We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24–275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.

We report the in-plane microwave surface impedance of a high-quality single crystal of $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br. In the superconducting state, we find three independent signatures of $d$-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate $\Gamma \sim T^3$; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above $T_c$, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below $\simeq 23$ K, yet resilient quasiparticles dominate the transport up to $\simeq 50$ K.

Electron transport in electric-field-driven tight-binding lattice coupled to fermion baths is comprehensively studied. We reformulate the problem by using the scattering-state method within the Coulomb gauge. Calculations show that the formulation justifies direct access to the steady state bypassing the time-transient calculations, which then makes the steady-state methods developed for quantum dot theories applicable to lattice models. We show that the effective temperature of the hot-electron induced by a dc electric field behaves as $T_{\mathrm{eff}}=C\gamma (\Omega /\Gamma )$ with a numerical constant $C$, tight-binding parameter $\gamma$, the Bloch oscillation frequency $\Omega$, and the damping parameter $\Gamma$. In the small damping limit $\Gamma /\Omega \to 0$, the steady state has a singular property with the electron becoming extremely hot in an analogy to the short-circuit effect. This leads to the conclusion that the dissipation mechanism cannot be considered as an implicit process, as treated in equilibrium theories. Finally, using the energy flux relation, we derive a steady-state current for interacting models where only on-site Green's functions are necessary.

The magnetic properties and Mott transition of the Hubbard model on the square lattice with frustration are studied at half-filling and zero temperature by the variational cluster approximation. When the onsite repulsion $U$ is large, the magnetically disordered state is realized in a highly frustrated region between the Néel and collinear phases, and the magnetic ordering with longer periodicity in space corresponding to the ordering vector $Q=(\pi ,\pi /2)$ is not found there. As for the Mott transition, in addition to the Mott gap and double occupancy, which clarify the nature of the transition, the structure of the self-energy in the spectral representation is studied in detail below and above the Mott transition point. The spectral structure of the self-energy is almost featureless in the metallic phase, but clear single dispersion appears in the Mott insulator phase and this gives rise to the Mott gap.

We extend the Fermi-liquid (FL) theory to include spin-orbit (SO) splitting of the energy bands, focusing on the Rashba SO coupling as an example. We construct the phenomenological Landau interaction function for such a system using the symmetry arguments and verify this construction by an explicit perturbative calculation. The Landau function is used to obtain the effective mass, compressibility, and stability conditions of the FL. It is shown that although the charge-sector properties, such as the effective mass and compressibility, are determined solely by well-defined quasiparticles, the spin-sector properties, such as the spin susceptibility, contain a contribution from damped states between the spin-split Fermi surfaces and thus cannot be fully described by the FL theory, except for the case of weak SO coupling. We derive some specific properties of a chiral FL and show, in particular, that for contact interaction spin-splitting of the Fermi velocities of Rashba subbands occurs because of the Kohn anomaly, also modified by SO coupling.

We implemented the calculation of hyperfine tensors into such plane wave supercell code working with the projector augmentation wave method that incorporates hybrid density functional theory and the contribution of the spin polarization of the core states. We show that the combination of HSE06 hybrid density functional together with the contribution of the core spin polarization provides accurate results on prominent point defects in various semiconductors, where the latter effect may be enormously large, in contrast to previous expectations. We briefly discuss the relevance of our results in the light of realization of solid-state quantum bits by paramagnetic point defects.

We study the thermoelectric effect between a conducting lead and a Majorana edge state. In the tunneling limit, we first use the Landauer-Büttiker formalism to derive the Mott formula relating the thermopower and the differential conductance between a conducting lead and a superconductor. When the tunneling takes place between a conducting lead and a Majorana edge state, we show that a nonvanishing thermopower can exist. Combining measurements of the differential conductance and the voltage induced by the temperature difference between the conducting lead and the edge state, the Mott formula provides a unique way to infer the temperature of the Majorana edge state.

We investigate near-equilibrium ballistic transport through a quantum point contact (QPC) along a GaAs/AlGaAs heterojunction with a transfer matrix technique as a function of temperature and the shape of the potential barrier in the QPC. Our analysis is based on a three-dimensional (3D) quantum-mechanical variational model within the Hartree-Fock approximation that takes into account the vertical depletion potential from ionized acceptors in GaAs and the gate-induced transverse confinement potential that reduce to an effective slowly varying one-dimensional (1D) potential along the narrow constriction. The calculated zero-temperature transmission exhibits a shoulder ranging from 0.3 to 0.6 depending on the length of the QPC and the profile of the barrier potential. The effect is a consequence of the compressibility peak in the 1D electron gas and is enhanced for antiferromagnetic interaction among electrons in the QPC, but is smeared out once temperature is increased by a few tenths of a Kelvin.

