The nucleation and growth properties of domains of molecules of the same state in open boundary three-dimensional (3D) spin-crossover systems of various shapes are discussed within the framework of the mechanoelastic model. The molecules are situated on face-centered-cubic lattices and are linked by springs through which they interact. Monte Carlo simulations imply that clusters nucleate from corners in the case of systems having well-developed faces and from kinks in the case of spherical samples, in accordance with available experimental data. In addition, a method to characterize the cooperativity in these systems is proposed, which by scanning the fluctuations in the 3D samples can be related directly to powder x-ray-diffraction experiments.

We reveal that inhomogeneous deformations (stretching, compression, twisting, or bending) of anharmonic lattices can lead to a local change of the coupling coefficients and induce the energy localization of high-frequency phonon modes. We consider a linear chain of particles interacting via the Lennard-Jones potentials under the action of a constant external force, and demonstrate that high-frequency oscillations can be localized at the edge of the inhomogeneously deformed chain. We also show stable propagation of an acoustic soliton in such chains that only changes its velocity due to the deformations. Additionally, we demonstrate that this mechanism is responsible for the formation of spatially localized phonon states in twisted graphene nanoribbons and the topological Möbius-like graphene structures through stretching of the valent bonds between carbon atoms. We argue that these anharmonic effects can be employed for rectification and control of heat flows in stretched lattices at the nanoscale.

We propose analytic expressions for fieldlike, $T_{\perp }$, and spin-transfer, $T_{\parallel }$, spin torque components in the spin-filter-based magnetic tunnel junction (SFMTJ), by using the single-band tight-binding model with the nonequilibrium Keldysh formalism. In consideration of multireflection processes between noncollinear magnetization of the spin-filter (SF) barrier and the ferromagnetic (FM) electrode, the central spin-selective SF barrier plays an active role in the striking discovery $T_{\perp }\gg T_{\parallel }$, which can be further identified by the unusual barrier thickness dependence of giant $T_{\perp }$. Our general expressions reveal the sinusoidal angular dependence of both spin torque components, even in the presence of the SF barrier.

We develop a theory of collective modes in a model of strongly disordered $s$-wave superconductor with a localization-induced pseudogap ${\mathrm{\Delta }}_P$, that is much larger than superconducting gap $\mathrm{\Delta }$. Then we applied the obtained results to the calculation of the ultrasound decay rate $\alpha \left(\omega \right)$ at low-frequencies $\omega \ll k_{B}T/\hslash$. We show that at low temperatures $T\ll T_c$ the magnitude of the decay rate $\alpha \left(\omega \right)$ is controlled by the ratio of $T/\mathrm{\Delta }$, while single-particle gap ${\mathrm{\Delta }}_P$ does enter the result for $\alpha \left(\omega \right)$. Thus, we propose a new method to measure the collective gap $\mathrm{\Delta }$ in a situation when strong pseudogap is present.

Specific heat has been measured in FeSe single crystals down to 0.414 K under magnetic fields up to 16 T. A sharp specific-heat anomaly is observed at the superconducting transition temperature $T_c=8.2$ K. Another jump of specific-heat coefficient $C/T$ is observed at about 1.08 K which is argued to be the long sought antiferromagnetic transition in bulk FeSe. Global fitting to electronic specific heat in a wide temperature region shows that the models with a single contribution with isotropic $s$-wave, anisotropic $s$-wave, and $d$-wave gaps all do not work well, nor the two isotropic $s$-wave gaps. We then fit the data by a model with two components in which one has the gap function of ${\mathrm{\Delta }}_0(1+\alpha cos2\theta )$. To have a good global fitting and the entropy conservation for the low-temperature transition, we conclude that the gap minimum should be smaller than 0.15 meV ($\alpha =0.9$ to 1), indicating that the superconducting gap(s) are nodeless but highly anisotropic. Our results are quite consistent with the gap structure derived recently from scanning tunneling spectroscopy measurements and yield specific-heat contributions of about 32% weight from the hole pocket and 68% from the electron pockets.

