We propose a method to separate the inverse spin Hall effect (ISHE) from galvanomagnetic effects in spin pumping experiments on metallic bilayer systems by measuring the dc electromotive force in two orthogonal directions. Calculations of dc voltages in longitudinal and Hall directions induced in ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}\text{/}\mathrm{Pt}$ films at ferromagnetic resonance in a microwave cavity predict that contributions from ISHE and from the galvanomagnetic effects, i.e., the anisotropic magnetoresistance and the anomalous Hall effect, exhibit distinct signal symmetry as well as angular dependence when changing the direction of the external field with respect to the film plane. According to measurements on ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}\text{/}\mathrm{Pt}$, only that dc voltage component which includes ISHE is more than five times larger than purely galvanomagnetic components. This is corroborated by results on ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3\text{/}\mathrm{Pt}$ samples, demonstrating universality of this method.

We study the spin transport in the low-temperature regime (often referred to as the precession-dominated regime) between a ferromagnetic Fermi liquid (FFL) and a normal metal metallic Fermi liquid (NFL), also known as the F/N junction, which is considered as one of the most basic spintronic devices. In particular, we explore the propagation of spin waves and transport of magnetization through the interface of the F/N junction where nonequilibrium spin polarization is created on the normal metal side of the junction by electrical spin injection. We calculate the probable spin wave modes in the precession-dominated regime on both sides of the junction especially on the NFL side where the system is out of equilibrium. Proper boundary conditions at the interface are introduced to establish the transport of the spin properties through the F/N junction. A possible transmission conduction electron spin resonance (CESR) experiment is suggested on the F/N junction to see if the predicted spin wave modes could indeed propagate through the junction. Potential applications based on this novel spin transport feature of the F/N junction are proposed in the end.

We study the binding of a holon to a doublon in a half-filled Hubbard model as the mechanism of the zero-temperature metal-insulator transition. In a spin polarized system a single holon-doublon (HD) pair exhibits a binding transition on a 3D lattice, or a sharp crossover on a 2D lattice, corresponding well to the standard Mott transition in unpolarized systems. We extend the HD-pair study towards nonpolarized systems by considering more general spin background and by treating the finite HD density within a BCS-type approximation. Both approaches lead to a discontinuous transition away from the fully polarized system and give density correlations consistent with numerical results on a triangular lattice.

Quasiparticle collapsing is a central issue in the study of strongly correlated electron systems. In the one-dimensional case, the quasiparticle collapsing in a form of spin-charge separation has been well established, but the problem remains elusive in dimensions higher than one. By using the density matrix renormalization group (DMRG) algorithm, we show that in an anisotropic two-leg $t-J$ ladder, an injected single hole behaves like a well-defined quasiparticle in the strong rung limit but undergoes a “phase transition” with the effective mass diverging at a quantum critical point (QCP) towards the isotropic limit. After the transition, the quasiparticle collapses into a loosely bound object of a charge (holon) and a spin-1/2 (spinon) accompanied by an unscreened phase string as well as a substantially enhanced binding energy between two doped holes. A phase diagram of multileg ladders is further obtained, which extrapolates the QCP towards the two-dimensional limit. The underlying mechanism generic for any dimensions is also discussed.

Non-Hermitian systems with parity-time- ($\mathcal{P}\mathcal{T}$)- symmetric complex potentials can exhibit a phase transition when the degree of non-Hermiticity is increased. Two eigenstates coalesce at a transition point, which is known as the exceptional point (EP) for a discrete spectrum and spectral singularity for a continuous spectrum. The existence of an EP is known to give rise to a great variety of novel behaviors in various fields of physics. In this work, we study the complex band structures of one-dimensional photonic crystals with $\mathcal{P}\mathcal{T}$-symmetric complex potentials by setting up a Hamiltonian using the Bloch states of the photonic crystal without loss or gain as a basis. As a function of the degree of non-Hermiticity, two types of $\mathcal{P}\mathcal{T}$ symmetry transitions are found. One is that a $\mathcal{P}\mathcal{T}$-broken phase can reenter into a $\mathcal{P}\mathcal{T}$-exact phase at a higher degree of non-Hermiticity. The other is that two EPs, one originating from the Brillouin zone center and the other from the Brillouin zone boundary, can coalesce at some $k$ point in the interior of the Brillouin zone and create a singularity of higher order. Furthermore, we can induce a band inversion by tuning the filling ratio of the photonic crystal, and we find that the geometric phases of the bands before and after the inversion are independent of the amount of non-Hermiticity as long as the $\mathcal{P}\mathcal{T}$-exact phase is not broken. The standard concept of topological transition can hence be extended to non-Hermitian systems.

