We study the role of different orientations of an applied magnetic field as well as the interplay of structural asymmetries on the characteristics of eigenstates in a quantum ring system. We use a nearly analytical model description of the quantum ring, which allows for a thorough study of elliptical deformations and their influence on the spin content and Berry phase of different quantum states. The diamagnetic shift and Zeeman interaction compete with the Rashba spin-orbit interaction, induced by confinement asymmetries and external electric fields, to change spin textures of the different states. Smooth variations in the Berry phase are observed for symmetric quantum rings as a function of applied magnetic fields. Interestingly, we find that asymmetries induce nontrivial Berry phases, suggesting that defects in realistic structures would facilitate the observation of geometric phases.

We show that infinite matrix product states (MPSs) constructed from conformal field theories can describe ground states of one-dimensional critical systems with open boundary conditions. To illustrate this, we consider a simple infinite MPS for a spin-1/2 chain and derive an inhomogeneous open Haldane-Shastry model. For the spin-1/2 open Haldane-Shastry model, we derive an exact expression for the two-point spin correlation function. We also provide an $\mathrm{SU}\left(n\right)$ generalization of the open Haldane-Shastry model and determine its twisted Yangian generators responsible for the highly degenerate multiplets in the energy spectrum.

A strong diamagnetic response of a wire medium with a finite wire radius is reported. Contrary to the previous works where it was assumed that the wire medium exhibits only an electric response, we show that the nonzero magnetic susceptibility has to be taken into account for a proper effective medium description of the wire medium. Analytical and numerical results are supported by experimental measurements.

We identify qualitative trends in the stacking sequence dependence of carrier-carrier interaction phenomena in multilayer graphene. Our theory is based on an approach which explicitly exhibits the important role in interaction phenomena of the momentum-direction-dependent intersite phases determined by the stacking sequence. Using this method, we calculate and compare the self-energies, density-density response functions, collective modes, and ground-state energies of several different few-layer graphene systems. The influence of electron-electron interactions on important electronic properties can be understood in terms of competition between intraband exchange, interband exchange, and correlation contributions that vary systematically with the stacking arrangement.

We measured the polarized optical conductivity of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ from room temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid regime, and the hidden-order phase. The normal state is characterized by an anisotropic behavior between the $ab$ plane and $c$-axis responses. The $ab$-plane optical conductivity is strongly influenced by the formation of the coherent Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV leads to a spectral weight transfer to mid-infrared energies. The $c$-axis conductivity has a different behavior: the Drude peak already exists at 300 K and no particular anomaly or gap signature appears in the coherent Kondo liquid regime. When entering the hidden-order state, both polarizations see a dramatic decrease in the Drude spectral weight and scattering rate, compatible with a loss of about 50% of the carriers at the Fermi level. At the same time a density-wave-like gap appears along both polarizations at about 6.5 meV at 5 K. This gap closes respecting a mean-field thermal evolution in the $ab$ plane. Along the $c$ axis it remains roughly constant and it “fills up” rather than closing.

Using x-ray emission spectroscopy and absorption spectroscopy, it has been possible to directly access the states in the unoccupied conduction bands that are involved with $5f$ and $6d$ covalency in oxidized uranium. By varying the oxidizing agent, the degree of $5f$ covalency can be manipulated and monitored, clearly and irrevocably establishing the importance of $5f$ covalency in the electronic structure of the key nuclear fuel, uranium dioxide.

We describe nonlinear phonon-thermoelectric devices where charge current and electronic and phononic heat currents are coupled, driven by voltage and temperature biases, when phonon-assisted inelastic processes dominate the transport. Our thermoelectric transistors and rectifiers can be realized in a gate-tunable double quantum-dot system embedded in a nanowire which is realizable within current technology. The inelastic electron-phonon scattering processes are found to induce pronounced charge, heat, and cross rectification effects, as well as a thermal transistor effect that, remarkably, can appear in the present model even in the linear-response regime without relying on the onset of negative differential thermal conductance.