High-energy and $k$-space resolution angle-resolved photoemission spectroscopy experiments were achieved on nominally single and bilayer graphene grown by Si-flux assisted molecular beam epitaxy (MBE) on the C-face of SiC. This material shows the same structure as the graphene grown by standard high-temperature annealing of SiC, noticeably the rotational disorder and the very weak electronic coupling between stacked layers. The SiC substrate induces a strong doping by charge transfer, with a Dirac point located 320 meV below the Fermi level for monolayer graphene. The efficient screening by the successive graphene layers results in a reduction of this value to 190 meV for bilayer graphene. The opening of an energy band gap, whose width is inversely dependent on the thickness, is also reported. These measurements emphasize the potentialities of the Si-flux assisted MBE technique, more particularly for homogeneous low thickness graphene growth on the C-face of SiC.

Potter and Lee have demonstrated the presence of Majorana zero modes at the ends of quasi-one-dimensional (1D) $p+ip$ superconductors [Potter and Lee, Phys. Rev. Lett. 105, 227003 (2010)]. Here, we use conformal field theory methods to show that interedge tunneling of the vortex excitations along the length of the channel is crucial for such localization. We then show that localization of Majorana modes occurs also in quasi-1D channels of the $5/2$ fractional quantum Hall systems when modeled, following Moore and Read, as the $p+ip$ paired state of composite fermions. Finally, we propose a tunnel-interferometry experiment to detect these modes, which should show a $\pi$ phase shift of oscillations depending on whether or not a localized Majorana zero mode is present, which, in turn, can be controlled by varying the tunneling strength.

Transport measurements made on films of thallium-oxide ($n$-type semiconductor) are presented and discussed. The focus in this work is on the strongly-localized regime where charge transport is by variable-range hopping. It is demonstrated that, at liquid-helium temperatures, these films exhibit all the characteristic features of intrinsic electron glasses. These include a slow (logarithmic in time) conductance relaxation that may be induced by any of the following protocols: Quench cooling from high temperatures, sudden change of gate voltage, exposure to infrared radiation, and stressing the system with a non-Ohmic field. The microstructure of the films are characterized by electron microscopy and their carrier concentration are measured by Hall effect. Field-effect experiments reveal a memory dip that has a width compatible with the carrier concentration of the system as compared with previously studied electron glasses. It is observed that the common ingredient in all the systems that exhibit electron-glass effects is high carrier concentration suggesting that their localized sites may be multioccupied even when deep into the insulating regime. That lightly-doped semiconductors do not show intrinsic electron-glass effects is consistent with this empirical observation. The connection between the memory dip and the Coulomb gap is discussed in light of these findings.

Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than changing into the ferromagnetic metallic characteristics, which are expected from the conventional double-exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a principle for optically manipulating magnetism.

Periodic density functional theory (DFT)+U calculations using sufficiently large supercells to explicitly account for dopant disorder are reported for undoped Ca${}_2$CuO${}_2$Cl${}_2$ and for the doped Ca${}_{2-x}$Na${}_x$CuO${}_2$Cl${}_2$ ($x$ $=$ 0.125 and $x$ $=$ 0.250). For the undoped material, the charge-transfer antiferromagnetic character is properly reproduced. For the doped systems, a metallic character is predicted with the conduction band dominated by the O(2p) states, with doping having a small effect on the antiferromagnetic order of the Cu${}^{2+}$ cations within the CuO${}_2$ planes while maintaining the diamagnetic character in the rest of centers. The analysis of the spin density plots for the doped material evidences the appearance of a new feature reminiscent of the so-called Zhang-Rice singlet, thus providing unbiased independent support to the corresponding model Hamiltonian. However, it is also worth pointing out that the present DFT picture provides only an average static representation of what is expected to be a dynamic electronic structure.

One of the main challenges in spin qubits' studies associated with nitrogen vacancy impurities in diamond is to increase the coupling strength between the spins. With this task in mind we suggest a new type of a hybrid magneto-nano-electromechanical resonator, the functionality of which is based on a magnetic-field-induced deflection of an appropriate cantilever that oscillates between nitrogen vacancy (NV) spins in diamond. Specifically, we consider a $\mathrm{Si}\left(100\right)$ cantilever coated with a thin magnetic $\mathrm{Ni}$ film. As a new aspect of this study we utilize magnetoelastic stress and magnetic-field-induced torque to induce a controlled cantilever deflection. It is shown that, depending on the value of the system parameters, the induced asymmetry of the cantilever deflection substantially modifies the characteristics of the system. In particular, the coupling strength between the NV spins and the degree of entanglement can be controlled through magnetoelastic stress and magnetic-field-induced torque effects. Our theoretical proposal can be implemented experimentally with the potential of increasing several times the coupling strength between the NV spins. It finds that the coupling strength achieved by using our proposal enhances several times the maximal coupling strength reported before by Rabl et al. [P. Rabl, P. Cappellaro, M. V. Gurudev Dutt, L. Jiang, J. R. Maze, and M. D. Lukin, Phys. Rev. B 79, 041302(R) (2009)].

