We investigate theoretically the spin purity of single holes confined in vertically coupled GaAs/AlGaAs quantum dots (QDs) under longitudinal magnetic fields. A unique behavior is observed for triangular QDs, by which the spin is largely pure when the hole is in one of the dots, but it becomes strongly mixed when an electric field is used to drive it into molecular resonance. The spin admixture is due to the valence-band spin-orbit interaction, which is greatly enhanced in $C_{3h}$ symmetry environments. The strong yet reversible electrical control of hole spin suggests that molecules with $C_3$-symmetry QDs, like those obtained with [111] growth, can outperform the usual $C_2$-symmetry QDs obtained with [001] growth for the development of scalable qubit architectures.

We investigate relaxation dynamics in the system of two identical quantum dipole emitters (QDEs) located near a metal nanoparticle (MNP) exhibiting a dipolar localized surface plasmon (LSP) resonance at the frequency of the QDE radiative transition. Considering one QDE to be brought into an optically active excited state and weakly coupled to the resonant LSP, we show that a stable superposition state of two QDEs is formed during the transition time, which is much shorter than the QDE spontaneous decay time and determined by the efficiency of resonant interaction between the QDEs and induced LSP. It is elucidated that the superposition state is established as a result of redistribution of the energy of the initially excited QDE so that the corresponding steady-state QDE fields induced at the MNP site cancel each other. The degree of steady-state entanglement characterized by the concurrence is found dependent only on the ratio of distances between the QDEs and the MNP, reaching its maximum value of $\sim 0.65$, when the separation between the MNP and the initially excited QDE is larger by $\sim 20\%$ than the distance from the other QDE to the MNP.

We study the vibrational properties of graphene under combined shear and uniaxial tensile strain using density-functional perturbation theory. Shear strain always causes rippling instabilities with strain-dependent direction and wavelength; armchair strain contrasts this instability, enabling graphene stability in a large range of combined strains. A complementary description based on membrane elasticity theory nicely clarifies the competition of shear-induced instability and uniaxial tension. We also report the large strain-induced shifts of the split components of the $G$ optical phonon line, which may serve as a shear diagnostic. As to the electronic properties, we find that conical intersections move away from the Brillouin zone border under strain, and they tend to coalesce at large strains, making the opening of gaps difficult to assess. By a detailed search, we find that even at large strains, only small gaps in the tens-of-meV range open at the former Dirac points.

We have compared and contrasted magnetic, magnetocaloric, and magnetoresistive properties of Gd and Dy members of the rare-earth $\left(R\right)$ series $R\mathrm{F}{\mathrm{e}}_5\mathrm{A}{\mathrm{l}}_7$, crystallizing in $\mathrm{ThM}{\mathrm{n}}_{12}$ structure, known to order antiferromagnetically. Among other observations, we would like to emphasize the following findings: (i) There are multiple sign crossovers in the temperature $\left(T\right)$ dependence of isothermal entropy change $\left(\mathrm{\Delta }S\right)$ in the case of Dy compound. In addition to nil $\mathrm{\Delta }S$ at the magnetic compensation point known for two-magnetic-sublattice systems, there is an additional sign crossover at low temperatures, as though there is a “reentrant inverse-magnetocaloric” phenomenon. Corresponding sign reversals could also be observed in the magnetoresistance data. (ii) The plots of magnetoresistance versus magnetic field are found to be highly asymmetric with the reversal of the direction of magnetic field $\left(H\right)$ well below $T_N$ for both compounds, similar to those known for antiferromagnetic tunnel junctions. We attribute these to subtle changes in spin orientations of $R$ and Fe moments induced by $T$ and $H$.

Local temperature variations induced by spin-wave propagation are studied using a model that couples nonuniform magnetization dynamics and heat flow. We show that the remote heating at the sample edge reported recently [T. An et al., Nat. Mater. 12, 549 (2013)] is due to the geometry-induced gradual reduction of the effective field. We demonstrate that the same effect can be achieved by a reduction in the external field instead of a constriction at the edge and, furthermore, that both the location and the amount of energy to be delivered to the lattice can be controlled accurately this way.

We synthesized the novel $S=1/2{\mathrm{Mo}}_3$ cluster magnet ${\mathrm{Li}}_2{\mathrm{ScMo}}_3{\mathrm{O}}_8$ as well as the isomorphic compound ${\mathrm{Li}}_2{\mathrm{InMo}}_3{\mathrm{O}}_8$, and investigated their macroscopic and microscopic physical properties. ${\mathrm{Li}}_2{\mathrm{InMo}}_3{\mathrm{O}}_8$ showed magnetic ordering at $T_N\sim 12$ K with a ${120}^{\circ }$ structure, while no magnetic ordering is observed down to 0.5 K in ${\mathrm{Li}}_2{\mathrm{ScMo}}_3{\mathrm{O}}_8$ despite the strong antiferromagnetic interaction among clusters probed by the Weiss temperature. Results of the ${}^7\mathrm{Li}$ nuclear magnetic resonance spectra and the nuclear spin-lattice relaxation rate $\left(1/T_1\right)$ measurement show direct microscopic proof of the spin-disordered ground state. We discuss the difference in the ground state between ${\mathrm{Li}}_2{\mathrm{ScMo}}_3{\mathrm{O}}_8$ and ${\mathrm{Li}}_2{\mathrm{InMo}}_3{\mathrm{O}}_8$ using the description of localized magnetism based on spin frustration within the triangular lattice and that of inter- and intracluster charge fluctuations.

