Controlling material properties by modulating the crystalline structure has been attempted using various techniques, e.g., hydrostatic pressure, chemical pressure, and epitaxy. These techniques succeed to improve properties and achieve desired functionalities by changing the unit cell in all dimensions. In order to obtain a more detailed understanding on the relation between the crystal lattice and material properties, it is desirable to investigate the influence of a smaller number of parameters. Here, we utilize the combination of chemical pressure and epitaxy to modify a single lattice parameter of the multiferroic orthorhombic $R\mathrm{Mn}{\mathrm{O}}_3$ ($R=\mathrm{rare}\text{−}\mathrm{earth}$; $\mathrm{o}\text{−}R\mathrm{Mn}{\mathrm{O}}_3$) system. By growing a series of $\mathrm{o}\text{−}R\mathrm{Mn}{\mathrm{O}}_3$ ($R=\mathrm{Gd}–\mathrm{Lu}$) films coherently on (010)-oriented $\mathrm{YAl}{\mathrm{O}}_3$ substrates, the influence of chemical pressure is reflected only along the $b$ axis. Thus a series of $\mathrm{o}\text{−}R\mathrm{Mn}{\mathrm{O}}_3$ with $a\sim 5.18\AA ,5.77\AA <b<5.98\AA$, and $c\sim 7.37\AA$ were obtained. Raman spectra analysis reveals that the change of the $b$-axis parameter induces a shift of the oxygen in the nominally “fixed” $ca$ plane. Their ferroelectric ground state is independent on the $b$-axis parameter showing polarization of $\sim 1\mu \mathrm{C}\mathrm{c}{\mathrm{m}}^{-2}$ along the $a$ axis for the above-mentioned range, except for $b\sim 5.94\AA$, which corresponds to $\mathrm{TbMn}{\mathrm{O}}_3$ showing $\sim 2\mu \mathrm{C}\mathrm{c}{\mathrm{m}}^{-2}$. This result implies that multiferroic order of $\mathrm{o}\text{−}R\mathrm{Mn}{\mathrm{O}}_3$ is almost robust against the $b$-axis parameter provided that the dimension of the $ca$ plane is fixed to $7.37\AA \times 5.18\AA$.

We developed a technique to determine suitable spin models for small embedded clusters of arbitrary geometry by combining the spin-cluster expansion with the relativistic disordered local moment scheme. We present results for uncovered and covered hexagonal Co clusters on Au(111) surface, and use classical Monte Carlo simulations to study the temperature dependent properties of the systems. To test the new method we compare the calculated spin-model parameters of the uncovered clusters with those of a Co monolayer deposited on Au(111). In general, the isotropic and Dzyaloshinsky-Moriya interactions are larger between atoms at the perimeter than at the center of the clusters. For Co clusters covered by Au, both the contribution to the magnetic anisotropy and the easy axis direction of the perimeter atoms differ from those of the inner atoms due to reduced symmetry. We investigate the spin reversals of the covered clusters with perpendicular magnetic anisotropy and based on the variance of the magnetization component parallel to the easy direction we suggest a technique to determine the blocking temperature of superparamagnetic particles. We also determine the Néel relaxation time from the Monte Carlo simulations and find that it satisfies the Néel-Arrhenius law with an energy barrier close to the magnetic anisotropy energy of the clusters.

