We have investigated the low-temperature magnetic state of face-centered-cubic (fcc) ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$, a Mott insulator and the first molecular analog of a geometrically frustrated Heisenberg fcc antiferromagnet with $S=1/2$ spins. Specific heat studies reveal the presence of both long-range antiferromagnetic ordering and a magnetically disordered state below $T_N=2.2$ K, which is in agreement with local probe experiments. These results together with the strongly suppressed $T_N$ are unexpected for conventional atom-based fcc antiferromagnets, implying that the fulleride molecular degrees of freedom give rise to the unique magnetic ground state.

Spin waves (or magnons) interact with magnetic domain walls (DWs) in a complicated way that a DW can propagate either along or against magnon flow. However, thermally activated magnons always drive a DW to the hotter region of a nanowire of magnetic insulators under a temperature gradient. We theoretically illustrate why it is surely so by showing that DW entropy is always larger than that of a domain as long as material parameters do not depend on spin textures. Equivalently, the total free energy of the wire can be lowered when the DW moves to the hotter region. The larger DW entropy is related to the increase of magnon density of states at low energy originated from the gapless magnon bound states.

We study the equilibrium properties of an Ising model on a disordered random network where the disorder can be quenched or annealed. The network consists of fourfold coordinated sites connected via variable length one-dimensional chains. Our emphasis is on nonuniversal properties and we consider the transition temperature and other equilibrium thermodynamic properties, including those associated with one-dimensional fluctuations arising from the chains. We use analytic methods in the annealed case, and a Monte Carlo simulation for the quenched disorder. Our objective is to study the difference between quenched and annealed results with a broad random distribution of interaction parameters. The former represents a situation where the time scale associated with the randomness is very long and the corresponding degrees of freedom can be viewed as frozen, while the annealed case models the situation where this is not so. We find that the transition temperature and the entropy associated with one-dimensional fluctuations are always higher for quenched disorder than in the annealed case. These differences increase with the strength of the disorder up to a saturating value. We discuss our results in connection to physical systems where a broad distribution of interaction strengths is present.

Polycrystalline samples of layered perovskite-related ${\mathrm{La}}_2{\mathrm{Ru}}_{1-y}{\mathrm{Mn}}_y{\mathrm{O}}_5$ ($0\le y\le 0.25$) were prepared by a soft-chemistry synthesis based on the thermal decomposition of a citrate precursor. The crystal structures of the compounds were determined by Rietveld analysis of x-ray diffraction patterns recorded at different temperatures. Additional neutron diffraction patterns for ${\mathrm{La}}_2{\mathrm{Ru}}_{0.75}{\mathrm{Mn}}_{0.25}{\mathrm{O}}_5$ were recorded between 1.5 K and 300 K. A structural phase transition from a monoclinic room-temperature to a triclinic low-temperature phase was observed for $y\le 0.20$, while for $y=0.25$ a phase separation at low temperatures was identified. The structural phase-transition temperatures decrease with increasing Mn concentration. Selected area electron diffraction of ${\mathrm{La}}_2{\mathrm{Ru}}_{0.75}{\mathrm{Mn}}_{0.25}{\mathrm{O}}_5$ documented the absence of cationic ordering of Ru and Mn. The Mn and Ru oxidation states were determined using x-ray absorption spectroscopy and showed a stable +4 valence for both elements in all samples investigated, at room temperature as well as at 110 K.

