectrics although they are often neglected in statistical considerations. Analysis of statistical distributions and correlations of polarization and electric field during the field-driven polarization reversal in a bulk polycrystalline ferroelectric by means of the two-dimensional self-consistent mesoscopic switching (SMS) model has revealed that correlations, mediated by electrostatic fields, are mostly isotropic and short range at a typical scale of the mean grain size [Phys. Rev. B 96, 054113 (2017)]. However, the magnitude of emerging depolarization fields remains substantial and strongly influences the switching kinetics. It is known, on the other hand, that the effect of inhomogeneities, such as a granular structure, on the electric field pattern and local field magnitudes is considerably overestimated in two-dimensional simulations. Three-dimensional extension of the SMS model in the current study allows a realistic evaluation of the impact of spatial correlations on the polarization switching in ferroelectric ceramics and opens a possibility to consider materials of different phase symmetries. It is shown that bound charges at grain boundaries due to mismatching grain polarizations as well as the subsequent depolarization fields are essentially dependent on the crystalline symmetry. This explains great differences in statistical field distributions and polarization kinetics observed in ceramics of different phase symmetries. Field correlations are anisotropic, depend on the material symmetry, but remain in all cases short range at the scale of a grain size. This sheds light on the success of models assuming statistically independent switching of different regions. Evolution of the statistical field distributions in the course of polarization reversal is also symmetry dependent but temporal changes in distributions are not substantial which clarifies a good performance of models neglecting the feedback via depolarization fields.

Carbon dioxide exhibits a richness of high-pressure polymorphs ranging from typical molecular solids to fully extended covalent solids, which, in turn, makes it a very appealing topic of fundamental research in condensed-matter physics and simultaneously provides valuable insights into the routes of developing possibly novel materials with advanced properties. The single-crystal x-ray diffraction (XRD) and Brillouin scattering spectroscopy of $\mathrm{C}{\mathrm{O}}_2-\mathrm{I}$ were performed under high temperature and pressure. Densities, acoustic velocities, and elastic moduli of $\mathrm{C}{\mathrm{O}}_2-\mathrm{I}$ were obtained along 300-, 400-, and 580-K isotherms up to the phase-transition boundaries. $\mathrm{C}{\mathrm{O}}_2-\mathrm{I}$ transforms to phase III and phase IV at room temperature (at 12.19 GPa) and 580 K (at 10.83 GPa), respectively. It was observed that high temperature suppresses pressure-induced stress in single-crystal $\mathrm{C}{\mathrm{O}}_2-\mathrm{I}$. All elastic constants and thermal elasticity parameters of $\mathrm{C}{\mathrm{O}}_2-\mathrm{I}$ were obtained and analyzed using finite-strain theory and thermal equation of state modeling. The $C_{11}$, $C_{12}$, and $K_S$ increase almost linearly with pressure, while shear moduli $C_{44}$ and G exhibit a downward trend with pressure, showing a noticeable reduction at higher temperature. Elastic anisotropy A is practically independent of pressure along each isotherm and increases from 1.75 to 1.9.

We study out-of-time-ordered correlators (OTOCs) in hard-core boson models with short-range and long-range hopping and compare the results to the OTOCs in the Luttinger-liquid model. For density-density correlations, a related expectation value of the squared commutator starts at zero and decays back to zero after the passage of the wavefront in all three models, while the wavefront broadens as $t^{1/3}$ in the short-range model and shows no broadening in the long-range model and the Luttinger-liquid model. For the boson creation operator, the corresponding commutator function shows saturation inside the light cone in all three models, with similar wavefront behavior as in the density-density commutator function, despite the presence of a nonlocal string in terms of Jordan-Wigner fermions. For the long-range model and the Luttinger-liquid model, the commutator function decays as a power law outside the light cone in the long-time regime when following different fixed-velocity rays. In all cases, the OTOCs approach their long-time values in a power-law fashion, with different exponents for different observables and short-range versus long-range cases. Our long-range model appears to capture exponents in the Luttinger-liquid model (which are found to be independent of the Luttinger parameter in the model). This conclusion also comes to bear on the OTOC calculations in conformal field theories, which we propose correspond to long-ranged models.

We present a protocol for the study of the dynamics and thermodynamics of quantum systems strongly coupled to a bath and subject to an external modulation. Our protocol quantifies the evolution of the system-bath composite by expanding the full density matrix as a series in the powers of the modulation rate, from which the functional form of work, heat, and entropy rates can be obtained. Under slow driving, thermodynamic laws are established. The entropy production rate is positive and is found to be related to the excess work dissipated by friction, at least up to second order in the driving speed. As an example of the present methodology, we reproduce the results for the quantum thermodynamics of the driven resonance level model. We also emphasize that our formalism is quite general and allows for electron-electron interactions, which can give rise to exotic Kondo resonances appearing in thermodynamic quantities.