We develop the theory of hydrodynamic charge and heat transport in strongly interacting quasirelativistic systems on manifolds with inhomogeneous spatial curvature. In solid-state physics, this is analogous to strain disorder in the underlying lattice. In the hydrodynamic limit, we find that the thermal and electrical conductivities are dominated by viscous effects and that the thermal conductivity is most sensitive to this disorder. We compare the effects of inhomogeneity in the spatial metric to inhomogeneity in the chemical potential and discuss the extent to which our hydrodynamic theory is relevant for experimentally realizable condensed-matter systems, including suspended graphene at the Dirac point.

Strong spin-orbit coupling (SOC) can result in ground states with nontrivial topological properties. The situation is even richer in magnetic systems where the magnetic ordering can potentially have strong influence over the electronic band structure. The class of $A{\mathrm{MnBi}}_2$ ($A$ = Sr, Ca) compounds are important in this context as they are known to host massive Dirac fermions with strongly anisotropic dispersion, which is believed to be due to the interplay between strong SOC and magnetic degrees of freedom. We report the optical conductivity of ${\mathrm{YbMnBi}}_2$, a newly discovered member of this family and a proposed Weyl semimetal (WSM) candidate with broken time reversal symmetry. Together with density functional theory (DFT) band-structure calculations, we show that the complex conductivity can be interpreted as the sum of an intraband Drude response and interband transitions. We argue that the canting of the magnetic moments that has been proposed to be essential for the realization of the WSM in an otherwise antiferromagnetically ordered system is not necessary to explain the optical conductivity. We believe our data is explained qualitatively by the uncanted magnetic structure with a small offset of the chemical potential from strict stochiometry. We find no definitive evidence of a bulk Weyl nodes. Instead, we see signatures of a gapped Dirac dispersion, common in other members of $A{\mathrm{MnBi}}_2$ family or compounds with similar 2D network of Bi atoms. We speculate that the evidence for a WSM seen in ARPES arises through a surface magnetic phase. Such an assumption reconciles all known experimental data.

$\mathrm{C}{\mathrm{u}}_x\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is known for superconductivity due to Cu intercalation in the van der Waals gaps between the quintuple layers of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ at $x>0.10$. Here we report the synthesis and transport properties of Cu-doped $\mathrm{C}{\mathrm{u}}_x\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ films prepared by the chemical-vapor-deposition (CVD) method with $0.11\ge x\ge 0$. It is found that the insulatinglike temperature-dependent resistivity of polycrystalline $\mathrm{C}{\mathrm{u}}_x\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ films exhibits a marked metallic downturn and an increase of carrier concentration below ∼37 K. There is also a time-dependent slow relaxation behavior in the resistance at low temperature. These effects might be related to the strong hybridization between $\mathrm{C}{\mathrm{u}}^{+}$ and $\mathrm{C}{\mathrm{u}}^{2+}$ conduction bands from the intercalated $\mathrm{C}{\mathrm{u}}^{+}$ and substituted $\mathrm{C}{\mathrm{u}}^{2+}$ sites in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ films. The findings here have important implications for the understanding and development of doping-induced superconductivity in topological insulators.

The entanglement entropy in one-dimensional critical systems with boundaries has been associated with the noninteger ground-state degeneracy. This quantity, being a characteristic of boundary fixed points, decreases under renormalization group flow, as predicted by the g theorem. Here, using conformal field theory methods, we exactly calculate the entanglement entropy in the boundary Ising universality class. Our expression can be separated into the well-known bulk term and a boundary entanglement term, displaying a universal flow between two boundary conditions, in accordance with the g theorem. These results are obtained within the replica trick approach, where we show that the associated twist field, a central object generating the geometry of an $n$-sheeted Riemann surface, can be bosonized, giving simple analytic access to multiple quantities of interest. We argue that our result applies to other models falling into the same universality class. This includes the vicinity of the quantum critical point of the two-channel Kondo model, allowing one to track in real space the presence of a region containing one-half of a qubit with entropy $\frac{1}{2}log\left(2\right)$, associated with a free local Majorana fermion.

Temperature-dependent infrared spectroscopy has been investigated for ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ which undergoes a ferromagnetic transition at $T_C\approx 177\mathrm{K}$. It is observed that the spectral weight is transferred from $\sim 4800–14\hspace{0.17em}000{\mathrm{cm}}^{-1}$ to $\sim 0$–$4800{\mathrm{cm}}^{-1}$ as the temperature is lowered around $T_C$. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound ${\mathrm{CaFeO}}_3$. The optical conductivity also reveals a broad peak structure in the range of $\sim 700–1500{\mathrm{cm}}^{-1}$. Fit by the model of single-polaron absorption, the broad peak is interpreted by the excitation of polarons. From the fitting parameters of the polaron peak, we estimate the electron-phonon coupling constant $\alpha \sim 0.4–0.5$, implying that ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_3$ falls into the weak-coupling regime.