Magnetic properties as well as the specific heat of the spin-dimer system ${\mathrm{La}}_2{\mathrm{Ru}}_{1-y}{\mathrm{Mn}}_y{\mathrm{O}}_5$ were investigated for manganese concentrations $0\le y\le 0.25$. The magnetic (dc) susceptibility of the unsubstituted ($y=0$) system shows a steplike decrease close to 160 K reflecting a magnetostructural transition into a dimerized ground state. With increasing manganese concentration this behavior (typical for singlet formation) becomes continuously suppressed and the susceptibility bears the signatures of the emergence of new magnetic ground states. The high-temperature Curie-Weiss susceptibility can be described by ${\mathrm{Ru}}^{4+}$ ($S=1$) and ${\mathrm{Mn}}^{4+}$ ($S=\frac{3}{2}$) spin moments, with a dramatic decrease of the Curie-Weiss temperatures by almost 30% close to $y=0.1$, indicating significant changes in the average mean magnetic exchange. Field-cooled and zero-field-cooled experiments as well as ac-susceptibility measurements provide clear evidence for the formation of a spin-glass state, well below the characteristic dimerization temperature. The relaxation dynamics can be described by a Vogel-Fulcher-Tammann behavior and indicates high fragility when characterized in terms of glassy dynamics of canonical supercooled liquids. Additional electron-spin resonance experiments indicate different spin-glass regimes and a rather dynamic nature of the dimerized phase. In the Mn-substituted compounds, a linear contribution to the heat capacity at low temperatures can be ascribed to the spin-glass formation. With increasing manganese concentration, the anomaly in the specific heat caused by the spin-singlet formation is shifted to lower temperatures and becomes continuously suppressed and smeared out. On the basis of these results, we propose a ($y,T$)-phase diagram indicating the competition of the spin-glass and the dimerized states. We stress the similarities with doped $\mathrm{Cu}\mathrm{Ge}{\mathrm{O}}_3$, the canonical inorganic spin-Peierls system.

We consider a simple model of a quasi-one-dimensional conductor in which two order parameters (OP) may coexist, i.e., the superconducting OP $\Delta$ and the OP $W$ that characterize the amplitude of a charge-density wave (CDW). In the mean field approximation, we present equations for the matrix Green's functions $G_{ik}$, where the first subscript $i$ relates to the one of the two Fermi sheets and the other, $k$, operates in the Gor'kov-Nambu space. Using the solutions of these equations, we find stationary states for different values of the parameter describing the curvature of the Fermi surface $\mu$, which can be varied, e.g., by doping. It is established, in particular, that in the interval ${\mu }_1<\mu <{\mu }_2$, the self-consistency equations have a solution for coexisting OPs $\Delta$ and $W$. However, this solution corresponds to a saddle point in the energy functional $\Phi (\Delta ,W)$, i.e., it is unstable. Stable states are (1) the W state, i.e., the state with the CDW ($W\ne 0$, $\Delta =0$) at $\mu <{\mu }_2$ and (2) the S state, i.e., the purely superconducting state ($\Delta \ne 0$, $W=0$) at ${\mu }_1<\mu$. These states correspond to minima of $\Phi$. At $\mu <{\mu }_0=({\mu }_1+{\mu }_2)/2$, the state (1) corresponds to a global minimum, and at ${\mu }_0<\mu$, the state (2) has a lower energy, i.e., only the superconducting state survives at large $\mu$. We study the dynamics of the variations $\delta \Delta$ and $\delta W$ from these states in the collisionless limit. It is characterized by two modes of oscillations, the fast and the slow one. The fast mode is analogous to damped oscillations in conventional superconductors. The frequency of slow modes depends on the curvature $\mu$ and is much smaller than $2\Delta /\hslash$ if the coupling constants for superconductivity and CDW are close to each other. The considered model can be applied to high-$T_{\text{c}}$ superconductors where the parts of the Fermi surface near the “hot” spots may be regarded as the considered two Fermi sheets. We also discuss relation of the considered model to the simplest model for Fe-based pnictides.

