We analyze the time-dependent energy and heat flows in a resonant level coupled to a fermionic continuum. The level is periodically forced with an external power source that supplies energy into the system. Based on the tunneling Hamiltonian approach and scattering theory, we discuss the different contributions to the total energy flux. We then derive the appropriate expression for the dynamical dissipation, in accordance with the fundamental principles of thermodynamics. Remarkably, we find that the dissipated heat can be expressed as a Joule law with a universal resistance that is constant at all times.

A series of cerium-based heavy fermion materials is studied using a combination of local density functional theory and the many-body Gutzwiller approximation. Computed orbital-dependent electronic mass enhancement parameters are compared with available experimental data extracted from measured values of the Sommerfeld coefficient. Gutzwiller density functional theory is shown to remarkably follow the trends across a variety of Ce compounds and to give important insights into the phenomenon of orbital-selective mass renormalization which in turn allows for a better understanding of a wide spread of data.

We examine the existence of the interaction-generated quantum anomalous Hall phase on the honeycomb lattice. For the spinless model at half-filling, the existence of a quantum anomalous Hall phase (Chern insulator phase) has been predicted using mean-field methods. However, recent exact diagonalization studies for small clusters with periodic boundary conditions have not found a clear sign of an interaction-driven Chern insulator phase. We use the exact diagonalization method to study properties of small clusters with open boundary conditions, and contrary to previous studies, we find clear signatures of the topological phase transition for finite-size clusters. We also examine the applicability of the entangled-plaquette-state (correlator-product-state) ansatz to describe the ground states of the system. Within this approach the lattice is covered with plaquettes and the ground-state wave function is written in terms of the plaquette coefficients. Configurational weights can then be optimized using a variational Monte Carlo algorithm. Using the entangled-plaquette-state ansatz we study the ground-state properties of the system for larger system sizes and show that the results agree with the exact diagonalization results for small clusters. This confirms the validity of the entangled-plaquette-state ansatz to describe the ground states of the system and provides further confirmation of the existence of the quantum anomalous Hall phase in the thermodynamic limit, as predicted by mean-field theory calculations.

We study the edge-mode excitations of a fractional quantum Hall droplet by expressing the edge-state wave functions as linear combinations of Jack polynomials with a negative parameter. We show that the exact diagonalization within a subspace of Jack polynomials can be used to generate the chiral edge-mode excitation spectrum in the Laughlin phase and the Moore-Read phase with realistic Coulomb interaction. The truncation technique for the edge excitations simplifies the procedure to reliably extract the edge-mode velocities, which avoids the otherwise complicated analysis of the full spectrum that contains both edge and bulk excitations. Generalization to the Read-Rezayi state is also discussed.

We construct a parent Hamiltonian for the family of non-Abelian chiral spin liquids proposed recently by two of us [Phys. Rev. Lett. 102, 207203 (2009)] which includes the Abelian chiral spin liquid proposed by Kalmeyer and Laughlin as the special case $s=\frac{1}{2}$. As we use a circular disk geometry with an open boundary, both the annihilation operators we identify and the Hamiltonians we construct from these are exact only in the thermodynamic limit.

In conventional superconductors the Cooper pairs have a zero center-of-mass momentum. In this paper we present a theory of superconducting states where the Cooper pairs have a nonzero center-of-mass momentum, inhomogeneous superconducting states known as a pair-density-waves (PDWs) states. We show that in a system of spin-$\frac{1}{2}$ fermions in two dimensions in an electronic nematic spin-triplet phase where rotational symmetry is broken in both real- and spin-space PDW phases arise naturally in a theory that can be analyzed using controlled approximations. We show that several superfluid phases that may arise in this phase can be treated within a controlled BCS mean-field theory, with the strength of the spin-triplet nematic order parameter playing the role of the small parameter of this theory. We find that in a spin-triplet nematic phase, in addition to a triplet $p$-wave and spin-singlet $d$-wave (or $s$ depending on the nematic phase) uniform superconducting states, it is also possible to have a $d$-wave (or $s$) PDW superconductor. The PDW phases found here can be either unidirectional, bidirectional, or tridirectional depending on the spin-triplet nematic phase and which superconducting channel is dominant. In addition, a triple-helix state is found in a particular channel. We show that these PDW phases are present in the weak-coupling limit, in contrast to the usual Fulde-Ferrell-Larkin-Ovchinnikov phases, which require strong coupling physics in addition to a large magnetic field (and often both).

