A structural phase transition has been discovered in the synthetic tetrahedrite ${\mathrm{Cu}}_{12}{\mathrm{Sb}}_4{\mathrm{S}}_{13}$ at approximately 88 K. Upon cooling, the material transforms from its known cubic symmetry to a tetragonal unit cell that is characterized by an in-plane ordering that leads to a doubling of the unit cell volume. Specific heat capacity measurements demonstrate a hysteresis of more than two degrees in the associated anomaly. A similar hysteresis was observed in powder x-ray diffraction measurements, which also indicate a coexistence of the two phases, and together these results suggest a first-order transition. This structural transition coincides with a recently-reported metal-insulator transition, and the structural instability is related to the very low thermal conductivity $\kappa$ in these materials. Inelastic neutron scattering was used to measure the phonon density of states in ${\mathrm{Cu}}_{12}{\mathrm{Sb}}_4{\mathrm{S}}_{13}$ and ${\mathrm{Cu}}_{10}{\mathrm{Zn}}_2{\mathrm{Sb}}_4{\mathrm{S}}_{13}$, both of which possess a localized, low-energy phonon mode associated with strongly anharmonic copper displacements that suppress $\kappa$. In ${\mathrm{Cu}}_{12}{\mathrm{Sb}}_4{\mathrm{S}}_{13}$, signatures of the phase transition are observed in the temperature dependence of the localized mode, which disappears at the structural transition. In contrast, in the cubic Zn-doped material, the mode is at slightly higher-energy but observable for all temperatures, though it softens upon cooling.

We address dissipation effects on the nonequilibrium quantum dynamics of an ensemble of spins-$\frac{1}{2}$ coupled via an Ising interaction. Dissipation is modeled by a (Ohmic) bath of harmonic oscillators at zero temperature and correspond either to the sound modes of a one-dimensional Bose-Einstein (quasi-)condensate or to the zero-point fluctuations of a long transmission line. We consider the dimer comprising two spins and the quantum Ising chain with long-range interactions and develop an (mathematically and numerically) exact stochastic approach to address nonequilibrium protocols in the presence of an environment. For the two-spin case, we first investigate the dissipative quantum phase transition induced by the environment through quantum quenches and study the effect of the environment on the synchronization properties. Then we address Landau-Zener-Stueckelberg-Majorana protocols for two spins and for the spin array. In this latter case, we adopt a stochastic mean-field point of view and present a Kibble-Zurek-type argument to account for interaction effects in the lattice. Such dissipative quantum spin arrays can be realized in ultracold atoms, trapped ions, and mesoscopic systems and are related to Kondo lattice models.

Based on first-principles calculations, we demonstrate that substitutions of transition metals Ru and Ir, neighboring and same group elements in the periodic table, for the Rh site in the vicinity of surface can induce a substantially large perpendicular magnetic anisotropy (PMA), up to an order of magnitude of 20 $\mathrm{erg}/{\mathrm{cm}}^2$, in FeRh films on MgO. The main driving mechanism for this huge PMA is the interplay between the $d_{xy}$ and $d_{x^2-y^2}$ orbital states of the substitutional $4d$ and $5d$ transition metal atoms with large spin-orbit coupling. Further investigations demonstrate that magnetization direction of PMA undergoes a transition into an in-plane magnetization at the antiferromagnet $\to$ ferromagnet phase transition, which provides a viable route for achieving large and switchable PMA associated with the magnetic-phase transition in antiferromagnet spintronics.

We report a study of the relaxation time of the restoration of the resistive superconducting state in single crystalline boron-doped diamond using amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown on an insulating diamond substrate have a low carrier density of about $2.5\times {10}^{21}{\mathrm{cm}}^{-3}$ and a critical temperature of about $2\mathrm{K}$. By changing the modulation frequency we find a high-frequency rolloff which we associate with the characteristic time of energy relaxation between the electron and the phonon systems or the relaxation time for nonequilibrium superconductivity. Our main result is that the electron-phonon scattering time varies clearly as $T^{-2}$, over the accessible temperature range of $1.7$ to $2.2$ K. In addition, we find, upon approaching the critical temperature $T_c$, evidence for an increasing relaxation time on both sides of $T_c$.

