We investigate the properties of Dirac electrons in a finite graphene sample under a perpendicular magnetic field that emerge when an in-plane electric bias is also applied. The numerical analysis of the Hofstadter spectrum and of the edge-type wave functions evidence the presence of shortcut edge states that appear under the influence of the electric field. The states are characterized by a specific spatial distribution, which follows only partially the perimeter, and exhibit ridges that connect opposite sides of the graphene plaquette. Two kinds of such states have been found in different regions of the spectrum, and their particular spatial localization is shown along with the diamagnetic moments that reveal their chirality. By simulating a four-lead Hall device, we investigate the transport properties and observe unconventional plateaus of the integer quantum Hall effect that are associated with the presence of the shortcut edge states. We show the contributions of the novel states to the conductance matrix that determine the new transport properties. The shortcut edge states resulting from the splitting of the $n=0$ Landau level represent a special case, giving rise to nontrivial transverse and longitudinal resistance.

We investigate both experimentally and theoretically a simple yet more robust and flexible alternative to Rabi oscillation-type biexciton preparation protocols traditionally used for semiconductor quantum dots. The quantum dot is excited by a strong laser pulse positively detuned from the two-photon resonance yielding an on demand initialization of the biexciton state by making use of the phonon-induced thermalization of the photon dressed states. It is shown that for excitation pulses in the picosecond range, a stable and high occupation of up to $C_{XX}=0.95\pm 0.02$ is reached. Notably, the generated photons show similar coherence properties as measured in the resonant two-photon scheme. This protocol is a powerful tool for the control of complex solid state systems combining radiative cascades, entanglement, and resonant cavity modes.

We study the equilibrium geometry and electronic structure of thin films of layered gray arsenic using ab initio density functional theory. In contrast to bulk gray As that is semimetallic, thin films display a significant band gap that depends sensitively on the number of layers, in-layer strain, layer stacking, and interlayer spacing. We find that metallic character can be introduced by increasing the number of layers beyond two or by subjecting semiconducting monolayers and bilayers to moderate tensile strain. The strain-induced metal-semiconductor transition is triggered by changes in the band ordering near the top of the valence band that causes an abrupt change from $\sigma$ to $\pi$ character of the frontier states.

We develop schemes for designing pulses that implement fast and precise entangling quantum gates in superconducting qubit systems despite the presence of nearby harmful transitions. Our approach is based on purposely involving the nearest harmful transition in the quantum evolution instead of trying to avoid it. Using analytical tools, we design simple microwave control fields that implement maximally entangling gates with fidelities exceeding 99% in times as low as 40 ns. We demonstrate our approach in a two-qubit circuit QED system by designing the two most important quantum entangling gates: a conditional-not gate and a conditional-z gate. Our results constitute an important step toward overcoming the problem of spectral crowding, one of the primary challenges in controlling multiqubit systems.

We study the extended Anderson model on the two-dimensional Penrose lattice, combining the real-space dynamical mean-field theory with the noncrossing approximation. It is found that the Coulomb repulsion between localized and conduction electrons does not induce a valence transition, but the crossover between the Kondo and mixed valence states is in contrast to the conventional periodic system. In the mixed-valence region close to the crossover, nontrivial valence distributions appear, characteristic of the Penrose lattice, demonstrating that the mixed-valence state coexists with local Kondo states in certain sites. The electric reconstruction in the mixed valence region is also addressed.

The electronic band structure of a prototype ${\text{Ni}}_2\text{MnGa}$ magnetic shape-memory alloy was studied in the weakly off-stoichiometric regime, using ${\text{Ni}}_{49.7}{\text{Mn}}_{29.1}{\text{Ga}}_{21.2}$ single-crystal samples. In the austenitic phase, isoenergetic $k$-space cuts through the entire Brillouin zone were measured by momentum-resolved photoelectron microscopy in the energy range $(E_{\mathrm{F}}-3\mathrm{eV},E_{\mathrm{F}})$, where $E_{\mathrm{F}}$ is the Fermi energy. The results are compared to simulations based on the ab initio density functional theory. Two regions were identified in the band structure. In the first one, between $E_{\mathrm{F}}-1\mathrm{eV}$ and $E_{\mathrm{F}}$, the minority spin $d\text{electrons}$ of Ni dominate the density of states showing a significant dispersion as a function of the $k$ vector. The second one, between $E_{\mathrm{F}}-3\mathrm{eV}$ and $E_{\mathrm{F}}-1\mathrm{eV}$, is controlled by majority-spin Mn $d\text{electrons}$. The dispersion is perturbed by a relatively high degree of disorder, producing an image of electronic states in the form of a spectral density of states.