We investigated the normal-state resistivity ${\rho }_{xx}\left(T\right)$ and the Hall effect in Zn- and Co-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ single-crystalline microbridges. A crossover temperature $T^{*}$ was observed in the temperature dependency of the longitudinal resistivity ${\rho }_{xx}\left(T\right)$, which separates ${\rho }_{xx}\left(T\right)$ into temperature-linear and temperature-nonlinear regions. Above $T^{*}$, the carriers in ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ and Co-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{1.94}{\mathrm{Co}}_{0.06}{\mathrm{As}}_2$ demonstrate electronlike behavior and an anomalous nonlinear magnetic field dependence of the Hall voltage with a sign reversal. By contrast, the Zn-doped ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{1.94}{\mathrm{Zn}}_{0.06}{\mathrm{As}}_2$ behaves like a hole type and the Hall coefficient is independent of the magnetic field. The field-induced sign reversal of the Hall coefficient of undoped and Co-doped samples depends on the field modification on the mobility and hole/electron concentration ratio. The $T^2$-dependent Hall angle of a nonmagnetic Zn-doped crystal is observed as a nearly parallel shift from that of the impurity-free crystal in the low-temperature region, indicating that the Zn induces a weak change of the spinons excitations, while increasing the number of scattering centers. The Co works as a nonmagnetic impurity as well, while it provides both spinons excitations and impurity scattering.

We describe a tunable-cavity quantum electrodynamics (QED) architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both microwave readout of tunneling and dispersive measurements of the qubit. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling, and detuning. A tunable-cavity frequency provides a way to strongly vary both the qubit-cavity detuning and coupling strength, which can reduce Purcell losses, cavity-induced dephasing of the qubit, and residual bus coupling for a system with multiple qubits. With our qubit-cavity system, we show that dynamic control over the cavity frequency enables one to avoid Purcell losses during coherent qubit evolutions and optimize state readout during qubit measurements. The maximum qubit decay time $T_1=1.5\mu$s is found to be limited by surface dielectric losses from a design geometry similar to planar transmon qubits.

The microscopic modeling of spin-orbit entangled $j=1/2$ Mott insulators such as the layered hexagonal iridates ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\mathrm{Li}}_2{\mathrm{IrO}}_3$ has spurred an interest in the physics of Heisenberg-Kitaev models. Here we explore the effect of lattice distortions on the formation of the collective spin-orbital states that include not only conventionally ordered phases but also gapped and gapless spin-orbital liquids. In particular, we demonstrate that in the presence of distortions, i.e., spatial anisotropies of the exchange couplings, conventionally ordered states are formed through an order-by-disorder selection, which is not only sensitive to the type of exchange anisotropy but also to the relative strength of the Heisenberg and Kitaev couplings. The spin-orbital liquid phases of the Kitaev limit—a gapless phase in the vicinity of spatially isotropic couplings and a gapped $Z_2$ phase for a dominant spatial anisotropy of the exchange couplings—show vastly different sensitivities to the inclusion of a Heisenberg exchange. While the gapless phase is remarkably stable, the gapped $Z_2$ phase quickly breaks down in what might be a rather unconventional phase transition driven by the simultaneous condensation of its elementary excitations.

We describe a simple quantum-mechanical approach to calculating equilibrium particle current along the edge of a system with nontrivial band spectrum topology. The approach does not require any a priori knowledge of the band topology and, as a matter of fact, treats topological and nontopological contributions to the edge currents on the same footing. We illustrate its usefulness by demonstrating the existence of “topologically nontrivial” particle currents along the edges of three different physical systems: two-dimensional electron gas with spin-orbit coupling and Zeeman magnetic field, surface state of a topological insulator, and kagomé antiferromagnet with Dzyaloshinskii-Moriya interaction. We describe the relationship of our results to the notion of orbital magnetization.

${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ is an interesting compound showing the coexistence of structural phase transition and superconductivity. The structural phase transition at 147 K leads to the formation of a superlattice. We perform optical spectroscopy measurements across the structural phase transition on single-crystal samples of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$. The optical spectroscopy study reveals an unusual temperature-induced spectral weight transfer over a broad energy scale, yielding evidence for the presence of electron correlation effects. Below the structural phase transition temperature an energy gap-like suppression in optical conductivity was observed, leading to the removal of partial itinerant carriers near the Fermi level. Unexpectedly, the suppression appears at a much higher energy scale than expected for the usual charge-density-wave phase transition.