Quantum anomalies are the breaking of a classical symmetry by quantum fluctuations. They dictate how physical systems of diverse nature, ranging from fundamental particles to crystalline materials, respond topologically to external perturbations, insensitive to local details. The anomaly paradigm was triggered by the discovery of the chiral anomaly that contributes to the decay of pions into photons and influences the motion of superfluid vortices in ${}^3\mathrm{He}$-A. In the solid state, it also fundamentally affects the properties of topological Weyl and Dirac semimetals, recently realized experimentally. In this work we propose that the most identifying consequence of the chiral anomaly, the charge density imbalance between fermions of different chirality induced by nonorthogonal electric and magnetic fields, can be directly observed in these materials with the existing technology of photoemission spectroscopy. With angle resolution, the chiral anomaly is identified by a characteristic note-shaped pattern of the emission spectra, originating from the imbalanced occupation of the bulk states and a previously unreported momentum dependent energy shift of the surface state Fermi arcs. We further demonstrate that the chiral anomaly likewise leaves an imprint in angle averaged emission spectra, facilitating its experimental detection. Thereby, our work provides essential theoretical input to foster the direct visualization of the chiral anomaly in condensed matter, in contrast to transport properties, such as negative magnetoresistance, which can also be obtained in the absence of a chiral anomaly.

The Hubbard model, which augments independent-electron band theory with a single parameter to describe electron-electron correlations, is widely regarded to be the “standard model” of condensed-matter physics. The model has been remarkably successful at addressing a range of correlation phenomena in solids, but it neglects many behaviors that occur in real materials, such as phonons, long-range interactions, and, in its simplest form, multiorbital effects. Here, we use ab initio electronic structure methods to design a material whose Hamiltonian matches as closely as possible that of the single-band Hubbard model. Our motivation is to compare the measured properties of our new material to those predicted by reliable theoretical solutions of the Hubbard model to determine the relevance of the model in the description of real materials. After identifying an appropriate crystal class and several appropriate chemistries, we use density-functional theory and dynamical mean-field theory to screen for the desired electronic band structure and metal-insulator transition. We then explore the most promising candidates for structural stability and suitability for doping, and we propose specific materials for subsequent synthesis. Finally, we identify a regime—that should manifest in our bespoke material—in which the single-band Hubbard model on a triangular lattice exhibits exotic $d$-wave superconductivity.

The in-plane electrical resistivity, $\rho$, and thermal conductivity, $\kappa$, of the heavy-fermion superconductor ${\mathrm{CeIrIn}}_5$ were measured down to 40 mK in magnetic fields up to 11 T applied along the $c$ axis. For all fields above $H_{c2}=4$ T of filamentary superconductivity, we find that the ratio of heat and charge conductivities in the $T\to 0$ limit obeys the Wiedemann-Franz law, $\kappa /T=L_0/\rho$, where $L_0=2.45\times {10}^{-8} \text{W}\mathrm{\Omega }{\text{K}}^{-2}$ is the Sommerfeld value of the Lorenz number. The temperature-dependent parts of both the electrical and thermal resistivity, $w\equiv T/L_0\kappa$, follow the functional dependence expected for the Fermi liquid theory of metals with $\rho -{\rho }_0=A{T}^2, w-w_0=B{T}^2$, with ${\rho }_0=w_0$ and $B\approx 2A$. The coefficient $B$ does not show a significant field dependence even upon approaching $H_{c2}=0.4$ T of the bulk superconducting state. The weak response to the magnetic field is in stark contrast with the behavior found in the closely related ${\mathrm{CeCoIn}}_5$, in which the field-tuned quantum critical point coincides with $H_{c2}$. The value of the electron-electron mass enhancement, as judged by the $A$ and $B$ coefficients, is about one order of magnitude reduced in ${\mathrm{CeIrIn}}_5$ as compared to ${\mathrm{CeCoIn}}_5$ (in spite of the fact that the zero field ${\gamma }_0$ in ${\mathrm{CeIrIn}}_5$ is twice as large as ${\gamma }_0$ in ${\mathrm{CeCoIn}}_5$), which suggests that the material is significantly farther away from the magnetic quantum critical point at bulk $H_{c2}$ and at all of the studied fields. A suppressed Kadowaki-Woods ratio in ${\mathrm{CeIrIn}}_5$ compared to ${\mathrm{CeCoIn}}_5$ suggests a notably more localized nature of $f$ electrons in the compound.

