We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ (PCMO) with the help of x-ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the “positive up negative down” method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron (ZP) ordered $P{2}_{1}nm$ and disordered $Pbnm$ structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroyed while the long-range antiferromagnetic ordering at low temperature in bulk system is replaced by ferromagnetic correlations in nanoparticles. The conclusion—that by reducing the crystallite size it might be possible to make ferromagnetism and ferroelectricity coexist near room temperature—should be generally applicable to all ZP ordered manganites.

We consider single-particle properties in the one-dimensional repulsive Hubbard model at commensurate fillings in the metallic phase. We determine the real-time evolution of the retarded Green's function by matrix-product state methods. We find that at sufficiently late times the numerical results are in good agreement with predictions of nonlinear Luttinger liquid theory. We argue that combining the two methods provides a way of determining the single-particle spectral function with very high frequency resolution.

In this work, the Einstein relation between the diffusivity and mobility of charge carriers for disordered organic semiconductors is analyzed. We formulate an analytic theory that allows predicting the Einstein relation for charge carrier hopping in disordered organic semiconductors with Gaussian density of states distribution as a function of disorder, temperature, bias field, and Fermi level, i.e., concentration of occupied states of the DOS under the condition of quasiequilibrium. By scanning the Fermi across the DOS, we calculate the charge carrier mobility and diffusivity as well the $qD/\mu k_{B}T$ ratio. We are thus able to identify the role of mobile and localized states on the interplay of diffusion and drift and can determine under which condition Einstein relation is valid or not.

The magnetism of magnetoelectric $S=\frac{3}{2}$ pyroxenes ${\mathrm{LiCrSi}}_2{\mathrm{O}}_6$ and ${\mathrm{LiCrGe}}_2{\mathrm{O}}_6$ is studied by density functional theory calculations, quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as low- and high-field magnetization measurements. In contrast with earlier papers, we find that the two compounds feature remarkably different, albeit nonfrustrated magnetic models. In ${\mathrm{LiCrSi}}_2{\mathrm{O}}_6$, two relevant exchange integrals, $J_1$ $\simeq 9$ K along the structural chains and $J_{\text{ic1}}$ $\simeq 2$ K between the chains, form a two-dimensional anisotropic honeycomb lattice. In contrast, the spin model of ${\mathrm{LiCrGe}}_2{\mathrm{O}}_6$ is constituted of three different exchange couplings. Surprisingly, the leading exchange $J_{\text{ic1}}$ $\simeq 2.3$ K operates between the chains, while $J_1$ $\simeq 1.2$ K is about two times smaller. The additional interlayer coupling $J_{\text{ic2}}$ $\simeq$ $J_1$ renders this model three dimensional. QMC simulations reveal excellent agreement between our magnetic models and the available experimental data. Underlying mechanisms of the exchange couplings, magnetostructural correlations, as well as implications for other pyroxene systems are discussed.

A frustrated spin-$\frac{1}{2}$ XXZ zigzag chain relevant to ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ is revisited in the light of symmetry-protected topological (SPT) phases. Using a density-matrix renormalization group method for infinite systems, we identify projective representations for four distinct time-reversal invariant SPT phases; two parity-symmetric dimer phases near the Heisenberg and XX limits and two parity-broken vector-chiral (VC) dimer phases in between. A small bond alternation in the nearest-neighbor ferromagnetic exchange coupling induces a direct SPT transition between the two distinct VC dimer phases. It is also found numerically that two Berezinskii-Kosterlitz-Thouless transitions, which occur from the gapless to the two distinct gapped VC phases in the case of $\delta =0$, meet each other in the case of $\delta >0$ at a Gaussian criticality of the same Tomonaga-Luttinger parameter value as in the SU(2)-symmetric case.

