We investigate the low-energy continuum limit theory for electrons in a graphene sheet under strain. We use the quantum field theory in curved spaces to analyze the effect of the system deformations into an effective gauge field. We study both in-plane and out-of-plane deformations and obtain a closed expression for the effective gauge field due to arbitrary nonuniform sheet deformations. The obtained results reveal a remarkable relation between the local-pseudomagnetic field and the Riemann curvature, so far overlooked.

We study lattice wave functions obtained from the SU(2)${}_1$ Wess-Zumino-Witten conformal field theory. Following Moore and Read's construction, the Kalmeyer-Laughlin fractional quantum Hall state is defined as a correlation function of primary fields. By an additional insertion of Kac-Moody currents, we associate a wave function with each state of the conformal field theory. These wave functions span the complete Hilbert space of the lattice system. On the cylinder, we study global properties of the lattice states analytically and correlation functions numerically using a Metropolis Monte Carlo method. By comparing short-range bulk correlations, numerical evidence is provided that the states with one current operator represent edge states in the thermodynamic limit. We show that the edge states with one Kac-Moody current of lowest order have a good overlap with low-energy excited states of a local Hamiltonian, for which the Kalmeyer-Laughlin state approximates the ground state. For some states, exact parent Hamiltonians are derived on the cylinder. These Hamiltonians are SU(2) invariant and nonlocal with up to four-body interactions.

We study current correlations in a $T$ junction composed of a grounded topological superconductor and of two normal-metal leads which are biased at a voltage $V$. We show that the existence of an isolated Majorana zero mode in the junction dictates a universal behavior for the cross correlation of the currents through the two normal-metal leads of the junction. The cross correlation is negative and approaches zero at high bias voltages as $-1/V$. This behavior is robust in the presence of disorder and multiple transverse channels, and persists at finite temperatures. In contrast, an accidental low-energy Andreev bound state gives rise to nonuniversal behavior of the cross correlation. We employ numerical transport simulations to corroborate our conclusions.

We present a theory of the electronic and optical properties of a charged artificial benzene ring (ABR). The ABR is described by the extended Hubbard model solved using exact diagonalization methods in both real and Fourier space as a function of the tunneling matrix element $t$, Hubbard on-site repulsion $U$, and interdot interaction $V$. In the strongly interacting case, we discuss exact analytical results for the spectrum of the hole in a half-filled ABR dressed by the spin excitations of the remaining electrons. The spectrum is interpreted in terms of the appearance of a topological phase associated with an effective gauge field piercing through the ring. We show that the maximally spin-polarized $\left(S=5/2\right)$ and maximally spin-depolarized $\left(S=1/2\right)$ states are the lowest energy, orbitally nondegenerate, states. We discuss the evolution of the phase diagram and level crossings as interactions are switched off and the ground state becomes spin nondegenerate but orbitally degenerate $S=1/2$. We present a theory of optical absorption spectra and show that the evolution of the ground and excited states, level crossings, and presence of artificial gauge can be detected optically.

Phonon properties of ${\mathrm{Mo}}_3{\mathrm{Sb}}_{7-x}{\mathrm{Te}}_x$ $\left(x=0,1.5,1.7\right)$, a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states, dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening and a large overall stiffening of interatomic force constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes nonuniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. Changes in phonon group velocities and phonon scattering rates are quantified, highlighting the large effect of electron-phonon coupling in this compound.

Domain wall resonance spectra in the weakly nonlinear regime for garnet films with a perpendicular anisotropy supporting parallel stripe domains have been investigated using micromagnetic simulations and zero-field broadband ferromagnetic resonance experiments. The main characteristics of the 2D numerical micromagnetic approach we developed is to solve the Landau-Lifshitz equation by an iterative method in the frequency domain and to incorporate a nonlinear phenomenological damping term. It is shown that the nonlinear damping affects simultaneously the driving field dependencies of the resonance frequency and the resonance linewidth for the fundamental domain wall resonance of parallel stripe domains, and the critical field for the domain wall resonance foldover. The micromagnetic simulations allow us to reproduce quantitatively both the nonlinear redshift of the domain wall resonance frequency and the nonlinear line broadening experimentally observed for increasing values of the input microwave power.