A comprehensive study of the room-temperature electrical and electrothermal transport of single-crystalline indium oxide (In${}_2$O${}_3$) and indium tin oxide (ITO) films over a wide range of electron concentrations is reported. We measured the room-temperature Hall mobility ${\mu }_H$ and Seebeck coefficient $S$ of unintentionally doped and Sn-doped high-quality, plasma-assisted molecular-beam-epitaxy-grown In${}_2$O${}_3$ for volume Hall electron concentrations $n_H$ from $7\times {10}^{16}$ cm${}^{-3}$ (unintentionally doped) to $1\times {10}^{21}$ cm${}^{-3}$ (highly Sn-doped, ITO). The resulting empirical $S\left(n_H\right)$ relation can be directly used in other In${}_2$O${}_3$ samples to estimate the volume electron concentration from simple Seebeck coefficient measurements. The mobility and Seebeck coefficient were modeled by a numerical solution of the Boltzmann transport equation. Ionized impurity scattering and polar optical phonon scattering were found to be the dominant scattering mechanisms. Acoustic phonon scattering was found to be negligible. Fitting the temperature-dependent mobility above room temperature of an In${}_2$O${}_3$ film with high mobility allowed us to find the effective Debye temperature (${\Theta }_D=700$ K) and number of phonon modes ($N_{\mathrm{OPML}}=1.33$) that best describe the polar optical phonon scattering. The modeling also yielded the Hall scattering factor $r_H$ as a function of electron concentration, which is not negligible ($r_H\approx 1.4$) at nondegenerate electron concentrations. Fitting the Hall-scattering-factor corrected concentration-dependent Seebeck coefficient $S\left(n\right)$ for nondegenerate samples to the numerical solution of the Boltzmann transport equation and to widely used, simplified equations allowed us to extract an effective electron mass of $m^{*}=(0.30\pm 0.03)m_e$ (with free electron mass $m_e$). The modeled mobility and Seebeck coefficient based on polar optical phonon and ionized impurity scattering describes the experimental results very accurately up to electron concentrations of ${10}^{19}$ cm${}^{-3}$, and qualitatively explains a mobility plateau or local maximum around ${10}^{20}$ cm${}^{-3}$. Ionized impurity scattering with doubly charged donors best describes the mobility in our unintentionally doped films, consistent with oxygen vacancies as unintentional shallow donors, whereas singly charged donors best describe our Sn-doped films. Our modeling yields a (phonon-limited) maximum theoretical drift mobility and Hall mobility of $\mu =190$ cm${}^2$/Vs and ${\mu }_H=270$ cm${}^2$/V$$s, respectively. Simplified equations for the Seebeck coefficient describe the measured values in the nondegenerate regime using a Seebeck scattering parameter of $r=-0.55$ (which is consistent with the determined Debye temperature), and provide an estimate of the Seebeck coefficient to lower electron concentrations. The simplified equations fail to describe the Seebeck coefficient around the Mott transition ($n_{\mathrm{Mott}}=5.5\times {10}^{18}$ cm${}^{-3}$) from nondegenerate to degenerate electron concentrations, whereas the numerical modeling accurately describes this region.

We demonstrate bias control of the efficiency of the hyperfine coupling between a single electron in an InAs quantum dot and the surrounding nuclear spins monitored through the positively charged exciton $X^{+}$ emission. In applied longitudinal magnetic fields, we vary simultaneously the correlation time of the hyperfine interaction and the nuclear spin relaxation time and thereby the amplitude of the achieved dynamic nuclear polarization under optical pumping conditions. In applied transverse magnetic fields, a change in the applied bias allows a switch from the anomalous Hanle effect to the standard electron spin depolarization curves.

We present detailed low-temperature transport measurements of a single quantum dot formed in an InGaAs/InP heterostructure with a strong tunnel coupling to the source and drain leads. The conventional spin-1/2 Kondo effect is observed for the quantum dot in the $N=9$ charge state. By changing the voltages applied to the quantum dot barrier gates, we find a zero-field splitting of the Kondo resonance and a zero-bias differential conductance, which shows a nonmonotonic in-plane magnetic field and temperature dependence. Using a two-site Hubbard model, we show that the main observed features can be explained in terms of Kondo correlation effects resulting from the exchange interaction between two localized spins.

During the graphitization of polar SiC(0001) surfaces through thermal decomposition, a strongly bound carbon-rich layer forms at the graphene/SiC interface. This layer is responsible for the system's high electron-doping and contributes to the degradation of the electrical properties of the overlying graphene. In this study, with the aid of photoelectron spectroscopy, low-energy electron microscopy, low-energy electron diffraction, and the density functional theory, we show that if the graphitization process starts from the nonpolar (11$\overline{2}$0) and (1$\overline{1}$00) surfaces instead, no buffer layer is formed. We correlate this direct growth of quasi-free-standing graphene over the substrate with the inhibited formation of tetrahedral bonds between the nonpolar surface and the carbon monolayer.