We study six different two-dimensional (2D) allotropes of carbon, silicon, germanium, and tin by means of the ab initio density functional theory for the ground state and approximate methods to calculate their electronic structures, including quasiparticle effects. Four of the investigated allotropes are based on dumbbell geometries, one on a kagome lattice, and one on the graphenelike hexagonal structure for comparison. Concerning carbon, our calculations of the cohesive energies clearly show that the hexagonal structure (graphene) is most stable. However, in the case of Si and Ge, the dumbbell structures, particularly the large honeycomb dumbbell (LHD) geometries, are energetically favored compared to the $s{p}^2/s{p}^3$-bonded hexagonal lattice (i.e., silicene and germanene). The main reason for this is the opening of a band gap in the honeycomb dumbbell arrangements. The LHD sheet crystals represent indirect semiconductors with a $K\to \mathrm{\Gamma }$ gap of about 0.5 eV. In the Sn case we predict the ${\mathrm{MoS}}_2$-like symmetry to be more stable, in contrast to the stanene and LHD geometries predicted in literature. Our results for freestanding group-IV layers shine new light on recent experimental studies of group-IV overlayers on various substrates.

We use an analytical model to describe the magnetocrystalline anisotropy energy (MAE) in solids as a function of band filling. The MAE is evaluated in second-order perturbation theory, which makes it possible to decompose the MAE into a sum of transitions between occupied and unoccupied pairs. The model enables us to characterize the MAE as a sum of contributions from different, often competing terms. The nitridometalates ${\mathrm{Li}}_2\left[\left({\mathrm{Li}}_{1-x}{T}_x\right)\mathrm{N}\right]$, with $T=$ Mn, Fe, Co, Ni, provide a system where the model is very effective because atomiclike orbital characters are preserved and the decomposition is fairly clean. Model results are also compared against MAE evaluated directly from first-principles calculations for this system. Good qualitative agreement is found.

The kagome spin-$\frac{1}{2}$ systems have attracted intensive attention in recent years as the primary candidate for hosting different gapped spin liquids (SLs). To uncover the nature of the novel quantum phase transition between the SL states, we study a minimum $XY$ model with the nearest-neighbor (NN) ($J_{xy}$), the second-NN, and the third-NN couplings ($J_{2xy}=J_{3xy}=J_{xy}^{\prime }$). We identify the time-reversal-symmetry-broken chiral SL (CSL) with the turn on of a small perturbation $J_{xy}^{\prime }\sim 0.06J_{xy}$, which is fully characterized by the fractionally quantized topological Chern number and the conformal edge spectrum as the $\nu =1/2$ fractional quantum Hall state. Interestingly, the NN $XY$ model ($J_{xy}^{\prime }=0$) is shown to be a critical SL state adjacent to the CSL, characterized by the gapless spin singlet and spin triplet excitations. The quantum phase transition from the CSL to the gapless critical SL is driven by the collapsing of the neutral (spin singlet) excitation gap. The effect of the NN spin-$z$ coupling $J_z$ is also studied, which leads to a quantum phase diagram with an extended regime for the gapless SL.

We determine the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet ${\mathrm{Ba}}_3{\mathrm{Mn}}_2{\mathrm{O}}_8$ using single-crystal neutron diffraction. We find that for magnetic fields between ${\mu }_{0}H=8.80\mathrm{T}$ and $10.56\mathrm{T}$ applied along the $\left[1\overline{1}0\right]$ direction the system exhibits spin density wave order with incommensurate wave vectors of type $(\eta ,\eta ,\epsilon )$. For ${\mu }_{0}H>10.56\mathrm{T}$, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the $\left[hh0\right]$ direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

Unraveling the influence that low dimensionality has upon the spin's stability in two-dimensional (2D) systems is instrumental for the efficient engineering of energy barriers in ultrathin magnetic layers. Taking rare-earth-based ultrathin multilayered nanostructures as a model system, we have investigated the dissimilar impact that low dimensionality and finite-size effects have upon the magnetic anisotropy energy (MAE) at the nanoscale. We conclusively show that the reduced dimensionality of the spin's system in 2D ferromagnetic layers imprints on the MAE constants a universal temperature decay as a quadratic power law of the reduced magnetization. This result is in agreement with predictions, although in marked contrast to the rank-dependent, thereby faster, decay of the MAE constants observed in three-dimensional nanostructures.

We study the interplay between Andreev (Majorana) bound states that form at the boundary of a (topological) superconductor and a train of microwave pulses. We find that the extra dynamical phase coming from the pulses can shift the phase of the Andreev reflection, resulting in the appearance of dynamical Andreev states. As an application we study the presence of the zero-bias peak in the differential conductance of a normal-topological superconductor junction—the simplest, yet somehow ambiguous, experimental signature for Majorana states. Adding microwave radiation to the measuring electrodes acts as a probe of the Andreev nature of the zero-bias peak.

