We consider ballistic transport through a lateral, two-dimensional superlattice with experimentally realizable, sinusoidally oscillating, Rashba-type spin-orbit interaction (SOI). The periodic structure of the rectangular lattice produces a spin-dependent miniband structure for static SOI. Using Floquet theory, transmission peaks are shown to appear in the mini-bandgaps as a consequence of the additional, time-dependent SOI. A detailed analysis shows that this effect is due to the generation of harmonics of the driving frequency, via which, e.g., resonances that cannot be excited in the case of static SOI become available. Additionally, the transmitted current shows space- and time-dependent partial spin polarization, in other words, polarization waves propagate through the superlattice.

Normally, quantum fluctuations enhance the escape from metastable states in the presence of dissipation. Here we show that dissipation can enhance the stability of a quantum metastable system, consisting of a particle moving in a strongly asymmetric double well potential, interacting with a thermal bath. We find that the escape time from the metastable region has a nonmonotonic behavior versus the system-bath coupling and the temperature, producing a stabilizing effect.

We develop a conceptually simple scheme based on a master-equation approach to evaluate the full-counting statistics (FCS) of elastic and inelastic off-resonant tunneling (cotunneling) in quantum dots (QDs) and molecules. We demonstrate the method by showing that it reproduces known results for the FCS and shot noise in the cotunneling regime. For a QD with an excited state, we obtain an analytic expression for the cumulant generating function (CGF) taking into account elastic and inelastic cotunneling. From the CGF we find that the shot noise above the inelastic threshold in the cotunneling regime is inherently super-Poissonian when external relaxation is weak. Furthermore, a complete picture of the shot noise across the different transport regimes is given. In the case where the excited state is a blocking state, strongly enhanced shot noise is predicted both in the resonant and cotunneling regimes.

We theoretically study charge noise generated by excited neutral modes, which impinge on the quantum point contact of a quantum Hall bar with filling fraction $\nu =2/3$. The noise is computed for thermally excited neutral modes as well as for biased neutral modes with dipole-fermion excitations. Within the dipole-fermion picture, we show that the noise arising from two colliding modes can be suppressed due to Pauli blocking and be nonuniversal due to random edge disorder, but becomes universal upon disorder averaging. The ratio of noise due to two colliding neutral modes and noise due to only one such mode is smaller for dipole fermions than for thermal excitations, thus providing evidence for the different quantum statistics of the two types of excitations.

We report on an observation of a fractional quantum Hall effect in an ultrahigh-quality two-dimensional hole gas hosted in a strained Ge quantum well. The Hall resistance reveals precisely quantized plateaus and vanishing longitudinal resistance at filling factors $\nu =2/3,4/3$, and $5/3$. From the temperature dependence around $\nu =3/2$ we obtain the composite fermion mass of $m^{☆}\approx 0.4m_e$, where $m_e$ is the mass of a free electron. Owing to large Zeeman energy, all observed states are spin polarized and can be described in terms of spinless composite fermions.

We describe the conductance of a normal-superconducting junction in systems with Landau levels that preserve time-reversal symmetry. Those Landau levels have been observed in strained honeycomb lattices. The current is carried along the edges in both the normal and superconducting regions. When the Landau levels in the normal region are half filled, the Andreev reflection is maximal and the conductance plateaus have a peak as a function of the filling factor. The height of those peaks is quantized at $4e^2/h$. The interface of the junction has Andreev edge states, which form a coherent superposition of electrons and holes that can carry a net valley current. We identify unique experimental signatures for superconductivity in time-reversal-invariant Landau levels.

A nodal topological lattice is a form of magnetic crystal with topologically nontrivial spin texture, which further exhibits a periodic array of nodes with vanishing magnetization. An electronic structure for conduction electrons strongly Hund coupled to such a nodal topological lattice is examined. Our analysis shows that each node attracts two localized states which form narrow bands through internode hybridization within the mid-gap region. Nodal bands carry a Chern number under suitable perturbations, suggesting their potential role in the topological Hall effect. Enhancement of the density of states near zero energy observable in a tunneling experiment will provide a signature of the formation of a nodal topological lattice.

Two Luttinger liquids, with an equal density and opposite sign of charge carriers, may exhibit enhanced excitonic correlations. We term such a system an exciton quasicondensate, with a possible realization being two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that this quasiexciton condensate can be stabilized in an extended range of parameters, in both spinless and spinful systems. We calculate the interwire tunneling current-voltage characteristic, and find that a negative differential conductance is a signature of excitonic correlations. For spinful electrons, the excitonic regime is shown to be distinct from the usual quasi-long-range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their interwire tunneling current-voltage characteristics. In the quasiexciton condensate regime the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both the Wigner crystal and the excitonic regime are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one-dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.

Mn is used as a dopant to improve the electromechanical properties of perovskite oxides. We investigate the effects of Mn defects and associated vacancies on the electronic and atomic properties of ${\mathrm{BaTiO}}_3$. Using density functional theory (DFT) and $\mathrm{DFT}+U$ we investigate the equilibrium geometry and electronic properties of the Mn ion on $A$ or $B$ sites and with compensating oxygen vacancies. We study the change in the oxidation state of Mn in response to local environment changes, such as the presence of oxygen vacancies.