The antiferromagnetic spin-1 chain has a venerable history and has been thought to be well understood. Here, we show that inclusion of both next-nearest-neighbor $\left(\alpha \right)$ and biquadratic $\left(\beta \right)$ interactions results in a rich phase diagram with a multicritical point that has not been observed before. We study the problem using a combination of the density matrix renormalization group (DMRG), an analytic variational matrix product state wave function, and conformal field theory. For negative $\beta <{\beta }^{*}$, we establish the existence of a spontaneously dimerized phase, separated from the Haldane phase by the critical line ${\alpha }_c\left(\beta \right)$ of second-order phase transitions. In the opposite regime, $\beta >{\beta }^{*}$, the transition from the Haldane phase becomes first order into the next-nearest-neighbor (NNN) AKLT phase. Based on the field theoretical arguments and DMRG calculations, we find that these two regimes are separated by a multicritical point $({\beta }^{*},{\alpha }^{*})$ of a different universality class, described by the level-4 SU(2) Wess-Zumino-Witten conformal theory. From the DMRG calculations, we estimate this multicritical point to lie in the range $-0.2<{\beta }^{*}<-0.15$ and $0.47<{\alpha }^{*}<0.53$. We further find that the dimerized and NNN-AKLT phases are separated from each other by a line of first-order phase transitions that terminates at the multicritical point. We establish that transitions out of the Haldane phase into the dimer or NNN-AKLT phases are topological in nature and occur either with or without closing of the bulk gap, respectively. We also study short-range incommensurate-to-commensurate transitions in the resulting phase diagram. Inside the Haldane phase, we show the existence of two incommensurate crossovers: the Lifshitz transition and the disorder transition of the first kind, marking incommensurate correlations in momentum and real space, respectively. Notably, these crossover lines stretch across the entire $(\beta ,\alpha )$ phase diagram, merging into a single incommensurate-to-commensurate transition line for negative $\beta \lesssim {\beta }^{*}$ inside the dimer phase. This behavior is qualitatively similar to that seen in classical frustrated two-dimensional spin models, by way of the quantum (1+1)D to classical 2D correspondence.

Thin superconducting films form a unique platform for geometrically confined, strongly interacting electrons. They allow an inherent competition between disorder and superconductivity, which in turn enables the intriguing superconducting-to-insulating transition and is believed to facilitate the comprehension of high-$T_c$ superconductivity. Furthermore, understanding thin film superconductivity is technologically essential, e.g., for photodetectors and quantum computers. Consequently, the absence of established universal relationships between critical temperature $\left(T_c\right)$, film thickness $\left(d\right)$, and sheet resistance $\left(R_s\right)$ hinders both our understanding of the onset of the superconductivity and the development of miniaturized superconducting devices. We report that in thin films, superconductivity scales as $d{T}_c\left(R_s\right)$. We demonstrated this scaling by analyzing the data published over the past 46 years for different materials (and facilitated this database for further analysis). Moreover, we experimentally confirmed the discovered scaling for NbN films, quantified it with a power law, explored its possible origin, and demonstrated its usefulness for nanometer-length-scale superconducting film-based devices.

Entanglement properties of excited eigenstates (or of thermal mixed states) are difficult to study with conventional analytical methods. We approach this problem for random spin chains using a recently developed real-space renormalization group technique for excited states (“RSRG-X”). For the random XX and quantum Ising chains, which have logarithmic divergences in the entanglement entropy of their (infinite-randomness) critical ground states, we show that the entanglement entropy of excited eigenstates retains a logarithmic divergence while the mutual information of thermal mixed states does not. However, in the XX case the coefficient of the logarithmic divergence extends from the universal ground-state value to a universal interval due to the degeneracy of excited eigenstates. These models are noninteracting in the sense of having free-fermion representations, allowing strong numerical checks of our analytical predictions.

High intensity pulses obtained by modern extreme ultraviolet (EUV) and x-ray photon sources allows the observation of peculiar phenomena in condensed matter. Experiments performed at the Fermi@Elettra FEL-1 free-electron-laser source at 23.7, 33.5, and 37.5 eV on Al thin films, for an intermediate-fluence range up to about $20\mathrm{J}/{\mathrm{cm}}^2$, show evidence for a nonmonotonic EUV transmission trend. A decreasing transmission up to about $5–10\mathrm{J}/{\mathrm{cm}}^2$ is followed by an increase at higher fluence, associated with saturable absorption effects. The present findings are interpreted within a simplified three-channel model, showing that an account of the interplay between ultrafast electron heating and saturation effects is required to explain the observed transmission trend.

