Without access to the full quantum state, modeling dissipation in an open system requires approximations. The physical soundness of such approximations relies on using realistic microscopic models of dissipation that satisfy completely positive dynamical maps. Here we present an approach based on the use of the Bohmian conditional wave function that, by construction, ensures a completely positive dynamical map for either Markovian or non-Markovian scenarios while allowing the implementation of realistic dissipation sources. Our approach is applied to compute the current-voltage characteristic of a resonant tunneling device with a parabolic-band structure, including electron-lattice interactions. A stochastic Schrödinger equation is solved for the conditional wave function of each simulated electron. We also extend our approach to (graphenelike) materials with a linear band structure using Bohmian conditional spinors for a stochastic Dirac equation.

We extend our recent result for the spin-relaxation time due to acoustic electron-phonon scattering in degenerate bands with spin mixing [New J. Phys. 18, 023012 (2016)] to include interactions with optical phonons, and present a numerical evaluation of the spin-relaxation time for intraband hole-phonon scattering in the heavy-hole (HH) bands of bulk GaAs. Comparing our computed spin-relaxation times to the conventional Elliott-Yafet result quantitatively demonstrates that the latter underestimates the spin-relaxation time because it does not correctly describe how electron-phonon interactions change the (vector) spin expectation value of the single-particle states. We show that the conventional Elliott-Yafet spin relaxation time is a special case of our result for weak spin mixing.

A relationship is derived between differences in electric polarization between bands and the “shift vector” that controls part of a material's bulk photocurrent, then demonstrated in several models. Electric polarization has a quantized gauge ambiguity and is normally observed at surfaces via the surface charge density, while shift current is a bulk property and is described by shift vector gauge invariant at each point in momentum space. They are connected because the same optical transitions that are described in shift currents pick out a relative gauge between valence and conduction bands. We also discuss subtleties arising when there are points at the Brillouin zone where optical transitions are absent. We conclude that two-dimensional materials with significant interband polarization differences should have high bulk photocurrent, meaning that the modern theory of polarization can be used as a straightforward way to search for bulk photovoltaic material candidates.

Rabi oscillations of a two-level atom appear as a quantum interference effect between the amplitudes associated with atomic superpositions, in analogy with the classic double-slit experiment which manifests a sinusoidal interference pattern. By extension, through direct detection of time-resolved resonance fluorescence from a quantum-dot neutral exciton driven in the Rabi regime, we experimentally demonstrate triple-slit-type quantum interference via quantum erasure in a V-type three-level artificial atom. This result is of fundamental interest in the experimental studies of the properties of V-type three-level systems and may pave the way for further insight into their coherence properties as well as applications for quantum information schemes. It also suggests quantum dots as candidates for multipath-interference experiments for probing foundational concepts in quantum physics.

We investigate the effects of spin-orbit coupling (SOC) in a three-orbital impurity model with a Kanamori interaction using the numerical renormalization group method. We focus on the impurity occupancy $N_d=2$ relevant to the dynamical mean-field theory studies of Hund's metals. Depending on the strength of SOC $\lambda$, we identify three regimes: the usual Hund's impurity for $\left|\lambda \right|<{\lambda }_c$, the van Vleck nonmagnetic impurity for $\lambda >{\lambda }_c$, and a $J=2$ impurity for $\lambda <-{\lambda }_c$. They all correspond to a Fermi liquid but with very different quasiparticle phase shifts and different physical properties. The crossover between these regimes is controlled by an emergent scale, the orbital Kondo temperature ${\lambda }_c=T_K^{\mathrm{orb}}$, that drops with increasing interaction strength. This implies that oxides with strong electronic correlations are more prone to the effects of spin-orbit coupling.

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in recent years. One of the leading contenders is the negatively charged nitrogen-vacancy (NV) defect in diamond with visible emission, but an alternative solution in a technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host a NV defect that could be an alternative qubit candidate with emission in the near infrared region. However, there is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including $3C, 4H$, and $6H$, and we also provide new experimental data in $4H$ SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC, providing detailed ionization energies of NV defects in SiC, which reveals the critical optical excitation energies for ionizing these qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

We have used angle-resolved photoemission spectroscopy to investigate the band structure of ${\mathrm{ReS}}_2$, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along $k_z$. These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ${\mathrm{ReS}}_2$, and how it can be expected to evolve when thinned to a single layer.