The magnetic structure and magnetoelectric effect have been investigated for single crystals of the antiferromagnet ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Single-crystal neutron diffraction and magnetic susceptibility measurement have revealed that the magnetic structure is different from a collinear arrangement with spin parallel to the trigonal axis as proposed previously. ${\mathrm{Co}}^{2+}$ magnetic moments are found to be almost lying in the basal plane, which lowers the magnetic symmetry to $C2/c^{\prime }$ with the propagation vector $\mathit{k}=0$. Associated with the magnetic phase transition, a sharp anomaly in the dielectric constant and displacement current indicate the appearance of the magnetoelectric below Néel temperature $T_N$ with a large coupling constant up to 30 ps/m. The existence of off-diagonal components in a magnetoelectric tensor indicate the formation of ferrotoroidic order in ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Such a magnetoelectric effect can be ascribed to the reduction of symmetry caused by simple antiferromagnetic order in a honeycomb network.

We consider quantum Hamiltonians of the form $H=H_0-U{\sum }_{j}cos\left(C_j\right)$, where $H_0$ is a quadratic function of position and momentum variables $\{x_1,p_1,x_2,p_2,\cdots \}$ and the $C_j$'s are linear in these variables. We allow $H_0$ and $C_j$ to be completely general with only two restrictions: we require that (1) the $C_j$'s are linearly independent and (2) $[C_j,C_k]$ is an integer multiple of $2\pi i$ for all $j,k$ so that the different cosine terms commute with one another. Our main result is a recipe for solving these Hamiltonians and obtaining their exact low-energy spectrum in the limit $U\to \infty$. This recipe involves constructing creation and annihilation operators and is similar in spirit to the procedure for diagonalizing quadratic Hamiltonians. In addition to our exact solution in the infinite $U$ limit, we also discuss how to analyze these systems when $U$ is large but finite. Our results are relevant to a number of different physical systems, but one of the most natural applications is to understanding the effects of electron scattering on quantum Hall edge modes. To demonstrate this application, we use our formalism to solve a toy model for a fractional quantum spin Hall edge with different types of impurities.

Transport experiments on the two-dimensional electron gas (2DEG) confined into a semiconductor quantum well and subjected to a quantizing magnetic field have uncovered a rich variety of uniform and nonuniform phases such as the Laughlin liquids, the Wigner, bubble, and Skyrme crystals, and the quantum Hall stripe state. Optically pumped nuclear magnetic resonance (OP-NMR) has also been extremely useful in studying the magnetization and dynamics of electron solids with exotic spin textures such as the Skyrme crystal. Recently, it has been demonstrated that a related technique, resistively-detected nuclear magnetic resonance (RD-NMR), could be a good tool to study the topography of the electron solids in the fractional and integer quantum Hall regimes. In this work, we compute theoretically the RD-NMR line shapes of various crystal phases of the 2DEG and study the relation between their spin density and texture and their NMR spectra. This allows us to evaluate the ability of the RD-NMR to discriminate between the various types of crystal states.