Using ideas from Landau's Fermi-liquid theory, we apply irreversible thermodynamics to conducting and insulating ferromagnets with magnetic variables $\widehat{M}$ for the quantization axis and for the spin accumulation $\vec{m}$ of the nonequilibrium excitations; thus the total magnetization is taken to be $\vec{\mathcal{M}}=\vec{M}+\vec{m}$. The resulting theory closely corresponds to the theory of Silsbee et al. [Silsbee, Janossy, and Monod, Phys. Rev. B 19, 4382 (1979)]. For the bulk, in addition to confirming the usual Landau-Lifshitz equation for $\widehat{M}$ and a Bloch-like equation for $\vec{m}$ (with a nonuniform precession term), there are two related cross-relaxation terms between the transverse parts of the nonequilibrium $\vec{m}$ and $\vec{M}$. Unlike the s-d model, where in a field $\vec{H}$ the equilibrium magnetizations ${\vec{M}}_s$ and ${\vec{M}}_d$ are both nonzero, for this m-M model in a field $\vec{H}$, only the equilibrium magnetization $\vec{M}$ is nonzero. For the interface, the boundary condition for $\widehat{M}$ is given by micromagnetics, and that for $\vec{m}$ is given by irreversible thermodynamics, where the current of transverse spins crossing the interface is proportional to the discontinuity in the transverse part of the vector spin chemical potential. $\widehat{M}, \vec{m}$, and $\vec{H}$ are coupled; in the decoupled approximation, we find the wave vectors for the modes of $\widehat{M}$ and the transverse $\vec{m}$. We discuss reciprocity between spin pumping ($\vec{\mathcal{M}}$ driven out of the ferromagnet) and spin transfer torque ($\vec{\mathcal{M}}$ driven into the ferromagnet).

We generalize the string-net construction to multiple flavors of strings, each of which is labeled by the elements of an Abelian group $G_i$. The same flavor of strings can branch, while different flavors of strings can cross one another and thus they form intersecting string nets. We systematically construct the exactly soluble lattice Hamiltonians and the ground-state wave functions for the intersecting string-net condensed phases. We analyze the braiding statistics of the low-energy quasiparticle excitations and find that our model can realize all the topological phases as the string-net model with group $G={\prod }_i{G}_i$. In this respect, our construction provides various ways of building lattice models which realize topological order $G$, corresponding to different partitions of $G$ and thus different flavors of string nets. In fact, our construction concretely demonstrates the Künneth formula by constructing various lattice models with the same topological order. As an example, we construct the $G={\mathbb{Z}}_2\times {\mathbb{Z}}_2\times {\mathbb{Z}}_2$ string-net model which realizes a non-Abelian topological phase by properly intersecting three copies of toric codes.

This paper considers a model consisting of a kinetic term, Rashba spin-orbit coupling and short-range Coulomb interaction at zero temperature. The Coulomb interaction is decoupled by a mean-field approximation in the spin channel using field theory methods. The results feature a first-order phase transition for any finite value of the chemical potential and quantum criticality for vanishing chemical potential. The Hall conductivity is also computed using the Kubo formula in a mean-field effective Hamiltonian. In the limit of infinite mass the kinetic term vanishes and all the phase transitions are of second order; in this case the spontaneous symmetry-breaking mechanism adds a ferromagnetic metallic phase to the system and features a zero-temperature quantization of the Hall conductivity in the insulating one.

Weak antilocalization (WAL) effects in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ single crystals have been investigated at high and low bulk charge-carrier concentrations. At low charge-carrier density the WAL curves scale with the normal component of the magnetic field, demonstrating the dominance of topological surface states in magnetoconductivity. At high charge-carrier density the WAL curves scale with neither the applied field nor its normal component, implying a mixture of bulk and surface conduction. WAL due to topological surface states shows no dependence on the nature (electrons or holes) of the bulk charge carriers. The observations of an extremely large nonsaturating magnetoresistance and ultrahigh mobility in the samples with lower carrier density further support the presence of surface states. The physical parameters characterizing the WAL effects are calculated using the Hikami-Larkin-Nagaoka formula. At high charge-carrier concentrations, there is a greater number of conduction channels and a decrease in the phase coherence length compared to low charge-carrier concentrations. The extremely large magnetoresistance and high mobility of topological insulators have great technological value and can be exploited in magnetoelectric sensors and memory devices.