We investigate the time evolution towards the asymptotic steady state of a one-dimensional interacting system after a quantum quench. We show that at finite times the latter induces entanglement between right- and left-moving density excitations, encoded in their cross-correlators, which vanishes in the long-time limit. This behavior results in a universal time decay $\propto t^{-2}$ of the system spectral properties, in addition to nonuniversal power-law contributions typical of Luttinger liquids. Importantly, we argue that the presence of quench-induced entanglement clearly emerges in transport properties, such as charge and energy currents injected in the system from a biased probe and determines their long-time dynamics. In particular, the energy fractionalization phenomenon turns out to be a promising platform to observe the universal power-law decay $\propto t^{-2}$ induced by entanglement and represents a novel way to study the corresponding relaxation mechanism.

The temperature-induced structural and electronic transformation in ${\mathrm{VO}}_2$ between the monoclinic M1 and tetragonal rutile phases was studied by means of ab initio molecular dynamics, based on density functional theory with Hubbard correction (DFT+$U$). We compare the structure of both phases, transition temperature and atomic fluctuations both above and below the transition, as well as the phonon density of states and scattering intensity of centroid position, with experimental data. The good quantitative agreement indicates that the chosen DFT+$U$ scheme is able to provide a fairly good description of the energetics of the system. Analysis of the dynamical processes associated with the structural transformation was carried out on the atomic scale by following the time evolution of dimerization amplitudes of vanadium atom chains and the twisting angle of vanadium dimers. The electronic transition was studied by tracing the changes in projected densities of states and their correlation with the evolution of the structural transformation. Our results reveal a strong interconnection between the structural and electronic transformations and show that they take place on the same time scale.

We investigate the effects in the spectral properties of a massive Dirac system of the dynamical renormalization induced by disorder/impurity scattering within the self-consistent Born approximation. We show how these effects lead to a remarkable closing of the bandgap edge. Above a critical value $U_c$ of the impurity scattering the gap eventually closes, giving rise to a finite density of states at zero energy. We show that the bandgap closing stems from the quasiparticle dynamical renormalization, and it is not associated with the vanishing of the effective massive term. Incoherent processes are fundamental to describe such physics.

Magnetic aftereffect measurements on a synthetic antiferromagnet with strong perpendicular anisotropy are presented. Two types of magnetic transitions are observed in the CoFeB/Ta/CoFeB bilayer where the two ferromagnetic CoFeB layers are antiferromagnetically coupled: the transition from a parallel to an antiparallel CoFeB layers magnetization alignment and the transition between two possible antiparallel magnetization states. Magnetic relaxation measurements show a complete reversal in the case of the transition between the two antiferromagnetic configurations. The experimental data can be well fitted in the frame of extended exponential relaxation using the Fatuzzo-Labrune model. Consequently, the domain nucleation and domain-wall propagation can be studied as a function of temperature and applied field.

We analyze the formation of Majorana zero modes at the edge of a two-dimensional topological superconductor. In particular, we study a time-reversal-invariant triplet phase that is likely to exist in doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Upon the introduction of an in-plane magnetic field to the superconductor, a gap is opened in the surface modes, which induces localized Majorana modes. The position of these modes can be simply manipulated by changing the orientation of the applied field, yielding novel methods for braiding these states with possible applications to topological quantum computation.

We report the results of multifrequency high-magnetic-field electron-spin resonance (ESR) measurements on the highly frustrated antiferromagnet ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$. This compound is magnetically composed of two subsystems A and B, which are described as a spin-1/2 triangular-lattice Heisenberg antiferromagnet and a honeycomb-lattice $J_1-J_2$ Ising antiferromagnet, respectively. ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$ undergoes successive magnetic phase transitions at $T_{\mathrm{N}1}=12.0$ K and $T_{\mathrm{N}2}=3.0$ K. For a magnetic field $H$ parallel to the $c$ axis, subsystem B exhibits successive metamagnetic transitions with magnetization plateaus at one-third and one-half of the saturation magnetization. Below $T_{\mathrm{N}2}$, we observed collective ESR modes for $H\parallel c$, which are characteristic of a triangular-lattice Heisenberg antiferromagnet with weak easy-plane anisotropy. We also observed a local excitation mode, which can be assigned as a single flip of the Ising-like spin of subsystem B. From a detailed analysis of the collective and local ESR modes, combined with the magnetization process, we determined the magnetic parameters of subsystems A and B, and confirmed that the two subsystems are almost decoupled.