A combined theoretical and experimental study of the natural Cu${}^{2+}$-mineral callaghanite is presented. Its crystal structure features well separated ${\mathrm{Cu}}_2$${\mathrm{(oH)}}_6$ structural dimers with weakly bonded carbonate groups and water molecules in between. Susceptibility, field-dependent magnetization and specific-heat measurements reveal a compound with a small spin gap of about 7 K. The observed magnetic properties are well described by a model of isolated antiferromagnetic spin dimers. Possible ferromagnetic interactions between the dimers amount to $-1.5$ K, at most. Different flavors of electronic structure calculations have been employed to locate the magnetic dimers in the crystal structure, i.e., to determine whether they coincide with the structural dimers or not. Calculations of the coupling between the structural dimers clearly show that magnetic and structural dimers are the same. For the intradimer coupling, however, the computational results confirmed a coupling strength close to zero but the sign of the coupling could not be determined unambiguously. Based on this finding, we then discuss how the reliability of the numerical methods depends on the characteristics of exchange pathways and on structural features of the compound in general. Eventually, we try to provide a minimum coupling strength that is needed for a reliable computational description.

We examine the effect of a spin-active interface on the symmetry of proximity-induced superconducting pairing amplitudes in topological insulators. We develop a model to investigate the leading-order contribution to the pairing amplitude considering three different kinds of spin-active interfaces: (i) those that induce spin-dependent scattering phases, (ii) those that flip the spin of incident electrons, and (iii) interfaces that both induce spin-dependent phases and flip the spins of incident electrons. We find that in cases (i) and (iii) odd-frequency triplet pairing is induced in the TI while for case (ii) no odd-frequency pairing is induced to leading order. We compare our results to those for normal metals and ferromagnetic materials finding that the nontrivial spin structure of the TI leads to qualitatively different behavior.

We study a superconducting quantum point contact made of a narrow In${}_{0.75}$Ga${}_{0.25}$As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor-superconductor hybrid Josephson junction involving only a single ballistic channel.

We investigate the effect of quantum phase slips on a helical quantum wire coupled to a superconductor by proximity. The effective low-energy description of the wire is that of a Majorana chain minimally coupled to a dynamical ${\mathbb{Z}}_2$ gauge field. Hence the wire emulates a matter-coupled gauge theory, with fermion parity playing the role of the gauged global symmetry. Quantum phase slips lift the ground-state degeneracy associated with unpaired Majorana edge modes at the ends of the chain, a change that can be understood as a transition between the confined and the Higgs-mechanism regimes of the gauge theory. We identify the quantization of thermal conductance at the transition as a robust experimental feature separating the two regimes. We explain this result by establishing a relation between thermal conductance and the Fredenhagen-Marcu string order parameter for confinement in gauge theories. Our work indicates that thermal transport could serve as a measure of nonlocal order parameters for emergent or simulated topological quantum order.

Out-of-plane spin-injection and detection through naturally stacked graphene layers were investigated in ferromagnet/graphene/ferromagnet (FGF) junctions. We obtained a maximum magnetoresistance (MR) of 4.6% at $T=4.2$ K in the junction of a four-layer graphene insertion, having a very small area$-$junction-resistance product of 0.2 $\Omega \mu$m${}^2$. According to resistance-temperature and current-voltage characteristics, the graphene layer in the FGF junction acted as a metal-like insertion rather than as an insulating barrier. A lower value for the interfacial spin asymmetry coefficient ($\gamma =0.25\pm 0.05$) obtained from the fitting of variations with interfacial resistance implies that the spin-injection efficiency along the out-of-plane direction was reduced by spin-flip scattering at graphene/ferromagnet interfaces. Our results showed that highly transparent graphene/ferromagnet interfaces with crystalline ferromagnet (FM) electrodes are required to achieve higher spin-injection efficiency through the graphene layer in a FGF junction along the out-of-plane direction.