We study the quantum phases and phase transitions of the Kane-Mele Hubbard (KMH) model on a zigzag ribbon of honeycomb lattice at a finite size via the weak-coupling renormalization group (RG) approach. In the noninteracting limit, the Kane-Mele (KM) model is known to support topological edge states where electrons show helical property with orientations of the spin and momentum being locked. The effective interedge hopping terms are generated due to finite-size effect. In the presence of an on-site Coulomb (Hubbard) interaction and the interedge hoppings, special focus is put on the stability of the topological edge states (TI phase) in the KMH model against (i) the charge and spin gaped (II) phase, (ii) the charge gaped but spin gapless (IC) phase, and (iii) the spin gaped but charge gapless (CI) phase depending on the number (even/odd) of the zigzag ribbons, doping level (electron filling factor) and the ratio of the Coulomb interaction to the interedge tunneling. We discuss different phase diagrams for even and odd numbers of zigzag ribbons. We find the TI-CI, II-IC, and II-CI quantum phase transitions are of the Kosterlitz-Thouless (KT) type. By computing various correlation functions, we further analyze the nature and leading instabilities of these phases. The relevance of our results for graphene is discussed.

Distinguishing different topologically ordered phases and characterizing phase transitions between them is a difficult task due to the absence of local order parameters. In this paper, we use a combination of analytical and numerical approaches to distinguish two such phases and characterize a phase transition between them. The “toric code” and “double semion” models are simple lattice models exhibiting ${\mathbb{Z}}_2$ topological order. Although both models express the same topological ground state degeneracies and entanglement entropies, they are distinct phases of matter because their emergent quasiparticles obey different statistics. For a 1D model, we tune a phase transition between these two phases and obtain an exact solution to the entire phase diagram, finding a second-order Ising$\times$Ising transition. We then use exact diagonalization to study the 2D case and find indications of a first-order transition. We show that the quasiparticle statistics provides a robust indicator of the distinct topological orders throughout the whole phase diagram.

We study electrons hopping on a kagome lattice at third filling described by an extended Hubbard Hamiltonian with on-site and nearest-neighbor repulsions in the strongly correlated limit. As a consequence of the commensurate filling and the large interactions, each triangle has precisely two electrons in the effective low-energy description, and these electrons form chains of different lengths. The effective Hamiltonian includes the ring exchange around the hexagons as well as the nearest-neighbor Heisenberg interaction. Using large-scale exact diagonalization, we find that the effective model exhibits two phases: If the charge fluctuations are small, the magnetic fluctuations confine the charges to short loops around hexagons, yielding a gapped charge-ordered phase. When the charge fluctuations dominate, the system undergoes a quantum phase transition to a resonating plaquette phase with ordered spins and gapless spin excitations. We find that a peculiar conservation law is fulfilled: the electron in the chains can be divided into two sublattices, and this division is conserved by the ring exchange term.

The Callaway model [J. Callaway, Phys. Rev. 113, 1046 (1959)], regarded as an improvement over the relaxation time approximation (RTA) for the phonon Boltzmann transport equation (BTE), is widely used in studying lattice thermal conductivity ($\kappa$). However, its accuracy needs to be systematically examined. By solving BTE accurately using an iterative method along with the first principles calculation of phonon scatterings, we conduct such an examination of the Callaway model as well as a modified version proposed by Allen [Phys. Rev. B 88, 144302 (2013)] for Si, diamond, and wurtzite AlN. At room temperature, the RTA underestimates $\kappa$ by 5%, 32%, 11%, and 12% for Si, diamond, and in-plane and cross-plane AlN, respectively. The deviation of the original Callaway model from the accurate $\kappa$ is $-1\%$, 25%, 1%, and $-12\%$, respectively, while the deviation of Allen's modified model is 7%, 44%, 13%, and $-8\%$, respectively. The room temperature anisotropy of AlN is 5%, and the anisotropy predicted by RTA, the Callaway model, and Allen's modified version is 7%, 19%, and 29%, respectively. We conclude that neither the original Callaway model nor Allen's modified version can generally guarantee an improvement over RTA. In these three systems, we also find that the relaxation times for umklapp processes scale as $1/{\omega }^3$ at low frequencies for both transverse acoustic (TA) and longitudinal acoustic (LA) modes, and those for normal processes scale as $1/\omega$ and $1/{\omega }^2$ for TA and LA modes, respectively.