We study a one-dimensional anisotropic XXZ Heisenberg spin-$\frac{1}{2}$ chain with weak random fields $h_i^z{S}_i^z$ by means of Jordan-Wigner transformation to spinless Luttinger liquid with disorder and bosonization technique. First, we reinvestigate the phase diagram of the system in terms of dimensionless disorder $\gamma =〈h^2〉/J^2\ll 1$ and anisotropy parameter $\mathrm{\Delta }=J_z/J_{xy}$, we find the range of these parameters where disorder is irrelevant in the infrared limit and spin-spin correlations are described by power laws, and compare it with previously obtained numerical and analytical results. Then we use the diagram technique in terms of plasmon excitations to study the low-temperature $\left(T\ll J\right)$ behavior of heat conductivity $\kappa$ and spin conductivity $\sigma$ in this power-law phase. The obtained Lorentz number $L\equiv \kappa /\sigma T$ differs from the value derived earlier by means of the memory function method. We argue also that in the studied region inelastic scattering is strong enough to suppress quantum interference in the low-temperature limit.

We consider a single-level quantum dot coupled to magnetic insulators (magnonic reservoirs) and magnetic metals (electronic reservoirs). The whole system is in an external magnetic field. In a general case, the system includes two magnonic and two electronic reservoirs, but we also present results for some specific situations, where only two or three reservoirs are effectively connected to the dot. The main objective is the analysis of the conversion of magnon current to electronic spin and charge currents, and vice versa. We consider the limiting case of large Coulomb energy in the dot (Coulomb blockade), as well as the case when the Coulomb energy is finite and double occupancy is allowed.

We study the twist defects in the toric code model introduced by Bombin [Phys. Rev. Lett. 105, 030403 (2010)]. Using a generalized two-dimensional Jordan-Wigner transformation and a projective construction, we show explicitly that twist defects carry unpaired Majorana zero modes. In addition, we propose a quantum nondemolition measurement scheme of the parity of Majorana modes. Such a scheme provides an alternative avenue to demonstrate the non-Abelian statistics of Majorana fermions. The braiding operation is simulated by a nonrepetitive measurement-based approach which removes the uncertainty associated with forced measurements.

We analyze the interplay between charge-density-wave (CDW) orders with axial momenta $(Q,0)$ and $(0,Q)$ (${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$, respectively), detected in the underdoped cuprates. The CDW order in real space can be unidirectional (either ${\mathrm{\Delta }}_x$ or ${\mathrm{\Delta }}_y$ is nonzero) or bidirectional (both ${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$ are nonzero). To understand which of the two orders develop, we adopt the magnetic scenario, in which the CDW order appears due to spin-fluctuation exchange, and derive the Ginzburg-Landau action to the sixth order in ${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$. We argue that at the mean-field level, the CDW order is bidirectional at the onset, with equal amplitudes of ${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$, but changes to unidirectional inside the CDW phase. This implies that at a given temperature, CDW order is unidirectional at smaller dopings but becomes bidirectional at larger dopings. This is consistent with recent x-ray data on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$, which detected tendency towards bidirectional order at larger dopings. We discuss the role of discrete symmetry breaking at a higher temperature for the interplay between bidirectional and unidirectional CDW orders and also discuss the role of pair-density-wave (PDW) order, which may appear along with CDW. We argue that PDW with the same momentum as CDW changes the structure of the bidirectional charge order by completely replacing either ${\mathrm{\Delta }}_x$ or ${\mathrm{\Delta }}_y$ CDW components by PDW. However, if a so-called Amperean PDW order, which pairs fermions with approximately the same momenta, is also present, both ${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$ remain nonzero in the bidirectional phase, albeit with nonequal amplitudes. This is again consistent with x-ray experiments, which at larger doping found nonequal ${\mathrm{\Delta }}_x$ and ${\mathrm{\Delta }}_y$ in every domain.