Extensions of dynamical mean-field theory (DMFT) make use of quantum impurity models as nonperturbative and exactly solvable reference systems which are essential to treat the strong electronic correlations. Through the introduction of retarded interactions on the impurity, these approximations can be made two-particle self-consistent. This is of interest for the Hubbard model because it allows to suppress the antiferromagnetic phase transition in two dimensions in accordance with the Mermin-Wagner theorem, and to include the effects of bosonic fluctuations. For a physically sound description of the latter, the approximation should be conserving. In this paper, we show that the mutual requirements of two-particle self-consistency and conservation lead to fundamental problems. For an approximation that is two-particle self-consistent in the charge and longitudinal spin channels, the double occupancy of the lattice and the impurity is no longer consistent when computed from single-particle properties. For the case of self-consistency in the charge and longitudinal as well as transversal spin channels, these requirements are even mutually exclusive so that no conserving approximation can exist. We illustrate these findings for a two-particle self-consistent and conserving DMFT approximation.

Organolead halide perovskites have emerged as potential building blocks for photovoltaic and optoelectronic devices. Yet the underlying fundamental physics is not well understood. There is lack of agreement on the electronic band structures and binding energies of coupled electron-hole pairs (excitons), which drive the photophysical processes. In this work, we conducted temperature-dependent reflectance and photoluminescence experiments on high-quality ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ single crystals. Two direct optical transitions corresponding to intrinsic free-excitons are clearly resolved, showing excellent consistence between the low-temperature ($T=10$ K) reflectance and photoluminescence spectra. Remarkably, the excitons have different binding energies and behave oppositely with temperature, suggesting distinctive origins. Moreover, the asymmetric photoluminescence profile is counterintuitively dominated by the high-energy exciton that is explained by a long relaxation time between levels and by the favorable generation rate of electron-hole pairs at the high-energy band. Our study opens access to the intrinsic properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ and sheds light to reconcile the large range of binding energies reported on these emergent direct band-gap semiconductors.

We study the time evolution of the optical spin Hall effect, which occurs when exciton polaritons undergo resonant Rayleigh scattering. The resulting spin pattern in momentum space is quantified by calculating the degree of circular polarization of the momentum space image for each point in time. We find the degree of circular polarization performing oscillations, which can be described within the framework of the pseudospin model by Kavokin et al. [A. Kavokin, G. Malpuech, and M. Glazov, Phys. Rev. Lett. 95, 136601 (2005)].

A family of topological semimetallic phases where twofold degenerate gapless points form linked rings is introduced. We refer to this phase as Weyl-link semimetals. A concrete two-band model with two linked nodal lines is constructed. We demonstrate that the Chern-Simons 3-form depends on the linking number of rings in a generic two-band model. In addition, we show the emergence of zero-energy modes in the Landau level spectrum can reveal the location of nodal lines, providing a method of probing their linking number.

Point defects in solids may realize solid state quantum bits. The spin-orbit coupling in these point defects plays a key role in the magneto-optical properties that determine the conditions of quantum bit operation. However, experimental data and methods do not directly yield these highly important data, particularly for such complex systems where the dynamic Jahn-Teller (DJT) effect damps the spin-orbit interaction. Here, we show for an exemplary quantum bit, the nitrogen-vacancy (NV) center in diamond, that ab initio supercell density functional theory provides a quantitative prediction for the spin-orbit coupling damped by DJT. We show that DJT is responsible for the multiple intersystem crossing rates of the NV center at cryogenic temperatures. Our results pave the way toward optimizing solid state quantum bits for quantum information processing and metrology applications.

We discuss positron-annihilation lifetimes for a set of illustrative bulk materials within the framework of the weighted-density approximation (WDA). The WDA can correctly describe electron-positron correlations in strongly inhomogeneous systems, such as surfaces, where the applicability of (semi-)local approximations is limited. We analyze the WDA in detail and show that the electrons which cannot screen external charges efficiently, such as the core electrons, cannot be treated accurately via the pair correlation of the homogeneous electron gas. We discuss how this problem can be addressed by reducing the screening in the homogeneous electron gas by adding terms depending on the gradient of the electron density. Further improvements are obtained when core electrons are treated within the LDA and the valence electron using the WDA. Finally, we discuss a semiempirical WDA-based approach in which a sum rule is imposed to reproduce the experimental lifetimes.