The superconductivity of the noncentrosymmetric system $\mathrm{LaNi}{\mathrm{C}}_2$ has been studied under high pressures up to 30 GPa with electrical resistivity measurements. For this superconducting state, the breaking of time-reversal symmetry was shown recently in a muon spin relaxation $\left(\mu \mathrm{SR}\right)$ experiment, which leads to nonunitary spin-triplet superconductive pairings. The present experiments on this superconductor reveal that the transition temperature $T_{\mathrm{c}}$ greatly increases at the rate of 0.25 (±0.01) K/GPa up to 3 GPa. However, above this pressure, $T_{\mathrm{c}}$ gradually decreases; and at the pressures over 8 GPa, the superconductivity disappears completely. With this disappearance of the superconductivity, a different state with a high-energy scale dramatically emerges. These results indicate that the system is not a simple normal metal, but is rather highly correlated with strong electronic interactions which would contribute to its superconductivity.

The emergence of carrier pairing from the electronically inhomogeneous phase of lightly hole-doped copper oxides has been investigated through magnetoresistance measurements on 1222-type ruthenocuprates ${\mathrm{RuSr}}_2{(R,\mathrm{Ce})}_2{\mathrm{Cu}}_2{\mathrm{O}}_{10-\delta }$, principally with $R=\mathrm{Gd}$, Sm, Nd. A well-defined transition at which superconducting fluctuations emerge is discovered at a remarkably low critical doping, $p_{\mathrm{c}}=0.0084$, deep within the antiferromagnetic phase. Systematic variations of the low-temperature fluctuation density with doping and cell volume demonstrate the intrinsic nature of the electronic inhomogeneity and provide new support for bosonic models of the superconducting mechanism.

Superparamagnetism appears when the Néel-Brown relaxation time of magnetic nanoparticles is shorter than the measurement time. Recent experimental studies of different types of magnetic nanoparticles revealed the existence of another paramagnetic region below the standard blocking temperatures. Here we elucidate the microscopic origin of this reentrant paramagnetism using a phenomenological model, which exploits the effects of weaker magnetic coupling strengths at the surfaces of ultrasmall nanoparticles. Within this picture, we have calculated the total magnetization of various nanoparticle arrays upon both finite-field and zero-field cooling processes via detailed classical Monte Carlo simulations, and found that the appearance of the reentrant phenomena necessarily invokes a drastic reduction of the magnetic coupling strengths at the surfaces of the nanoparticles. Our predictions can be readily tested experimentally using a micro-SQUID, and is expected to be beneficial in further applications of superparamagnetic nanoparticles.

The hidden order (HO) in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has been determined as a high-rank multipole formed by itinerant $5f$ electrons with distinct orbital structure imposed by the crystalline electric field. Because this can lead to a considerable number of different multipoles, it is of great importance to use microscopic techniques that are sensitive to their subtle physical differences. Here, we investigate whether the quasiparticle interference (QPI) method can distinguish between the two most frequently proposed HO parameter models: the even rank-4 hexadecapole and the odd-rank-5 dotriacontapole model. We obtain the quasiparticle dispersion and reconstructed Fermi surface in each HO phase adapting an effective two-orbital model of $5f$ bands that reproduces the main Fermi surface sheets of the para phase. We show that the resulting QPI spectrum reflects directly the effect of fourfold symmetry breaking in the rank-5 model, which is absent in the rank-4 model. Therefore we suggest that the QPI method should give the possibility of a direct discrimination between the two most investigated models of HO in ${\mathrm{URu}}_2{\mathrm{Si}}_2$. Furthermore, the signature of proposed chiral $d$-wave superconducting (SC) order parameter in QPI of the coexisting HO + SC phase is investigated.

We study numerically the universal conductance of Luttinger-liquid wire with a single impurity via the multiscale entanglement renormalization ansatz (MERA). The scale-invariant MERA provides an efficient way to extract scaling operators and scaling dimensions for both the bulk and the boundary conformal field theories. By utilizing the key relationship between the conductance tensor and ground-state correlation function, the universal conductance can be evaluated within the framework of the boundary MERA. We construct the boundary MERA to compute the correlation functions and scaling dimensions for the Kane-Fisher fixed points by modeling the single impurity as a junction (weak link) of two interacting wires. We show that the universal behavior of the junction can be easily identified within the MERA and argue that the boundary MERA framework has tremendous potential to classify the fixed points in general multiwire junctions.