When graphene is placed on a substrate of heavy metal, the Rashba spin-orbit interaction of substantial strength can occur. In an open system such as a quantum dot, the interaction can induce spin polarization. Would classical dynamics have any effect on the spin polarization? Here we consider the quantum-dot setting, where the Rashba interaction is confined within the central scattering region whose geometrical shape can be chosen to yield distinct types of dynamics, e.g., regular or chaotic, in the classical limit. We find that as compared with regular or mixed dynamics, chaos can lead to significantly smooth fluctuation patterns of the spin polarization in its variation with the Fermi energy. Strikingly, in the experimentally feasible range of the Rashba interaction strength, the average polarization for a chaotic dot can be markedly larger than that for a regular or mixed dot. From the semiclassical viewpoint, a key quantity that determines the average spin polarization is the angle distribution of the outgoing electrons at the interface between regions with and without the Rashba interaction, respectively. Classical chaos generates a different distribution which, in turn, leads to higher average spin polarization. There was little previous work on the interplay between classical chaos and electron spin, and the phenomenon of chaos-enhanced spin polarization uncovered here can be exploited for spintronics applications.

We identify features in the angle-resolved photoemission spectra (ARPES) arising from the periodic pattern characteristic for graphene heterostructure with hexagonal boron nitride $\left(h\mathrm{BN}\right)$. For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moiré superlattice in graphene/$h\mathrm{BN}$ systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and $h\mathrm{BN}$. We also analyze how the presence of a moiré-periodic strain in graphene or scattering of photoemitted electrons off $h\mathrm{BN}$ can be distinguished from the miniband formation.

We present a general hybrid model based upon the continuum generalized Peierls-Nabarro model (with density functional theory parametrization) to describe interlayer dislocations in bilayer systems. In this model, the bilayer system is divided into two linear elastic 2D sheets. The strains in each sheet can be relaxed by both elastic in-plane deformation and out-of-plane buckling; this deformation is described via classical linear elastic thin plate theory. The interlayer bonding between these two sheets is described by a three-dimensional generalized stacking-fault energy (GSFE) determined from first principle calculations and based upon the relative displacement between the sheets. The structure and energetics of various interlayer dislocations in bilayer graphene was determined by minimizing the elastic and bonding energy with respect to all displacements. The dislocations break into partials, and pronounced buckling is observed at the partial dislocation locations to relax the strain induced by their edge components. The partial dislocation core width is reduced by buckling. An analytical model is also developed based upon the results obtained in numerical simulation. We develop an analytical model for the bilayer structure and energy and show that these predictions are in excellent agreement with the numerical results.

We carry out inelastic electron tunneling spectroscopy (IETS) measurement on a clean Cu(110) surface at 2 K using a scanning tunneling microscope (STM). The spectrum exhibits a single peak at 6 meV, while the phonon density of states (DOS) cannot explain the appearance of this peak. In order to elucidate the origin of the observed peak, we derive a general expression of STM-IETS spectrum for the phonon excitation at a clean metal surface by employing the Keldysh-Green's functional theory. We reveal that the STM-IETS spectrum is proportional to the momentum-resolved Eliashberg function, and not solely to the phonon DOS. By ab initio calculations of the momentum-resolved Eliashberg function combined with the phonon dispersion, we assign the observed peak to the out-of-plane polarized surface phonon mode.

The spin texture of the unoccupied surface electronic structure of the metal-semiconductor hybrid system $\mathrm{Tl}/\mathrm{Ge}\left(111\right)-(1\times 1)$ is investigated by spin- and angle-resolved inverse photoemission as well as quasiparticle band-structure calculations. Spin-polarized surface bands with rotating spin and giant energy splitting are found along $\overline{\mathrm{\Gamma }}\overline{K}\left({\overline{K}}^{\prime }\right)$, forming valleys with alternating out-of-plane spin polarization at $\overline{K}$ and ${\overline{K}}^{\prime }$. This behavior is known from the equivalent hybrid system on Si(111). Along $\overline{\mathrm{\Gamma }}\overline{M}$, a pair of surface bands appears within a projected bulk band gap, whose equivalent on Tl/Si(111) is a surface resonance because, there, it overlaps with bulk states. Surprisingly, the spin splitting of these bands on Tl/Ge(111) is much smaller than on Tl/Si(111) despite the stronger surface localization and the heavier substrate. Our detailed analysis of the band structure and a tight-binding model including all relevant interactions show that a remarkable interplay between spin-orbit coupling and hybridization is responsible for this unexpected result. The comparison between the two similar hybrid systems demonstrates that the strength of the spin-orbit coupling alone, based on the atomic number of the respective elements, is not sufficient to estimate spin splittings of spin-orbit-influenced surface states.