Double quantum dot nanostructures embedded between two superconducting leads or in a superconducting ring have complex excitation spectra inside the gap which reveal the competition between different many-body phenomena. We study the corresponding two-impurity Anderson model using the nonperturbative numerical renormalization group (NRG) technique and identify the characteristic features in the spectral function in various parameter regimes. At half-filling, the system always has a singlet ground state. For large hybridization, we observe an inversion of excited interdot triplet and singlet states due to the level repulsion between two subgap singlet states. The Shiba doublet states split in two cases: (a) at nonzero superconducting phase difference and (b) away from half-filling. The most complex structure of subgap states is found when one or both dots are in the valence fluctuation regime. Doublet splitting can lead to a parity-changing quantum phase transition to a doublet ground state in some circumstances. In such cases, we observe very different spectral weights for the transitions to singlet or triplet excited Shiba states: the triplet state is best visible on the valence-fluctuating dot, while the singlets are more pronounced on the half-filled dot.

On the occasion of its 25th anniversary, we revise Chen's derivative rule for electron tunneling [C. J. Chen, Phys. Rev. B 42, 8841 (1990)] for the purpose of computationally efficient simulations of scanning tunneling microscopy (STM) based on first-principles electronic structure data. The revised model allows the weighting of tunneling matrix elements of different tip-orbital characters by an arbitrary energy-independent choice or based on energy-dependent weighting coefficients obtained by an expansion of the tip single-electron wave functions/density of states projected onto the tip-apex atom. Tip-orbital interference in the STM junction is included in the model by construction and can be analyzed quantitatively. As a further advantage, arbitrary tip geometrical orientations are included in the revised model by rotating the coordinate system of the tip apex using Euler angles and redefining the weighting coefficients of the tunneling matrix elements. We demonstrate the reliability of the model by applying it to two functionalized surfaces of recent interest where quantum interference effects play an important role in the STM imaging process: N-doped graphene and a magnetic ${\mathrm{Mn}}_2\mathrm{H}$ complex on the Ag(111) surface. We find that the proposed tunneling model is 25 times faster than the Bardeen method concerning computational time, while maintaining good agreement. Our results show that the electronic structure of the tip has a considerable effect on STM images, and the Tersoff-Hamann model does not always provide sufficient results in view of quantum interference effects. For both studied surfaces, we highlight the importance of interference between $s$ and $p_z$ tip orbitals that can cause a significant contrast change in the STM images. Our method, thus, provides a fast and reliable tool for calculating STM images based on Chen's derivative rule, taking into account the electronic structure and local geometry of the tip apex.

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic magnetic force microscopy, domain structures for the layered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature $T_C$, and appear either via the application of a $c$-axis magnetic field, or by decreasing the temperature further. At temperatures well below $T_C$, new domain walls, stable at zero field, can be formed by the application of a $c$-axis field. Magnetic structures are seen also at temperatures above $T_C$: these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.

We systematically investigated the anisotropic in-plane resistivity of iron tellurides including three kinds of impurity atoms: excess Fe, Se substituted for Te, and Cu substituted for Fe. Sizable resistivity anisotropy was found in the magnetostructurally ordered phase, whereas the sign is opposite (${\rho }_a>{\rho }_b$, where the $b$-axis parameter is shorter than the $a$-axis one) to that observed in the transition-metal doped iron arsenides $\left({\rho }_a<{\rho }_b\right)$. On the other hand, our results demonstrate that the magnitude of the resistivity anisotropy in the iron tellurides is correlated with the amount of impurities, implying that the resistivity anisotropy originates from an exotic impurity effect like that in the iron arsenides. This suggests that the anisotropic carrier scattering by impurities is a universal phenomenon in the magnetostructurally ordered phase of the iron-based materials.