We investigate the formation of nonequilibrium superconducting states in driven alkali-doped fullerides $A_3{\mathrm{C}}_{60}$. Within a minimal three-orbital model for the superconductivity of these materials, it was recently demonstrated theoretically that an orbital-dependent imbalance of the interactions leads to an enhancement of superconductivity at equilibrium [M. Kim et al., Phys. Rev. B 94, 155152 (2016)]. We investigate the dynamical response to a time-periodic modulation of this interaction imbalance and show that it leads to the formation of a transient superconducting state which survives much beyond the equilibrium critical temperature $T_c$. For a specific range of frequencies, we find that the driving reduces superconductivity when applied to a superconducting state below $T_c$ while still inducing a superconducting state when the initial temperature is larger than $T_c$. These findings reinforce the relevance of the interaction-imbalance mechanism as a possible explanation of the recent experimental observation of light-induced superconductivity in alkali-doped fullerenes.

We study interaction effect of quantum spin Hall state in InAs/GaSb quantum wells under an in-plane magnetic field by using the self-consistent mean field theory. We construct a phase diagram as a function of intralayer and interlayer interactions, and identify two novel phases, a charge-/spin-density wave phase and an exciton condensate phase. The charge-/spin-density wave phase is topologically nontrivial with helical edge transport at the boundary, while the exciton condensate phase is topologically trivial. The Zeeman effect is strongly renormalized due to interaction in certain parameter regimes of the system, leading to a much smaller $g$ factor, which may stabilize the helical edge transport.

We study systems made of periodic arrays of one-dimensional quantum wires coupled by Coulomb interaction. Using bosonization an interacting metallic fixed point is obtained, which is shown to be a higher-dimensional analog of the Tomonaga-Luttinger liquid, or a sliding Luttinger liquid. This non-Fermi liquid metallic state, however, is unstable in the presence of weak interwire backscatterings, which favor charge density wave states and suppress pairing. Depending on the effective strength of the Coulomb repulsion and the size of interwire spacing, various charge density wave states are stabilized, including Wigner crystal states. Our method allows for the determination of the specific ordering patterns and corresponding energy and temperature scales.

We did the resistivity and scanning tunneling microscope/spectroscopy (STM/STS) experiments at different temperatures and magnetic fields to investigate the origin of the turn-on (t-o) phenomenon of $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$. There are two interesting observations. Firstly, magnetoresistance (MR) follows the Kohler's rule scaling: $\mathrm{MR}\sim {\left(H/{\rho }_0\right)}^m$ with $m\approx 1.92$ and the t-o temperature $T^{*}$ under different magnetic fields can also be scaled by $T^{*}\sim {\left(H\text{−}{H}_c\right)}^{\upsilon }$ with $\upsilon =1/2$. Secondly, a combination of compensated electron-hole pockets and a possible electronic structure phase transition induced by the temperature have been validated in $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$ by the STM/STS experiments. Compared with the STS of $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$ single crystal under $H=0$, the STS hardly changes even when the applied field is up to 7 T. The origins of the t-o phenomenon in $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$ are discussed. Meanwhile, we analyzed the universality and applicability of the t-o phenomenon in the extreme MR materials with almost balanced hole and electron densities as well as with other systems where the density of hole or electron is in a dominant position.

Albeit difficult to access experimentally, the density of states (DOS) is a key parameter in solid-state systems, which governs several important phenomena including transport, magnetism, thermal, and thermoelectric properties. We study DOS in an ensemble of potassium intercalated single-wall carbon nanotubes and show, using electron spin resonance spectroscopy, that a sizable number of electron states are present, which gives rise to a Fermi-liquid behavior in this material. A comparison between theoretical and the experimental DOS indicates that it does not display significant correlation effects, even though the pristine nanotube material shows a Luttinger-liquid behavior. We argue that the carbon nanotube ensemble essentially maps out the whole Brillouin zone of graphene, thus it acts as a model system of biased graphene.