We study the Anderson single-level quantum dot attached to two BCS superconducting leads with the same gap size. We reveal that a system with asymmetric tunnel coupling to the leads (${\mathrm{\Gamma }}_L\ne {\mathrm{\Gamma }}_R$) can be related to the symmetric system with the same net coupling strength $\mathrm{\Gamma }={\mathrm{\Gamma }}_L+{\mathrm{\Gamma }}_R$. Surprisingly, it is the symmetric case which is the most general, meaning that all physical quantities in the case of asymmetric coupling are fully determined by the symmetric ones. We give ready-to-use conversion formulas for the $0\text{−}\pi$ phase transition boundary, on-dot quantities, and the Josephson current and illustrate them on the NRG results of Oguri, Tanaka and Bauer [Phys. Rev. B 87, 075432 (2013)] for the three-terminal setup. We apply our theory to the recent $0\text{−}\pi$ transition measurement of Delagrange et al. [Phys. Rev. B 93, 195437 (2016)] and determine the asymmetry of the experimental setup from the measured transition width. Finally, we establish that the widely assumed Kondo “universality” of physical quantities depending only on the ratio of the Kondo temperature and the superconducting gap $T_K/\mathrm{\Delta }$ cannot hold for asymmetric junctions.

We introduce a methodology to treat different degrees of freedom at different levels of approximation. We use cluster DMFT (dynamical mean field theory) for the $t_{2g}$ electrons and single site DMFT for the $e_g$ electrons to study the normal state of the iron pnictides and chalcogenides. In the regime of moderate mass renormalizations, the self-energy is very local, justifying the success of single site DMFT for these materials and for other Hunds metals. We solve the corresponding impurity model with CTQMC (continuous time quantum Monte Carlo) and find that the minus sign problem is not severe in regimes of moderate mass renormalization.

We characterize numerically and analytically the signatures of the spin-orbit interaction in a two-electron GaAs double quantum dot in the presence of an external magnetic field. In particular, we obtain the return probability of the singlet state by simulating Landau-Zener voltage detuning sweeps which traverse the singlet-triplet ($S-T_{+}$) resonance. Our results indicate that non-spin-conserving interdot tunneling processes arising from the spin-orbit interaction have well defined signatures. These allow direct access to the spin-orbit interaction scales and are characterized by a frequency shift and Fourier amplitude modulation of the Rabi flopping dynamics of the singlet-triplet qubits $S-T_0$ and $S-T_{+}$. By applying the Bloch-Feshbach projection formalism, we demonstrate analytically that the aforementioned effects originate from the interplay between spin-orbit interaction and processes driven by the hyperfine interaction between the electron spins and those of the GaAs nuclei.

We report on the observation of a circular photogalvanic current excited by terahertz laser radiation in helical edge channels of two-dimensional (2D) HgTe topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from a right-handed to a left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a $\approx 3\mu \mathrm{m}$ distance at CNP approaches $2e^2/h$, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with experimental data.

Oxygen-containing yttrium hydride thin films exhibit photochromic behavior: Transparent thin films reversibly switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. From optical spectrophotometry and ellipsometry measurements of the transparent state and photodarkened state, it is concluded that the photochromic effect can be explained by the gradual growth, under illumination, of metallic domains within the initial wide-band-gap semiconducting lattice. This conclusion is supported by Raman measurements.

Assembling different two-dimensional (2D) crystals, covering a very broad range of properties, into van der Waals (vdW) heterostructures enables unprecedented possibilities for combining the best of different ingredients in one objective material. So far, metallic, semiconducting, and insulating 2D crystals have been used successfully in making functional vdW heterostructures with properties by design. Here, we expand 2D superconducting crystals as a building block of vdW hererostructures. One-step growth of large-scale high-quality vdW heterostructures of graphene and 2D superconducting $\alpha \text{−}\mathrm{M}{\mathrm{o}}_2\mathrm{C}$ by using chemical vapor deposition is reported. The superconductivity and its 2D nature of the heterostructures are characterized by our scanning tunneling microscopy measurements. This adds 2D superconductivity, the most attractive property of condensed matter physics, to vdW heterostructures.