We theoretically study the magnetization inside a normal metal induced in an $s$-wave superconductor/ferromagnetic metal/normal metal/ferromagnetic metal/$s$-wave superconductor $\left(S/F1/N/F2/S\right)$ Josephson junction. Using the quasiclassical Green's function method, we show that the magnetization becomes finite inside the $N$. The origin of this magnetization is due to odd-frequency spin-triplet Cooper pairs formed by electrons of equal and opposite spins, which are induced by the proximity effect in the $S/F1/N/F2/S$ junction. We find that the magnetization $M(d,\theta )$ in the $N$ can be decomposed into two parts, $M(d,\theta )=M^{\mathrm{I}}\left(d\right)+M^{\mathrm{II}}(d,\theta )$, where $\theta$ is the superconducting phase difference between the two $S\mathrm{s}$ and $d$ is the thickness of $N$. The $\theta$-independent magnetization $M^{\mathrm{I}}\left(d\right)$ exists generally in $S/F$ junctions, while $M^{\mathrm{II}}(d,\theta )$ carries all $\theta$ dependence and represents the fingerprint of the phase coherence between the two $S\mathrm{s}$ in Josephson junctions. The $\theta$ dependence thus allows us to control the magnetization in the $N$ by tuning $\theta$ for a fixed $d$. We show that the $\theta$-independent magnetization $M^{\mathrm{I}}\left(d\right)$ weakly decreases with increasing $d$, while the $\theta$-dependent magnetization $M^{\mathrm{II}}(d,\theta )$ rapidly decays with $d$. Moreover, we find that the time-averaged magnetization $\langle M^{\mathrm{II}}(d,\theta )\rangle$ exhibits a discontinuous peak at each resonance dc voltage $V_n=n\hslash {\omega }_{\mathrm{S}}/2e$ $(n$: integer) when dc voltage $V$ as well as ac voltage $v_{\mathrm{ac}}\left(t\right)$ with frequency ${\omega }_{\mathrm{S}}$ are both applied to the $S/F1/N/F2/S$ junction. This is because $M^{\mathrm{II}}(d,\theta )$ oscillates generally in time $t$ (ac magnetization) with $d\theta /dt=2e[V+v_{\mathrm{ac}}\left(t\right)]/\hslash$ and thus $\langle M^{\mathrm{II}}(d,\theta )\rangle =0$, but can be converted into the time-independent dc magnetization for the dc voltage at $V_n$. We also discuss that the magnetization induced in the $N$ can be measurably large in realistic systems. Therefore, the measurement of the induced magnetization serves as an alternative way to detect the phase coherence between the two $S\mathrm{s}$ in Josephson junctions. Our results also provide a basic concept for tunable magnetization in superconducting spintronics devices.

We use fully self-consistent GW calculations on diamond and silicon carbide to reparametrize the Heyd-Scuseria-Ernzerhof (HSE) exact exchange density functional for use in band structure calculations of semiconductors and insulators. We show that the thus modified functional is able to calculate the band structure of bulk Si, Ge, GaAs, and CdTe with good quantitative accuracy at a significantly reduced computational cost as compared to $GW$ methods, and also gives significantly improved band gap predictions in wide-gap ionic crystals as compared to the HSE06 parametrization. We discuss the limitations of this functional in low dimensions by calculating the band structures of single-layer hexagonal BN and ${\mathrm{MoS}}_2$, and by demonstrating that the diameter scaling of curvature induced band gaps in single-walled carbon nanotubes is still physically incorrect using our functional; we consider possible remedies to this problem.

By combining density functional theory and nonequilibrium Green's function, we study the electronic and transport properties of monolayer black phosphorus nanoribbons (PNRs). First, we investigate the band gap of PNRs and its modulation by the ribbon width and an external transverse electric field. Our calculations indicate a giant Stark effect in PNRs, which can switch on transport channels of semiconducting PNRs under low bias, inducing an insulator-metal transition. Next, we study the transport channels in PNRs via the calculations of the current density and local electron transmission pathway. In contrast to graphene and ${\mathrm{MoS}}_2$ nanoribbons, the carrier transport channels under low bias are mainly located in the interior of both armchair and zigzag PNRs, and immune to a small amount of edge defects. Last, a device of the PNR-based dual-gate field-effect transistor, with high on/off ratio of ${10}^3$, is proposed based on the giant electric-field tuning effect.