${\mathrm{CeB}}_6$ is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with nonmagnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to $T_{\text{N}}$ upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.

We consider transport through a Josephson junction consisting of a conventional $s$-wave superconductor coupled via a double quantum dot to a noncentrosymmetric superconductor with both singlet and triplet pairing. We calculate the Andreev bound state energies and the associated Josephson current. We demonstrate that the current-phase relation is a sensitive probe of the singlet-triplet ratio in the noncentrosymmetric superconductor. In particular, in the presence of an inhomogeneous magnetic field the system exhibits a $\phi$-junction behavior.

We report a theoretical study of the macroscopic quantum tunneling (MQT) in small Josephson junctions containing randomly distributed two-level systems. We focus on a Josephson phase escape for switching from the superconducting (the zero-voltage) state to a resistive one. Above the crossover temperature $T_{cr}$ the thermal fluctuations of the Josephson phase induce such a switching, and as $T<T_{cr}$ the regime of the MQT occurs. In the absence of two-level systems (TLSs) a magnetic field applied parallel to the junction plane results in a smooth reduction of $T_{cr}\left(\mathrm{\Phi }\right)$, where $\mathrm{\Phi }$ is an applied magnetic flux. As the TLSs are present in Josephson junctions we obtain a resonant enhancement of the MQT. This phenomenon manifests itself by a narrow peak in the dependence of $T_{cr}\left(\mathrm{\Phi }\right)$ occurring in the intermediate range of $\mathrm{\Phi }$, i.e., $0<\mathrm{\Phi }<{\varphi }_0$ (${\varphi }_0$ is the magnetic flux quantum). We explain this effect quantitatively by a strong resonant suppression of the potential barrier for the Josephson phase escape that is due to the coherent quantum Rabi oscillations in two-level systems present in the junction.

We study the real-time evolution of large open quantum spin systems in two spatial dimensions, whose dynamics is entirely driven by a dissipative coupling to the environment. We consider different dissipative processes and investigate the real-time evolution from an ordered phase of the Heisenberg or XY model towards a disordered phase at late times, disregarding unitary Hamiltonian dynamics. The corresponding Kossakowski-Lindblad equation is solved via an efficient cluster algorithm. We find that the symmetry of the dissipative process determines the time scales, which govern the approach towards a new equilibrium phase at late times. Most notably, we find a slow equilibration if the dissipative process conserves any of the magnetization Fourier modes. In these cases, the dynamics can be interpreted as a diffusion process of the conserved quantity.

The Kibble-Zurek mechanism (KZM) is generalized to a class of multilevel systems and applied to study the quenching dynamics of one-dimensional (1D) topological superconductors (TS) with open ends. Unlike the periodic boundary condition, the open boundary condition, which is crucial for the zero-mode Majorana states localized at the boundaries, requires one to consider many coupled levels. Our generalized KZM predictions agree well with the numerically exact results for the 1D TS. In particular, the generalized KZM explains well the Majorana-mode contribution to the topological defects, which utterly defies the traditional KZM. The inherent bound-state character and multilevel structure of the Majorana mode, which play key roles in the TS, are efficiently captured by the generalized KZM.

${\mathrm{Hg}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ at a critical doping $x=x_c\approx 0.17$ has a bulk dispersion which includes two linear cones meeting at a single point at zero energy, intersecting a nearly flat band, similar to the pseudospin-1 Dirac-Weyl system. In the presence of a finite magnetic field, these bands condense into highly degenerate Landau levels. We have numerically calculated the frequency-dependent magneto-optical and zero-field conductivity of this material using the Kane model. These calculations show good agreement with recent experimental measurements. We discuss the signature of the flat band and the split peaks of the magneto-optics in terms of general pseudospin-$s$ models and propose that the system exhibits a non-$\pi$-quantized Berry phase, found in recent theoretical work.