A comprehensive analysis of the elementary processes behind the scanning tunneling microscope controlled rotation of ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{D}}_2$, isotopologues of a single acetylene molecule adsorbed on the Cu(001) surface, is given, with a focus on the isotope effects. With the help of density-functional theory we calculate the vibrational modes of ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{D}}_2$ on Cu(001) and estimate the anharmonic couplings between them, using a simple strings-on-rods model. The probability of the elementary processes, nonlinear and combination band, is estimated using the Keldysh diagram technique. This allows us to clarify the main peculiarities and the isotope effects of the ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{D}}_2$ on Cu(001) rotation, discovered in the pioneering work [B. C. Stipe et al., Phys. Rev. Lett. 81, 1263 (1998)], which have not been previously understood.

The origin of a high Neel temperature in a 5$d$ oxide, $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$, has been analyzed within the mean-field limit of a multiband Hubbard model and compared with the analogous 4$d$ oxide, $\mathrm{Sr}\mathrm{Tc}{\mathrm{O}}_3$. Our analysis shows that there are a lot of similarities in both of these oxides on the dependence of the effective exchange interaction strength ($J_0$) on the electron-electron interaction strength ($U$). However, the relevant value of $U$ in each system puts them in different portions of the parameter space. Although the Neel temperature for $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ is less than that for $\mathrm{Sr}\mathrm{Tc}{\mathrm{O}}_3$, our results suggest that there could be examples among other 5$d$ oxides that have a higher Neel temperature. We have also examined the stability of the G-type antiferromagnetic state found in $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ as a function of electron doping within GGA + $U$ calculations and find a robust G-type antiferromagnetic metallic state stabilized. The most surprising aspect of the doped results is the rigid bandlike evolution of the electronic structure, which indicates that the magnetism in $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ is not driven by Fermi surface nesting.

The fundamental aspects of spin-dependent transport processes and their interplay with temperature gradients, as given by the spin Seebeck coefficient, are still largely unexplored and a multitude of contributing factors must be considered. We used density functional theory together with a Monte-Carlo-based statistical method to simulate simple nanostructures, such as Co nanowires and films embedded in a Cu host or in vacuum, and investigated the influence of spin disorder scattering on electron transport at elevated temperatures. While we show that the spin-dependent scattering of electrons due to temperature-induced disorder of the local magnetic moments contributes significantly to the resistance, thermoelectric, and spin-caloric transport coefficients, we also conclude that the actual magnitude of these effects cannot be predicted, quantitatively or qualitatively, without such detailed calculations.

Phase separation of two different isosymmetric phases can occur as a result of their similar ground state energies. We have found such phase separation in a multiferroic system at a tensile-strain-driven morphotropic phase boundary of rhombohedral and orthorhombic bismuth ferrite. We utilize the emergent phase competition in order to implement three-step electric switching of ferroelectric polarization as well as rotation of the magnetic easy axis using electric fields. This observation provides a pathway to phase-competition-driven magnetoelectric devices.

At a pressure of around 13 GPa iron undergoes a structural phase transition from the bcc to the hexagonal close-packed phase. Atomistic simulations have provided important insights into this transition. However, while experiments in polycrystals show clear evidence that the $\alpha$-$\epsilon$ transition is preceded by plasticity, simulations up to now could not detect any plastic activity occurring before the phase change. Here we study shock waves in polycrystalline Fe using an interatomic potential which incorporates the $\alpha$-$\epsilon$ transition faithfully. Our simulations show that the phase transformation is preceded by dislocation generation at grain boundaries, giving a three-wave profile. The $\alpha$-$\epsilon$ transformation pressure is much higher than the equilibrium transformation pressure but decreases slightly with increasing loading ramp time (decreasing strain rate). The transformed phase is mostly composed of hcp grains with large defect density. Simulated x-ray diffraction displays clear evidence for this hcp phase, with powder-diffraction-type patterns as they would be seen using current experimental setups.