The Casimir effect is a general phenomenon in physics, which arises when the vacuum fluctuation of an arbitrary field is modified by static or a slowly varying boundary. However, its spin version is rarely addressed, mainly due to the fact that a macroscopic boundary in quantum spin systems is hard to define. In this paper, we explore the spin Casimir effect induced by the zero-point fluctuation of spin waves in a general noncollinear ordered quantum antiferromagnet. This spin Casimir effect results in a spin torque between local spins and further causes various singular and divergent results in the framework of spin-wave theory, which invalidate the standard $1/S$ expansion procedure. Based on the spin Casimir torque interpretation, we develop a spin-wave expansion approach named as torque equilibrium spin wave theory. In this approach, the spin Casimir effect is treated in a self-consistent way, and the spin-wave expansion results are free from singularities and divergences. A detailed spin-wave analysis of the antiferromagnetic spin-1/2 Heisenberg model on a spatially anisotropic triangular lattice is undertaken within our approach. Our results indicate that the spiral order is only stable in the region $0.5<\alpha <1.2$, where $\alpha$ is the ratio of the coupling constants. In addition, the instability in the region $1.2<\alpha <2$ is owing to the spin Casimir effect instead of the vanishing sublattice magnetization. Furthermore, our method provides an efficient and convenient tool that can estimate the correct exchange parameters and outline the quantum phase diagrams, which can be useful for experimental fitting processes in frustrated quantum magnets.

We theoretically analyzed the effects of damping on spin torque induced oscillation in a spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized reference layer. It was found that there exists an unexpectedly small critical damping constant, above which no steady-state oscillation can be excited by spin torque. It was also found that the critical damping constant is almost independent of the anisotropy constants but increases with the spin polarization of the injected current.

Frustrated magnetism on the kagome lattice has been a fertile ground for rich and fascinating physics, ranging from experimental evidence of a spin liquid to theoretical predictions of exotic superconductivity. Among experimentally realized spin-$\frac{1}{2}$ kagome magnets, herbertsmithite, kapellasite, and haydeeite $\left[(\mathrm{Zn},\mathrm{Mg}){\mathrm{Cu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2\right]$ are all well described by a three-parameter Heisenberg model, but they exhibit distinctly different physics. We address the problem using a pseudofermion functional renormalization-group approach and analyze the low-energy physics in the experimentally accessible parameter range. Our analysis places kapellasite and haydeeite near the boundaries between magnetically ordered and disordered phases, implying that slight modifications could dramatically affect their magnetic properties. Inspired by this, we perform ab initio density functional theory calculations of (Zn,Mg,$\mathrm{Cd}){\mathrm{Cu}}_3$ $({\mathrm{OH})}_6{\mathrm{Cl}}_2$ at various pressures. Our results suggest that by varying pressure and composition one can traverse a paramagnetic regime between different magnetically ordered phases.

Magnetic circular dichroism in the extreme ultraviolet (XUV) spectral range is a powerful technique for element-specific probing of magnetization in multicomponent magnetic alloys and multilayers. We combine a high-harmonic generation source with a $\lambda /4$ phase shifter to obtain circularly polarized XUV femtosecond pulses for ultrafast magnetization studies. We report on simultaneously measured resonant magnetic circular dichroism (MCD) of Co and Ni at their respective $M_{2,3}$ edges and of Pt at its O edge, originating from interface magnetism. We present a time-resolved MCD absorption measurement of a thin magnetic Pt/Co/Pt film, showing simultaneous demagnetization of Co and Pt on a femtosecond time scale.