The tunneling rates for spin-up and -down electrons are investigated for a GaAs quantum dot in an in-plane magnetic field by using a real-time single-electron counting scheme with a nearby charge detector. An extremely small spin-polarized current on the order of attoamperes is analyzed with the spin and energy dependences of the tunneling rates. Fully spin-polarized current is obtained when only a spin-up Zeeman sublevel is located in the transport window. When both Zeeman sublevels are allowed to contribute to the transport, we find that the tunneling rate for spin-up electrons is considerably higher than that for spin-down electrons. This partially spin-polarized current can be explained by the exchange-enhanced spin splitting in low-density regions near the tunneling barriers.

Majorana fermions (MFs) are appearing in pairs and nonlocally distributed at the two ends of the wire. The two MFs can combine together to form a nonlocal fermion state. Due to the nonlocal nature of Majorana fermions in the topological superconductor, there are two types of nonlocal transport processes supported by MFs: crossed Andreev reflection and electron transmission. In this paper, we study electron transport through a normal lead-quantum dot-topological superconductor-quantum dot-normal lead (N-QD-TS-QD-N) junction. We show that these two nonlocal processes can be directly controlled by gating the energy level of QDs. For instance, when both energies of QDs (labeled as ${\epsilon }_1$ and ${\epsilon }_2)$ are equal to the coupling energy of MFs (denoted as $E_M)$, the electron in the left lead can transport to the right lead via MFs while when ${\epsilon }_1=-{\epsilon }_2=E_M$ is satisfied, the electron in the left lead can combine one electron in the right lead to form a Cooper pair and tunnel into the topological superconductor via MFs.

Temperature-dependent interband transitions and exciton excitations of sol-gel derived ${\mathrm{CuCr}}_{1-x}$${\mathrm{Mg}}_x$${\mathrm{O}}_2$ $(2\%\le x\le 8\%)$ films have been investigated by transmittance spectra (8–300 K) and photoluminescence (PL) spectra (77–300 K). An abnormal dependence of the optical band gap with the temperature has been found for the films with $x=0.02$ and 0.04. At the low-temperature region, the gap energy shows a redshift trend with decreasing temperature. It is due to the strong Cr $3d\text{−}\text{O}$ $2p\text{-Cu}$ $3d$ interaction in the upper part of the valence band, which can be weaken by heavily Mg doping. The spin-orbit interactions of ${\mathrm{Cr}}^{3+}$ ions in an octahedral environment make the $3d$ states more disperse, which can contribute to the relatively high conductivity. A well-defined low-energy absorption has been assigned to the spin-allowed $3d$ $\to$ $3d$ transition. Moreover, a strong exciton excitation around 1.8 eV has been observed due to the naturally low-dimensional structure of the delafossite, which can be modulated by temperature and hole concentration.

The doping and strain effects on the electron transport of monolayer ${\mathrm{MoS}}_2$ are systematically investigated using the first-principles calculations with Boltzmann transport theory. We estimate the mobility has a maximum 275 ${\mathrm{cm}}^2$/(V$$s) in the low doping level under the strain-free condition. Applying a small strain ($\sim 3\%$) can improve the maximum mobility to 1150 ${\mathrm{cm}}^2$/(V$$s) and the strain effect is more significant in the high doping level. We demonstrate that the electric resistance mainly due to the electron transitions between $K$ and $Q$ valleys scattered by the $M$ momentum phonons. However, the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the $K$ and $Q$ valleys. This sensitivity of mobility to the external strain may direct the improving electron transport of ${\mathrm{MoS}}_2$.