We present measurements of the magnetic entropy landscape and Grüneisen parameter of the ${\mathrm{Cu}}^{2+}$ complex $({\mathrm{C}}_5{\mathrm{H}}_{12}\mathrm{N}{)}_2\mathrm{Cu}{\mathrm{Cl}}_4$ in a magnetic field. Our thermodynamic measurements are in very good agreement with a theoretical description by a $S=1/2$ Heisenberg ladder model. Due to its excellent experimental access, the compound crosses two quantum critical points in the applied range of the magnetic field, first from a gapped unpolarized state to a critical phase and then to a gapped fully polarized state. This behavior is reflected directly in the magnetic entropy map. Due to the remarkable properties of the magnetic Grüneisen parameter, we are able to discuss the validity of critical behavior with respect to temperature and magnetic field for this model quasi-one-dimensional system.

We use inelastic neutron scattering and dielectric constant measurements to study the doping influence on the spin dynamics and magnetoelectric (ME) effect in hexagonal${\mathrm{Y}}_{0.7}{\mathrm{Lu}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$. In undoped $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{Lu}\mathrm{Mn}{\mathrm{O}}_3$, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In ${\mathrm{Y}}_{0.7}{\mathrm{Lu}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$, at the antiferromagnetic zone center, we observed a much smaller ${\Delta }_{12}\approx 0.52$ meV gap (which is $\sim 2.5$ meV for both $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{Lu}\mathrm{Mn}{\mathrm{O}}_3$) that coincides with a weaker in-plane dielectric anomaly at $T_N$; both can be attributed to a weaker Mn trimerization distortion in ${\mathrm{Y}}_{0.7}{\mathrm{Lu}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ compared to $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{Lu}\mathrm{Mn}{\mathrm{O}}_3$. The results provide strong evidence that the magnitude of ME coupling is linked to the strength of the trimerization distortion, suggesting the Mn trimerization is responsible for the ME effect in ${\mathrm{Y}}_{1-y}{\mathrm{Lu}}_y\mathrm{Mn}{\mathrm{O}}_3$.

Genuinely homogeneous metamaterials, which may be characterized by local effective constitutive relations, are required for many spectacular metamaterial applications. Such metamaterials have to be made of meta-atoms, i.e., subwavelength resonators, which exhibit only electric and or magnetic dipole and negligible higher-order multipolar polarizabilities in the spectral range of interest. Here, we show that these desired meta-atoms can be designed by exploiting the extreme coupling regime. Appropriate meta-atoms are identified by performing a multipole analysis of the field scattered from the respective meta-atom. To design those particular meta-atoms it is important to disclose the frequency and angular-dependent polarizability of both dipole moments. We demonstrate the applicability of a purely analytical model to accurately calculate for a normally incident plane wave reflection and transmission from meta-surfaces made of periodically arranged meta-atoms. With our work we identify a possible route towards the engineering of artificial materials while only considering the response from its constituents. Our approach is generally applicable to all spectral domains and can be used to evaluate and design metamaterials made from different constituting materials, e.g., metals, dielectrics, or semiconductors.

We study the local equilibrium properties of two-dimensional electron gases at high magnetic fields in the presence of random smooth electrostatic disorder, Rashba spin-orbit coupling, and the Zeeman interaction. Using a systematic magnetic length ($l_B$) expansion within a Green's function framework we derive quantum functionals for the local spin-resolved particle and current densities which can be useful for future studies combining disorder and mean-field electron-electron interaction in the quantum Hall regime. We point out that the spin polarization presents a peculiar spatial dependence which can be used to determine the strength of the Rashba coupling by local probes. The spatial structure of the current density, consisting of both compressible and incompressible contributions, also essentially reflects the effects of Rashba spin-orbit interaction on the energy spectrum. We show that in the semiclassical limit $l_B\to 0$ the local Hall conductivity remains, however, still quantized in units of $e^2/h$ for any finite strength of the spin-orbit interaction. In contrast, it becomes half-integer quantized when the latter is infinite, a situation which corresponds to a disordered topological insulator surface consisting of a single Dirac cone. Finally, we argue how to define at high magnetic fields a spin Hall conductivity related to a dissipationless angular momentum flow, which is characterized by a sequence of plateaus as a function of the inverse magnetic field (thus free of resonances).