We present melting curves of aluminum, copper, and nickel calculated on the basis of a quasiharmonic approximation. The dependence of a phonon density of states on electron temperature is taken into account for both thermodynamic properties and a mean square displacement of atoms. Linear expansion coefficients are strongly dependent on an approximation of the exchange-correlation functional; the generalized gradient approximation gives better results at normal conditions. Using the Lindemann criterion we obtain good agreement with experimental pressure dependences of the melting temperature for Al and Cu. In the case of Ni we consider a spin polarization effect to reproduce a recent first-principle simulation and shock-wave data. However, our melting curve is located significantly higher than static experimental points. We also consider a thermal excitation of electrons in a crystal and investigate the dependence of the melting temperature on the electronic one at normal and elevated densities. Hardening of the crystal structure for all the metals is obtained in our simulation; this effect might be confirmed experimentally owing to a relatively long lifetime of the two-temperature state.

Rare earth based equimolar Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy (HEA) is a prototype of an ideal HEA, stabilized by the entropy of mixing at any temperature with random mixing of elements on the hexagonal close-packed lattice. In order to determine intrinsic properties of an ideal HEA characterized by the enormous chemical (substitutional) disorder on a weakly distorted simple lattice, we have performed measurements of its magnetic and electrical response and the specific heat. The results show that the Ho-Dy-Y-Gd-Tb hexagonal HEA exhibits a rich and complex magnetic field-temperature $\left(H,T\right)$ phase diagram, as a result of competition among the periodic potential arising from the electronic band structure that favors periodic magnetic ordering, the disorder-induced local random potential that favors spin glass-type spin freezing in random directions, the Zeeman interaction with the external field that favors spin alignment along the field direction, and the thermal agitation that opposes any spin ordering. Three characteristic temperature regions were identified in the $\left(H,T\right)$ phase diagram between room temperature and 2 K. Within the upper temperature region I (roughly between 300 and 75 K), thermal fluctuations average out the effect of local random pinning potential and the spin system behaves as a pure system of compositionally averaged spins, undergoing a thermodynamic phase transition to a long-range ordered helical antiferromagnetic state at the Néel temperature $T_N=180\mathrm{K}$ that is a compositional average of the Néel temperatures of pure Tb, Dy, and Ho metals. Region II (between 75 and 20 K) is an intermediate region where the long-range periodic spin order “melts” and the random ordering of spins in the local random potential starts to prevail. Within the low-temperature region III (below 20 K), the spins gradually freeze in a spin glass configuration. The spin glass phase appears to be specific to the rare earths containing hexagonal HEAs, sharing properties of site-disordered spin glasses and geometrically frustrated (site-ordered) spin systems, as a consequence of strongly interacting large abundant spins of four magnitudes (those of Gd, Tb, Dy, and Ho) on the hexagonal lattice, being weakly diluted by nonmagnetic yttrium atoms.

Reconstruction of the Fermi surface into small electron pockets in the normal ground state of the underdoped high-temperature superconducting cuprates has been related to charge ordering. It remains an open question, however, as to whether Fermi-surface reconstruction occurs from a more conventional starting large holelike Fermi surface yielding multiple pockets, or whether it occurs by connecting truncated “Fermi arcs” in an unconventional pseudogap state to yield a single pocket per ${\mathrm{CuO}}_2$ plane. Thus far, the observation of an in-plane magnetoresistance, and sign reversals and quantum oscillations in the Hall coefficient have been considered to provide support for the former conventional scenario. Here we show that in fact a single nodal diamond-shaped electron pocket with concave sides produced by the latter unconventional scenario yields the observed experimental signatures in magnetotransport, negative Hall coefficient, and quantum oscillations in the negative Hall effect. Taken in conjunction with complementary signatures such as the experimentally observed low value of heat capacity at high magnetic fields and the experimental observation of a charge ordering wave vector that connects the tips of the truncated Fermi arcs, an origin of Fermi-surface reconstruction from unconventional Fermi arcs is suggested.