We examine what happens when a strongly many-body localized system is coupled to a weak heat bath, with both system and bath containing similar numbers of degrees of freedom. Previous investigations of localized systems coupled to baths operated in regimes where the back action of the system on the bath is negligible and concluded that the bath generically thermalizes the system. In this paper we show that when the system is strongly localized and the bath is only weakly ergodic, the system can instead localize the bath. We demonstrate this both in the limit of weak coupling between system and bath and in the limit of strong coupling and also for two different types of “weak” bath—baths which are close to an atomic limit and baths which are close to a noninteracting limit. The existence of this “many-body localization proximity effect” indicates that many-body localization is more robust than previously appreciated, and it can not only survive coupling to a (weak) heat bath but can even destroy the bath.

Supersymmetries in quantum mechanics offer a way to obtain degeneracies in the excitation spectrum which do not originate from selection rules. The mechanism behind the degeneracies is the same as the one that leads to the miraculous cancellations of divergences in supersymmetric field theories found in the high energy physics context. There is up to now no realistic proposal of nonintegrable systems that show level degeneracies due to a supersymmetric structure. Here, we propose an implementation of a quantum-mechanical supersymmetry in a Cooper-pair box shunted by a Josephson junction rhombus which is effectively $\pi$ periodic in the superconducting phase difference. For a characteristic ratio between the strength of the $2\pi$- and the $\pi$-periodic junction, we find a twofold degeneracy of all the energy levels all the way from the weak junction/charge qubit limit to the strong junction/transmon regime. We provide explicit experimental values for the parameters of the system and show that tuning in and out of the supersymmetric point is easily achieved by varying an external gate voltage. We furthermore discuss a microwave experiment to detect the supersymmetry and conclude that it can indeed be implemented with currently existing Josephson junction technology.

${\mathrm{KH}}_2{\mathrm{PO}}_4$ (KDP) belongs to the class of hydrogen-bonded ferroelectrics, whose paraelectric to ferroelectric phase transition is driven by the ordering of the protons in the hydrogen bonds. We demonstrate that forbidden reflections of KDP, when measured at an x-ray absorption edge, are highly sensitive to the asymmetry of proton configurations. The change of average symmetry caused by the “freezing” of the protons during the phase transition is clearly evidenced. In the paraelectric phase, we identify in the resonant spectra of the forbidden reflections a contribution related to the transient proton configurations in the hydrogen bonds, which violates the high average symmetry of the sites of the resonant atoms. The analysis of the temperature dependence reveals a change of relative probabilities of the different proton configurations. They follow the Arrhenius law, and the activation energies of polar and Slater configurations are 18.6 and 7.3 meV, respectively.

The frustrated classical antiferromagnetic Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of an antiferromagnetic skyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating skyrmion crystals (one by sublattice), and most importantly, the AF-SkX state seems to survive in the limit of zero temperature. To characterize the phase diagram we compute the average of the topological order parameter which can be associated with the number of topological charges or skyrmions. As the magnetic field increases this parameter presents a clear jump, indicating a discontinuous transition from a spiral phase into the AF-SkX phase, where multiple Bragg peaks coexist in the spin structure factor. For higher fields, a second (probably continuous) transition occurs into a featureless paramagnetic phase.

We present results of precise measurements of the thermal and electrical transport in the optimally and overdoped $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ single crystals $\left(x=0.35,0.55,0.88\right)$ and compare them to the previously reported data on $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{C}{\mathrm{o}}_y\right)}_2\mathrm{A}{\mathrm{s}}_2$. A contraction of the electron pocket is observed upon substitution potassium for barium, but even at the extreme doping $\left(x=0.88\right)$ there is still a noticeable contribution from negative charge carriers to the electronic transport. The size of the electron pocket in all K-doped samples is small enough to cause a significant enhancement of the respective Hall-Lorenz number. Another observed characteristic is the emergence of a maximum in the transverse thermal conductivity below the superconducting critical temperature of the optimally $\left(x=0.35\right)$ and slightly overdoped $\left(x=0.55\right)$ samples. The evolution of this anomaly from the optimally electron-doped $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{0.94}\mathrm{C}{\mathrm{o}}_{0.06}\right)}_2\mathrm{A}{\mathrm{s}}_2$ to hole-overdoped $\mathrm{B}{\mathrm{a}}_{0.45}{\mathrm{K}}_{0.55}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ suggests formation of a uniform superconducting gap on the electron pocket in the former and regions of a depressed gap on the hole pocket in the latter.