Three-dimensional (3D) metals/semimetals under magnetic field have an instability toward a density wave (DW) ordering which breaks a translational symmetry along the field direction. Effective boson models for the DW phases take forms of $XY$ models with/without Potts terms. Longitudinal conductivity along the field direction is calculated in the DW phases with inclusion of effects of low-energy charge fluctuation (phason) and disorder. A single-particle imaginary-time Green function is identified with a partition function of 3D $XY$ models in the presence of pairs of magnetic monopoles. In terms of the celebrated electromagnetic duality, electronic spectral function is calculated near the DW phase transition. The calculated result shows that the single-particle spectral function acquires an additional low-energy feature due to the strong phason fluctuation. Relevance to an in-plane conductance due to surface chiral Fermi arc states is also discussed.

Variational methods are a common approach for computing properties of ground states but have not yet found analogous success in finite-temperature calculations. In this work, we develop a new variational finite-temperature algorithm (VAFT), which combines ideas from minimally entangled typical thermal states (METTS), variational Monte Carlo (VMC) optimization, and path integral Monte Carlo (PIMC). This allows us to define an implicit variational density matrix to estimate finite-temperature properties in two and three dimensions. We benchmark the algorithm on the bipartite Heisenberg model and compare to exact results.

The correspondence between the edge theory and the entanglement spectrum is firmly established for the chiral topological phases. We study a gapped, topologically ordered, nonchiral state with a conserved $\text{U}\left(1\right)$ charge and show that the entanglement Hamiltonian contains not only the information about topologically distinct edges such a phase may admit, but also which of them will be realized in the presence of symmetry-breaking/-conserving perturbations. We introduce an exactly solvable, charge-conserving lattice model of a ${\mathbb{Z}}_2$ spin liquid and derive its edge theory and the entanglement Hamiltonian, also in the presence of perturbations. We construct a field theory of the edge and study its renormalization group flow. We show the precise extent of the correspondence between the information contained in the entanglement Hamiltonian and the edge theory.

We investigate an exact nonequilibrium solution of a two-site electron-phonon model, where an infrared-active phonon that is nonlinearly coupled to the electrons is driven by a laser field. The time-resolved electronic spectrum shows coherence-incoherence spectral weight transfer, a clear signature of light-enhanced electron-phonon coupling. The present study is motivated by recent evidence for enhanced electron-phonon coupling in pump-probe terahertz and angle-resolved photoemission spectroscopy in bilayer graphene when driven near resonance with an infrared-active phonon mode [E. Pomarico et al., Phys. Rev. B 95, 024304 (2017)], and by a theoretical study suggesting that transient electronic attraction arises from nonlinear electron-phonon coupling [D. M. Kennes et al., Nat. Phys. 13, 479 (2017)]. We show that a linear scaling of light-enhanced electron-phonon coupling with the pump field intensity emerges, in accordance with a time-nonlocal self-energy based on a mean-field decoupling using quasiclassical phonon coherent states. Finally, we demonstrate that this leads to enhanced double occupancies in accordance with an effective electron-electron attraction. Our results suggest that materials with strong phonon nonlinearities provide an ideal playground to achieve light-enhanced electron-phonon coupling and possibly light-induced superconductivity.

The propagation of an electron beam over a graphene/dielectric sandwich structure is considered assuming the distance between the graphene layers in sandwich is large enough to prevent interlayer tunneling. A dispersion equation for the surface electromagnetic modes propagating along graphene sheets is derived and Čerenkov synchronism between a surface wave and a nonrelativistic electron beam is predicted at achievable parameters of the system. Generation frequency tuning is proposed by varying the graphene doping, the number of graphene sheets, the distance between sheets, etc.

Combining atomistic methods and effective mass approaches, the magnetic field response of Mn-doped deformed quantum dots has been characterized. The defect-induced asymmetry triggers unavoidable spin-orbit coupling effects that become more relevant the smaller the quantum confinement. Thus, the expected spin splitting modulation by the exchange interaction between the confined carriers and the Mn impurity is tuned by the intertwining of spatial geometry and magnetic field. Furthermore, ab initio calculations are presented for assessing the exchange interaction term and its response to confinement effects at the atomistic level.