Motivated by the rapid experimental progress of quantum spin ice materials, we study the dynamical properties of pyrochlore spin ice in the U(1) spin liquid phases. In particular, we focus on the spinon excitations that appear at high energies and show up as an excitation continuum in the dynamic spin structure factor. The keen connection between the crystal symmetry fractionalization of the spinons and the spectral periodicity of the spinon continuum is emphasized and explicitly demonstrated. When the spinon experiences a background $\pi$ flux and the spinon continuum exhibits an enhanced spectral periodicity with a folded Brillouin zone, this spectral property can then be used to detect the spin quantum number fractionalization and U(1) spin liquid. Our prediction can be immediately examined by inelastic neutron-scattering experiments among quantum spin ice materials with Kramers' doublets. Further application to the non-Kramers' doublets is discussed.

We investigate the origin and renormalization of the gradient ($Q^2$) term in the propagator of soft bosonic fluctuations in theories of itinerant fermions near a quantum critical point (QCP) with ordering wavevector ${\mathbf{Q}}_0=0$. A common belief is that (i) the $Q^2$ term comes from fermions with high energies (roughly of order of the bandwidth) and, as such, should be included into the bare bosonic propagator of the effective low-energy model, and (ii) fluctuations within the low-energy model generate Landau damping of soft bosons, but affect the $Q^2$ term only weakly. We argue that the situation is in fact more complex. First, we found that the high- and low-energy contributions to the $Q^2$ term are of the same order. Second, we computed the high-energy contributions to the $Q^2$ term in two microscopic models (a Fermi gas with Coulomb interaction and the Hubbard model) and found that in all cases these contributions are numerically much smaller than the low-energy ones, especially in 2D. This last result is relevant for the behavior of observables at low energies, because the low-energy part of the $Q^2$ term is expected to flow when the effective mass diverges near QCP. If this term is the dominant one, its flow has to be computed self-consistently, which gives rise to a novel quantum-critical behavior. Following up on these results, we discuss two possible ways of formulating the theory of a QCP with ${\mathbf{Q}}_0=0$.

A strictly two-dimensional (2D) material such as freestanding graphene (FSG) is rarely investigated at the atomic scale by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A basic difficulty in probing FSG by STM and STS is the mechanical instability when a highly compliant 2D atomic layer interacts with a proximal tip. Here we report a detailed method to conduct reliable STM and STS on FSG with atomic precision. We found that FSG is intrinsically rippled and exhibits a nonlinear strain-stress relation under applied normal forces; it shows a very soft region of bending strain and stiffer regions of in-plane tensile strain once the nanoscale ripples of FSG are eliminated. The elimination of the nanoripples can be controlled by tip-induced pulling or pushing force through the so-called closed-loop Z-V STS mode which can monitor the FSG deformation. A key factor for controllable STM and STS measurements is to select tunneling set points to place FSG in metastable configurations, as determined from stress-strain (i.e., Z-V) response. Atomic imaging and electronic states thus measured must be interpreted by considering the dynamical deformation of FSG as tunneling parameters, and therefore tip-FSG forces, are varied.

Rippling is an inherent quality of two-dimensional materials playing an important role in determining their properties. Here, we study the effect of structural corrugations on the electronic and transport properties of monolayer black phosphorus (phosphorene) in the presence of tilted magnetic field. We follow a perturbative approach to obtain analytical corrections to the spectrum of Landau levels induced by a long-wavelength corrugation potential. We show that surface corrugations have a non-negligible effect on the electronic spectrum of phosphorene in tilted magnetic field. Particularly, the Landau levels are shown to exhibit deviations from the linear field dependence. The observed effect become especially pronounced at large tilt angles and corrugation amplitudes. Magnetotransport properties are further examined in the low temperature regime taking into account impurity scattering. We calculate magnetic field dependence of the longitudinal and Hall resistivities and find that the nonlinear effects reflecting the corrugation might be observed even in moderate fields $(B<10$ T).