We study the role of thermal magnons in spin and heat transport across a normal-metal/insulating-ferromagnet interface, which is beyond an elastic electronic spin transfer. Using an interfacial exchange Hamiltonian, which couples spins of itinerant and localized orbitals, we calculate spin and energy currents for an arbitrary interfacial temperature difference and misalignment of spin accumulation in the normal metal relative to the ferromagnetic order. The magnonic contribution to spin current leads to a temperature-dependent torque on the magnetic order parameter; reciprocally, the coherent precession of the magnetization pumps spin current into the normal metal, the magnitude of which is affected by the presence of thermal magnons.

We use a tunable laser angle-resolved photoemission spectroscopy to study the electronic properties of the prototypical multiband BCS superconductor ${\mathrm{MgB}}_2$. Our data reveal a strong renormalization of the dispersion (kink) at $\sim 65\mathrm{meV}$, which is caused by the coupling of electrons to the $E_{2g}$ phonon mode. In contrast to cuprates, the 65 meV kink in ${\mathrm{MgB}}_2$ does not change significantly across $T_c$. More interestingly, we observe strong coupling to a second, lower energy collective mode at a binding energy of 10 meV. This excitation vanishes above $T_c$ and is likely a signature of the elusive Leggett mode.

We propose an approach to the problem of lattice distortions at low temperatures and ambient pressure in the solid hydrogens in their rotational ground states that explicitly accounts for the molecular nature of the constituent particles. The model is based on the idea that the second-order rotation-related correction to the ground-state energy depends on the lattice parameters. The calculated ground-state rotation-related contributions, ${\delta }_{\mathrm{gs}}=c/a-{(8/3)}^{1/2}$, are negative for all species, amounting to about $-1.5\times {10}^{-5}$ for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$, whereas for HD this contribution is about $-0.6\times {10}^{-3}$, which is roughly 50 times larger. This substantial difference stems from the fact that the rotational dynamics in the homonuclear solids and in HD differ appreciably. The approach can be generalized to high pressures.

Iridates represent a unique material system that possesses both strong spin-orbit coupling and electron correlation. The interplay between spin-orbit coupling and correlation could facilitate the emergence of novel electronic and magnetic states. In this work, we report on a systematic study of magnetism in the double perovskite iridate ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and its hole-doped compounds $\left({\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\right){}_2{\mathrm{ZnIrO}}_6$ via dc magnetization measurement, heat capacity characterization, electron spin resonance spectroscopy, and simple modeling. Undoped ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ undergoes two magnetic transitions, at $T_1\sim 7.3$ K and $T_2\sim 8.5$ K, respectively. While magnetic hysteresis loops with large remnant moments are observed below these two transition temperatures, the corresponding magnetic states are shown to be canted antiferromagnetic by the linear increase in magnetization in the high field region along with the observation of antiferromagnetic resonance. The nature of the canted antiferromagnetic states with large canting angles can be understood by a simple model that includes both the Heisenberg exchange interaction and the Dzyaloshinskii-Moriya interaction. With the introduction of ${\mathrm{Ir}}^{5+}$ by ${\mathrm{Sr}}^{2+}$ doping, the canted antiferromagnetic phases are suppressed, accompanied by an enhancement of electrical conductivity.

We report direct measurements of hole spin lifetimes in ferromagnetic GaMnAs carried out by time- and polarization-resolved spectroscopy. Below the Curie temperature, ultrafast photoexcitation of GaMnAs with linearly polarized light is shown to create a nonequilibrium hole spin population via dynamical polarization of the holes through $p\text{−}d$ exchange scattering with ferromagnetically ordered Mn spins. The system is then observed to relax in a distinct three-step recovery process: (i) a femtosecond hole spin relaxation, on the scale of 160–200 fs; (ii) a picosecond hole energy relaxation, on the scale of 1–2 ps; and (iii) a coherent, damped Mn spin precession with a period of 250 ps. The transient amplitude of the hole spin relaxation component diminishes with increasing temperature, directly following the ferromagnetic order of GaMnAs, while the hole energy amplitude shows negligible temperature change. Our results serve to establish the hole spin lifetimes in the ferromagnetic semiconductor GaMnAs, at the same time demonstrating a spectroscopic method for studying nonequilibrium hole spins in the presence of magnetic order and spin-exchange interaction.