The nucleation and growth of zinc phthalocyanine (ZnPc) thin films on a Ag(100) surface are studied employing in situ, real-time low-energy electron microscopy and complementary density functional theory (DFT) calculation to elucidate the role of incorporation kinetics of planar molecules in phase selection during nucleation and apply this knowledge to the fabrication of highly crystalline ZnPc films. We show that the nucleation of crystalline ZnPc islands requires a large concentration of diffusing molecules. The required amount of nominal deposition to initiate the growth of monolayer (ML) high two-dimensional crystalline islands is dependent on both growth temperature and crystalline phase. At room temperature (RT) and slightly above (RT to ∼430 K), ZnPc crystalline islands have double-domain $R33.69$ structures with average domain sizes in the submicrometer range. At higher temperatures, a 5 × 5 commensurate ZnPc structure nucleates. DFT calculations reveal significant differences in interfacial energies of an isolated ZnPc molecule on a substrate, depending on an adsorption site and azimuthal orientation of the molecule relative to the substrate atomic lattice. The observed delay in the onset of the nucleation of an island is caused by the existence of a large energy barrier for molecule incorporation into an island. At certain growth conditions it is possible to induce a structural transition from the 5 × 5 to the $R33.69$ phase when the nominal coverage reaches 1 ML. The resulting film has excellent crystallinity with individual domains of hundreds of micrometers in size.

Recently, optical probes have become available that can access and observe energy renormalization due to electron-phonon interaction in graphene away from the well-studied Dirac $K$ point. Using an expanded deformation potential approach, we present a theoretical study of the electron-phonon self-energy and scattering matrix elements across the entire Brillouin zone. We elucidate the roles of modulated hopping and conventional deformation potential coupling, parameterized via standard deformation potentials, the in-plane phonon modes, intra- and interband contributions, and umklapp processes. Applying the theory to nonlinear optical transmission spectroscopy in the vicinity of the $M$ point, we find very good agreement with recently published experimental data.

A theory for the epitaxial growth of Au on ${\mathrm{MoS}}_2$ is developed and analyzed. The theory combines continuum linear elasticity theory with density functional theory to analyze epitaxial growth in this system. It is demonstrated that if one accounts for interfacial energies and strains, the presence of misfit dislocations, and the compliance of the ${\mathrm{MoS}}_2$ substrate, the experimentally observed growth orientation is favored despite the fact that it represents a larger elastic mismatch than two competing structures. The stability of the experimentally preferred orientation is attributed to the formation of a large number of strong Au-S bonds, and it is noted that this strong bond may serve as a means to exfoliate and transfer large single layers sheets of ${\mathrm{MoS}}_2$, as well as to engineer strain within single layers of ${\mathrm{MoS}}_2$. The potential for using a van der Waals-bonded layered material as a compliant substrate for applications in 2D electronic devices and epitaxial thin film growth is discussed.

A recent claim of possible superfluid behavior of parahydrogen films intercalated within a crystalline matrix of Na atoms is examined. Quantum Monte Carlo simulations at finite temperature yield strong numerical evidence that the system forms at low temperature a nonsuperfluid crystalline phase, commensurate with the underlying impurity lattice. The physics of this system is therefore qualitatively identical to that observed in similar settings, extensively studied in precedence. Comparison of numerical results obtained here, with those of the reference in which the prediction of superfluidity (disproven here) was made, points to likely bias in the computational methodology adopted therein.