Electron-electron (e-e) and electron-hole (e-h) interactions are often associated with many exotic phenomena in correlated electron systems. Here, we report an observation of anomalous excitons at 3.75, 4.67, and 6.11 eV at 4.2 K in bulk ${\mathrm{SrTiO}}_3$. Fully supported by ab initio $GW$ Bethe-Salpeter equation calculations, these excitons are due to strong e-h and e-e interactions with different characters: 4.67 and 6.11 eV are resonant excitons and 3.75 eV is a bound Wannier-like exciton with an unexpectedly higher level of delocalization. Measurements and calculations on ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ for $0.0001\le x\le 0.005$ further show that the energy and spectral-weight of the excitonic peaks vary as a function of electron doping $\left(x\right)$ and temperature, which are attributed to screening effects. Our results show the importance of e-h and e-e interactions yielding to anomalous excitons and thus bring out a new fundamental perspective in ${\mathrm{SrTiO}}_3$.

A projected entangled pair state (PEPS) with ancillas can be evolved in imaginary time to obtain thermal states of a strongly correlated quantum system on a two-dimensional lattice. Every application of a Suzuki-Trotter gate multiplies the PEPS bond dimension $D$ by a factor $k$. It has to be renormalized back to the original $D$. In order to preserve the accuracy of the Suzuki-Trotter (ST) decomposition, the renormalization in principle has to take into account full environment made of the new tensors with the bond dimension $k\times D$. Here, we propose a self-consistent renormalization procedure operating with the original bond dimension $D$, but without compromising the accuracy of the ST decomposition. The iterative procedure renormalizes the bond using full environment made of renormalized tensors with the bond dimension $D$. After every renormalization, the new renormalized tensors are used to update the environment, and then the renormalization is repeated again and again until convergence. As a benchmark application, we obtain thermal states of the transverse field quantum Ising model on a square lattice, both infinite and finite, evolving the system across a second-order phase transition at finite temperature.

We report measurements of microwave spectra of two-dimensional electron systems hosted in dilute Al alloy ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ for a range of Landau level fillings $\nu$ around 1. For $\nu >0.8$ or $\nu <1.2$, the samples exhibit a microwave resonance whose frequency decreases as $\nu$ moves away from 1. A resonance with this behavior is the signature of solids of quasiparticles or quasiholes in the partially occupied Landau level, and was previously seen only in ultralow-disorder samples. For $\nu <0.8$ down to as low as $\nu =0.54$, a resonance in the spectra is still present in the Al alloy-disordered samples, although it is partially or completely suppressed at $\nu =\frac{3}{5}$ and $\frac{1}{2}$, and is strongly damped over much of this $\nu$ range. The resonance also shows a striking enhancement in peak frequency for $\nu$ just below $\frac{3}{4}$. We discuss possible explanations of the resonance behavior for $\nu <0.8$ in terms of the composite fermion picture.

For low-dimensional metallic structures, such as nanotubes, the exchange coupling between localized magnetic dopants is predicted to decay slowly with separation. The long-range character of this interaction plays a significant role in determining the magnetic order of the system. It has previously been shown that the interaction range depends on the conformation of the magnetic dopants in both graphene and nanotubes. Here we examine the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in carbon nanotubes in the presence of uniaxial strain for a range of different impurity configurations. We show that strain is capable of amplifying or attenuating the RKKY interaction, significantly increasing certain interaction ranges, and acting as a switch: effectively turning on or off the interaction. We argue that uniaxial strain can be employed to significantly manipulate magnetic interactions in carbon nanotubes, allowing an interplay between mechanical and magnetic properties in future spintronic devices. We also examine the dimensional relationship between graphene and nanotubes with regards to the decay rate of the RKKY interaction.

Experimental efforts to stabilize ferromagnetism in ultrathin films of transition metal oxides have so far failed, despite expectations based on density functional theory (DFT) and DFT+$U$. Here, we investigate one of the most promising materials, ${\mathrm{SrRuO}}_3$, and include correlation effects beyond DFT by means of dynamical mean-field theory. In agreement with experiment, we find an intrinsic thickness limitation for metallic ferromagnetism in ${\mathrm{SrRuO}}_3$ thin films. Indeed, we demonstrate that the realization of ultrathin ferromagnetic films is out of reach of standard thin-film techniques. Proposing charge carrier doping as an alternative route to manipulate thin films, we predict room-temperature ferromagnetism in electron-doped ${\mathrm{SrRuO}}_3$ ultrathin films.

Materials irradiated with multiple femtosecond laser pulses in subablation conditions are observed to develop various types of self-assembled morphologies that range from nanoripples to periodic microgrooves and quasiperiodic microspikes. Here, we present a physical scenario that couples electrodynamics, describing surface plasmon excitation, with hydrodynamics, describing Marangoni convection and counter-rolls, to elucidate this important subablation regime of light-matter interaction in which matter is being modified; however, the underlying process is not yet fully understood. The proposed physical mechanism could be generally applicable to practically any conductive material structured by ultrashort laser pulses; therefore it can be useful for the interpretation of further critical aspects of light-matter interaction.