We have performed longitudinal magnetoresistance measurements on heavily $n$-doped silicon for donor concentrations exceeding the critical value for the metal-nonmetal transition. The results are compared to those from a many-body theory where the donor electrons are assumed to reside at the bottom of the many-valley conduction band of the host. Good qualitative agreement between theory and experiment is obtained.

By comprehensive neutron diffraction measurements we have studied the magnetic structure of aegirine $\left({\mathrm{NaFeSi}}_2{\mathrm{O}}_6\right)$ in and above its multiferroic phase. Natural aegirine exhibits two magnetic transitions into incommensurate magnetic order with a propagation vector of ${\vec{k}}_{\mathrm{inc}}=(0,\sim 0.78,0)$. Between 9 and 6 K, we find a transverse spin-density wave with moments pointing near the $c$ direction. Below 6 K, magnetic order becomes helical and spins rotate in the $ac$ plane. The same irreducible representation is involved in the two successive transitions. In addition, the ferroelectric polarization $\vec{P}$ appearing along the $b$ direction cannot be described by the most common multiferroic mechanism but follows $\vec{P}\propto {\vec{S}}_i\times {\vec{S}}_j$. Synthetic ${\mathrm{NaFeSi}}_2{\mathrm{O}}_6$ does not exhibit the pure incommensurate helical order but shows coexistence of this order with a commensurate magnetic structure. By applying moderate pressure to natural aegirine, we find that the incommensurate magnetic ordering partially transforms to the commensurate one, underlining the nearly degenerate character of the two types of order in ${\mathrm{NaFeSi}}_2{\mathrm{O}}_6$.

We report comprehensive studies of the crystallographic, magnetic, and thermal properties of a spinel-type magnetically frustrated compound, $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$, and a magnetically diluted system, $\mathrm{C}{\mathrm{o}}_{\text{1}\text{−}x}\mathrm{Z}{\mathrm{n}}_x\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at $x<0.6$. A multicritical point was apparent at $T=3.4\mathrm{K}$ and $x=0.12$, where the antiferromagnetic, spin-glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below $x=0.6$. At $x\sim 0.6$, magnetic susceptibility and specific heat were described by temperature power laws, $\chi \sim C/T\sim T^{-\delta }$, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an $x$-temperature phase diagram in the range $0\le x\le 1$ for $\mathrm{C}{\mathrm{o}}_{\text{1}\text{−}x}\mathrm{Z}{\mathrm{n}}_x\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$. The transition temperature of $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.

We directly trace the near- and midinfrared transmission change of a $\mathrm{V}{\mathrm{O}}_2$ thin film during an ultrafast insulator-to-metal transition triggered by high-field multiterahertz transients. Nonthermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analog of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1 Å.

We study the two-dimensional Hubbard model in the weak-coupling regime and compare the self-energy obtained from various approximate diagrammatic schemes to the result of diagrammatic Monte Carlo simulations, which sum up all weak-coupling diagrams up to a given order. While dynamical mean-field theory provides a good approximation for the local part of the self-energy, including its frequency dependence, the partial summation of bubble and/or ladder diagrams typically yields worse results than second-order perturbation theory. Even widely used self-consistent schemes such as $GW$ or the fluctuation-exchange approximation (FLEX) are found to be unreliable. Combining the dynamical mean-field self-energy with the nonlocal component of $GW$ in $GW\hspace{0.17em}+\hspace{0.17em}\mathrm{DMFT}$ yields improved results for the local self-energy and nonlocal self-energies of the correct order of magnitude, but here, too, a more reliable scheme is obtained by restricting the nonlocal contribution to the second-order diagram. FLEX + DMFT is found to give accurate results in the low-density regime, but even worse results than FLEX near half-filling.

We investigate the influence of electrical charge noise on a resonant exchange (RX) qubit in a triple quantum dot. This RX qubit is a variation of the exchange-only spin qubit which responds to a narrow-band resonant frequency. Our noise model includes uncorrelated charge noise in each quantum dot giving rise to two independent (noisy) bias parameters $\varepsilon$ and $\mathrm{\Delta }$. We calculate the energy splitting of the two qubit states as a function of these two bias detuning parameters to find “sweet spots,” where the qubit is least susceptible to noise. Our investigation shows that such sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters. The location of the sweet spots in the $(\varepsilon ,\mathrm{\Delta })$ plane depends on the hopping strength and asymmetry between the quantum dots. In the regime of weak charge noise, we identify a new favorable operating regime for the RX qubit based on these sweet spots.

Strong Rashba spin-orbit coupling (SOC) of two-dimensional electron gas (2DEG) at the oxide interface ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ underlies a variety of exotic physics, but its nature is still under debate. We derive an effective Hamiltonian for the 2DEG at the oxide interface ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ and find a different anisotropic Rashba SOC for the $d_{xz}$ and $d_{yz}$ orbitals. This anisotropic Rashba SOC leads to anisotropic static spin susceptibilities and also distinctive behavior of the spin Hall conductivity. These unique spin responses may be used to determine the nature of the Rashba SOC experimentally and shed light on the orbital origin of the 2DEG.