The manipulation of single spins in double quantum dots by making use of the exchange interaction and a highly inhomogeneous magnetic field was discussed in Coish and Loss [Phys. Rev. B 75, 161302 (2007)]. However, such large inhomogeneity is difficult to achieve through the slanting field of a micromagnet in current designs of lateral double dots. Therefore, we examine an analogous spin manipulation scheme directly applicable to realistic GaAs double dot setups. We estimate that typical gate times, realized at the singlet-triplet anticrossing induced by the inhomogeneous micromagnet field, can be a few nanoseconds. We discuss the optimization of initialization, read-out, and single-spin gates through suitable choices of detuning pulses and an improved geometry. We also examine the effect of nuclear dephasing and charge noise. The latter induces fluctuations of both detuning and tunneling amplitude. Our results suggest that this scheme is a promising approach for the realization of fast single-spin operations.

We study the Rashba spin splitting of the $L$-gap surface state on the (111) surfaces of Au, Ag, and Cu, and that of the topologically protected surface state on Sb(111) by a first-principles calculation based on the embedded Green's function technique. By taking advantage of semi-infinite surface geometry, we address several issues that are difficult to handle with the finite slab calculation. For Au(111), we investigate the nonlinear dependence on wave vector $\mathbf{k}$ of the spin-splitting energy, and also at what wave number each dispersion curve of the two spin-split bands is merged into a surface-projected bulk band. Then, we discuss why Cu(111) exhibits a larger Rashba effect than Ag(111) despite the fact that the atomic orbitals of Ag have larger spin-orbit matrix elements than those of Cu. We reveal that, while the wave function of the $L$-gap surface state has strong $p_z$ character (the $z$ axis is surface normal), the Rashba spin splitting occurs through the spin-orbit matrix element between the $d_{z^2}$ and $d_{uz}$ orbital components hybridized with $p_z$, where $d_{uz}$ denotes a linear combination of $d_{xz}$ and $d_{yz}$ with its orbital lobes pointing to the $\mathbf{k}$ direction (defined as the $u$ axis). Cu(111) exhibits a larger spin splitting than Ag(111), since the amount of both the $d_{z^2}$ and $d_{uz}$ components mixed in its surface-state wave function is much larger than that of Ag(111). In contrast to the (111) surfaces of noble metals, the spin splitting of the topologically protected surface state on Sb(111) arises from the spin-orbit matrix element between the $p_z$ and $p_u$ orbital components. We also discuss under what condition the energy dispersion curve of a surface state intersects the boundary line between a projected bulk band and a projected bulk band gap at a large angle, instead of gradually approaching the boundary line with increasing $\left|\mathbf{k}\right|$. Finally, we comment on a recent theoretical work of Krasovskii [Phys. Rev. B 90, 115434 (2014)] on the microscopic origin of the Rashba spin splitting.

The influence of silicene's strong spin-orbit interaction and of an external electric field $E_z$ on the transport coefficients are investigated in the presence of a perpendicular magnetic field $B$. For finite $E_z$ the spin and valley degeneracy of the Landau levels is lifted and leads to additional plateaus in the Hall conductivity, at half-integer values of $4e^2/h$, due to spin intra-Landau-level transitions that are absent in graphene. These plateaus are more sensitive to disorder and thermal broadening than the main plateaus, occurring at integral values of $4e^2/h$, when the Fermi level passes through the Landau levels. We also evaluate the Hall and longitudinal resistivities and critically contrast the results with those for graphene on a substrate.

We extend the coupled-wire construction of quantum Hall phases, and search for fractional topological insulating states in models of weakly coupled wires at zero external magnetic field. Focusing on systems beyond double copies of fractional quantum Hall states at opposite fields, we find that spin-spin interactions can stabilize a large family of fractional topological phases with broken time reversal invariance. The latter is manifested by spontaneous spin polarization, by a finite Hall conductivity, or by both. This suggests the possibility that fractional topological insulators may be unstable to spontaneous symmetry breaking.

We show that in layered systems with electronic phase separation tendency, the long-range Coulomb interaction can drive the spontaneous formation of unidirectional superlattices of electronic charge in a completely homogeneous crystalline background. In this self-organized electronic heterostructure, the ratio among the number of crystalline planes in the minority and majority electronic phases corresponds to Farey fractions with the superlattice period controlled by the background charge density and the frustrating Coulomb interaction strength. The phase diagram displays Arnold tongues obeying a modified Farey tree hierarchy and a devil's staircase, typical of systems with frustration among different scales. We further discuss the competition of these electronic superlattices, recently observed in iron-based superconductors and mixed valence compounds, with in-plane electronically modulated phases.