We consider a nonstationary array of conductors, connected by resistances that fluctuate with time. The charge transfer between a particular pair of conductors is supposed to be dominated by electrical breakdowns—the moments when the corresponding resistance is close to zero. An amount of charge, transferred during a particular breakdown, is controlled by the condition of minimum for the electrostatic energy of the system. We find the conductivity, relaxation rate, and fluctuations for such a system within the classical approximation, valid, if the typical transferred charge is large compared to $e$. We discuss possible realizations of the model for colloidal systems and arrays of polymer-linked grains.

We performed angle-resolved photoemission experiments on ${\mathrm{CaC}}_6$ and measured $k_z$-dependent electronic structures to investigate the interlayer states. The results reveal a spherical interlayer Fermi surface centered at the $\mathrm{\Gamma }$ point. We also find that the graphene-driven band possesses a weak $k_z$ dispersion. The overall electronic structure shows a peculiar single-graphene-layer periodicity in the $k_z$ direction although the ${\mathrm{CaC}}_6$ unit cell is supposed to contain three graphene layers. This suggests that the $c$-axis ordering of Ca has little effect on the electronic structure of ${\mathrm{CaC}}_6$. In addition to ${\mathrm{CaC}}_6$, we also studied the a low-temperature superconductor ${\mathrm{BaC}}_6$. For ${\mathrm{BaC}}_6$, the graphene-band Dirac-point energy is smaller than that of ${\mathrm{CaC}}_6$. Based on data from ${\mathrm{CaC}}_6$ and ${\mathrm{BaC}}_6$, we rule out the ${\mathrm{C}}_{xy}$ phonon mode as the origin of the superconductivity in ${\mathrm{CaC}}_6$, which strongly suggests interlayer-state-driven superconductivity.

Using the nonequilibrium Keldysh formalism, we solve the equations of motion for electron-phonon superconductivity, including an ultrafast pump field. We present results for time-dependent photoemission spectra out of equilibrium which probe the dynamics of the superconducting gap edge. The partial melting of the order by the pump field leads to oscillations at twice the melted gap frequency, a hallmark of the Higgs or amplitude mode. Thus the Higgs mode can be directly excited through the nonlinear effects of an electromagnetic field and detected without requiring any additional symmetry breaking.

We examined the chemical doping effect on the Kondo semiconductor ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$ with a nonmagnetic ground state by means of the magnetic susceptibility, specific heat, electrical resistivity, and thermopower. The effect of Ru doping on the ground state is small. On the other hand, by a small amount of Rh doping, the magnetic susceptibility is strongly enhanced along the orthorhombic $a$ axis, and a Curie-Weiss behavior is observed in a wide temperature range. The low-temperature specific heat is also strongly enhanced by the doping, and a metallic ground state is realized at low temperatures. These results suggest the collapse of the spin and charge gap due to the suppression of the $c\text{−}f$ hybridization effect. From the results of a crystalline electric field analysis on the magnetic susceptibility of $\mathrm{Ce}({\mathrm{Fe}}_{1-x}{\mathrm{Rh}}_x){}_2{\mathrm{Al}}_{10}$, it was revealed that the Rh-doping effect on the $c\text{−}f$ hybridization effect is anisotropic, especially for the $a$ axis. Similar doping effects are seen in the Rh-doped ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$, Ir-doped ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$, and Si-doped ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$. From these results, we conclude that the collapse of the spin and charge gap by such an excess electron doping is one of the universal features of the Kondo semiconductor $\mathrm{Ce}{T}_2{\mathrm{Al}}_{10}$ ($T$ = Fe, Ru, and Os).