We study transport properties of the helical edge channels of a quantum spin Hall insulator, in the presence of electron-electron interactions and weak, local Rashba spin-orbit coupling. The combination of the two allows for inelastic backscattering that does not break time-reversal symmetry, resulting in interaction-dependent power-law corrections to the conductance. Here, we use a nonequilibrium Keldysh formalism to describe the situation of a long, one-dimensional edge channel coupled to external reservoirs, where the applied bias is the leading energy scale. By calculating explicitly the corrections to the conductance up to fourth order of the impurity strength, we analyze correlated single- and two-particle backscattering processes on a microscopic level. Interestingly, we show that the modeling of the leads together with the breaking of Galilean invariance has important effects on the transport properties. Such breaking occurs because the Galilean invariance of the bulk spectrum transforms into an emergent Lorentz invariance of the edge spectrum. With this broken Galilean invariance at the quantum spin Hall edge, we find a contribution to single-particle backscattering with a very low power scaling, while in the presence of Galilean invariance the leading contribution will be due to correlated two-particle backscattering only. This difference is further reflected in the different values of the Fano factor of the shot noise, an experimentally observable quantity. The described behavior is specific to the Rashba scatterer and does not occur in the case of backscattering off a time-reversal-breaking, magnetic impurity.

Layered perovskites ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ are regarded as the key materials for understanding the properties of magnetic relativistic insulators, mediated by the strong spin-orbit (SO) coupling. One of the most fundamental issues is to which extent these properties can be described by the superexchange (SE) model, formulated in the limit of the large Coulomb repulsion for some appropriately selected pseudospin states, and whether these materials themselves can be classified as Mott insulators. In this work, we address these issues by deriving the relevant models and extracting parameters of these models from the electronic-structure calculations with the SO coupling, based on the density functional theory. First, we construct the effective Hubbard-type model for the magnetically active $t_{2g}$ bands, by recasting the problem in the language of localized Wannier orbitals. Then, we map the obtained electron model onto the pseudospin model by applying the theory of SE interactions, which is based on the second-order perturbation theory with respect to the transfer integrals. We discuss the microscopic origin of anisotropic SE interactions, inherent to the compass Heisenberg model, and the appearance of the antisymmetric Dzyaloshinskii-Moriya term, associated with the additional rotation of the ${\mathrm{IrO}}_6$ octahedra in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. In order to solve the pseudospin Hamiltonian problem and evaluate the Néel temperature $\left(T_N\right)$, we employ the nonlinear sigma model. We have found that, while for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ our value of $T_N$ agrees with the experimental data, for ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ it is overestimated by a factor of 2. We argue that this discrepancy is related to limitations of the SE model: while for more localized $t_{2g}$ states in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ it works reasonably well, the higher-order terms in the perturbation theory expansion play a more important role in the more “itinerant” ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, giving rise to the new type of isotropic and anisotropic exchange interactions, which are not captured by the SE model. This conclusion is supported by unrestricted Hartree-Fock calculations for the same electron model, where in the case of ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, already on the mean-field level, we were able to reproduce the experimentally observed magnetic ground state, while for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ the main results are essentially the same as in the SE model.

Pressure- and temperature-induced changes in the Ce valence and $c-f$ hybridization of the Ce115 superconductors have been studied systematically. Resonant x-ray-emission spectroscopy indicated that the increase of the Ce valence with pressure was significant for ${\mathrm{CeCoIn}}_5$, and moderate for $\mathrm{CeIr}{\left({\mathrm{In}}_{0.925}{\mathrm{Cd}}_{0.075}\right)}_5$. We found no abrupt change of the Ce valence in the Kondo regime for $\mathrm{CeIr}{\left({\mathrm{In}}_{0.925}{\mathrm{Cd}}_{0.075}\right)}_5$, which suggests that valence fluctuations are unlikely to mediate the superconductivity in this material. X-ray-diffraction results were consistent with the pressure-induced change in the Ce valence. High-resolution photoelectron spectroscopy revealed a temperature-dependent reduction of the spectral intensity at the Fermi level, indicating enhanced $c-f$ hybridization on cooling.