The accuracy of the many-body perturbation theory $GW$ formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential $W$ upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene, and the ${\text{C}}_{60}$ fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories.

We study a string-net ladder in the presence of a string tension. Focusing on the simplest non-Abelian anyon theory with a quantum dimension larger than two, we determine the phase diagram and find a Russian doll spectrum featuring size-independent energy levels as well as highly degenerate zero-energy eigenstates. At the self-dual points, we compute the gap exactly by using a mapping onto the Temperley-Lieb chain. These results are in stark contrast with the ones obtained for Fibonacci or Ising theories.

We contrast the transport properties (dc resistivity, Seebeck coefficient), optical conductivity, spectral functions, dynamical magnetic susceptibility, and the nuclear magnetic resonance $1/T_1$ spin-lattice relaxation rate of the repulsive and attractive infinite-dimensional Hubbard models in the paramagnetic phase for a generic band filling. The calculations are performed in a wide temperature interval using the dynamical mean-field theory with the numerical renormalization group as the impurity solver. The attractive case exhibits significantly more complex temperature dependencies which can be explained by the behavior of the half-filled Hubbard model in external magnetic field with constant magnetization, to which the attractive Hubbard model maps through the partial particle-hole transformation. The resistivity is nonmonotonous for the strongly attractive case: it peaks significantly above the Mott-Ioffe-Regel value at a temperature $T_{\max }$ where the quasiparticle band disappears. For both signs of $U$ we find particle-hole asymmetry in the self-energy at low energies, but with the opposite kind of excitations having longer lifetime. This leads to a strong suppression of the slope of the Seebeck coefficient in the attractive case rather than an enhancement as in the repulsive case. The spin-lattice relaxation rate in the strongly attractive case has a nonmonotonic temperature dependence, thereby revealing the pairing fluctuations.

We investigate the spin-dependent thermoelectric effects in magnetic graphene in both diffusive and ballistic regimes. Employing the Boltzmann and Landauer formalisms we calculate the spin and charge Seebeck coefficients (thermopower) in magnetic graphene varying the spin splitting, temperature, and doping of the junction. It is found that while in normal graphene the temperature gradient drives a charge current, in the case of magnetic graphene a significant spin current is also established. In particular we show that in the undoped magnetic graphene in which different spin carriers belong to conduction and valence bands, a pure spin current is driven by the temperature gradient. In addition it is revealed that profound thermoelectric effects can be achieved at intermediate easily accessible temperatures when the thermal energy is comparable with Fermi energy $k_{B}T\lesssim \mu$. By further investigation of the spin-dependent Seebeck effect and a significantly large figure of merit for spin thermopower ${\mathcal{Z}}_{\mathrm{sp}}T$, we suggest magnetic graphene as a promising material for spin-caloritronics studies and applications.

We report low-temperature transport studies of parallel double quantum dots formed in GaSb/InAsSb core-shell nanowires. At negative gate voltages, regular patterns of Coulomb diamonds are observed in the charge stability diagrams, which we ascribe to single-hole tunneling through a quantum dot in the GaSb core. As the gate voltage increases, the measured charge stability diagram indicates the appearance of an additional quantum dot, which we suggest is an electron quantum dot formed in the InAsSb shell. We find that an electron-hole interaction induces shifts of transport resonances in the source-drain voltage from which an average electron-hole interaction strength of $2.9\pm 0.3\mathrm{meV}$ is extracted. We also carry out magnetotransport measurements of a hole quantum dot in the GaSb core and extract level-dependent $g$ factors and a spin-orbit interaction.