The response of thin films of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ to a strong perpendicular magnetic field is investigated by performing magnetic band-structure calculations for a realistic multiband tight-binding model. Several crucial features of Landau quantization in a realistic three-dimensional topological insulator are revealed. The $n=0$ Landau level is absent in ultrathin films, in agreement with experiment. In films with a crossover thickness of five quintuple layers, there is a signature of the $n=0$ level, whose overall trend as a function of magnetic field matches the established low-energy effective-model result. Importantly, we find a field-dependent splitting and a strong spin polarization of the $n=0$ level, which can be measured experimentally at reasonable field strengths. Our calculations reveal mixing between the surface and bulk Landau levels, which causes the character of the levels to evolve with magnetic field.

We report transport studies on ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x\right)}_2\mathrm{T}{\mathrm{e}}_3$ topological insulator thin films with tunable electronic band structure. We find a doping and temperature regime in which the Hall coefficient is negative indicative of electron-type carriers, whereas the Seebeck coefficient is positive indicative of hole-type carriers. This sign anomaly is due to the distinct transport behaviors of the bulk and surface states: the surface Dirac fermions dominate magnetoelectric transport while the thermoelectric effect is mainly determined by the bulk states. These findings may inspire new ideas for designing topological insulator-based high-efficiency thermoelectric devices.

A topological insulator is characterized by protected Dirac states on the edges due to the spin-orbit coupling. In this work we show that the topological phase can be driven by nanosize effects in Bi bilayers stacking. The nontrivial phase is a result of the interaction of Rashba-type Fermi gases coming from the Bi bilayer surfaces with opposite spin-orbit coupling. The surface states form a two-dimensional topological protected conductor, in contrast to the one-dimensional conduction observed in a single Bi bilayer. The three-dimensional (3D) topological phase takes place for a few Bi bilayers, where confinement effects open a band gap. The topological protected Dirac states, the helical spin texture, and Fermi velocities reveal a 3D topological insulator character, which is in agreement with first-principles results. Our findings help us to understand topological properties of nanostructured stacking of Rashba-type spin-orbit Fermi gases, as well provide a tool to find or construct new 3D topological insulators.

We study the effect of a perpendicular magnetic field on the electronic structure and charge transport of a monolayer ${\text{MoS}}_2$ nanoribbon at zero temperature. We particularly explore the induced valley Zeeman effect through the coupling between the magnetic field $B$ and the orbital magnetic moment. We show that the effective two-band Hamiltonian provides a mismatch between the valley Zeeman coupling in the conduction and valence bands due to the effective mass asymmetry and it is proportional to $B^2$ similar to the diamagnetic shift of exciton binding energies. However, the dominant term which evolves with $B$ linearly, originates from the multiorbital and multiband structures of the system. Besides, we investigate the transport properties of the system by calculating the spin-valley resolved conductance and show that, in a low-hole doped case, the transport channels at the edges are chiral for one of the spin components. This leads to a localization of the nonchiral spin component in the presence of disorder and thus provides a spin-valley polarized transport induced by disorder.

Inspired by recent experiments where electron transport was measured across graphene nanoribbons (GNRs) suspended between a metal surface and the tip of a scanning tunneling microscope [Koch et al., Nat. Nanotechnol. 7, 713 (2012)], we present detailed first-principles simulations of inelastic electron tunneling spectroscopy (IETS) of long pristine and defective armchair and zigzag nanoribbons under a range of charge carrier conditions. For the armchair ribbons we find two robust IETS signals around 169 and 196 mV corresponding to the $D$ and $G$ modes of Raman spectroscopy as well as additional fingerprints due to various types of defects in the edge passivation. For the zigzag ribbons we show that the spin state strongly influences the spectrum and thus propose IETS as an indirect proof of spin polarization.

A scheme based on coherent tunneling by adiabatic passage (CTAP) of exchange-only spin qubit quantum states in a linearly arranged double quantum dot chain is demonstrated. Logical states for the qubit are defined by adopting the spin state of three electrons confined in a double quantum dot. The possibility to obtain gate operations entirely with electrical manipulations makes this qubit a valuable architecture in the field of quantum computing for the implementation of quantum algorithms. The effect of the external control parameters as well as the effect of the dephasing on the coherent tunneling in the chain is studied. During adiabatic transport, within a constant energy degenerate eigenspace, the states in the double quantum dots internal to the chain are not populated, while transient populations of the mixed states in the external ones are predicted.

Charge generation in an organic photovoltaic blend was investigated using transient absorption spectroscopy. In films of pure electron donating material, subpicosecond spectral oscillations were observed and assigned to torsional modes associated with excited state relaxation and localization. These modes are systematically suppressed in the presence of fullerene, indicating that a significant fraction of charge transfer occurs prior to excited state localization.

We have used specific heat and neutron diffraction measurements on single crystals of ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ for Fe concentrations $x\le 0.7$ to establish that chemical substitution of Ru with Fe acts as “chemical pressure” $P_{ch}$ as previously proposed by Kanchanavatee et al. [Phys. Rev. B 84, 245122 (2011)] based on bulk measurements on polycrystalline samples. Notably, neutron diffraction reveals a sharp increase of the uranium magnetic moment at $x=0.1$, reminiscent of the behavior at the “hidden order” to large-moment-antiferromagnetic phase transition observed at a pressure $P_x\approx 0.5-0.7$ GPa in ${\mathrm{URu}}_2{\mathrm{Si}}_2$. Using the unit-cell volume determined from our measurements and an isothermal compressibility ${\kappa }_T=5.2\times {10}^{-3} {\mathrm{GPa}}^{-1}$ for ${\mathrm{URu}}_2{\mathrm{Si}}_2$, we determine the chemical pressure $P_{ch}$ in ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ as a function of $x$. The resulting temperature $\left(T\right)$–chemical pressure $\left(P_{ch}\right)$ phase diagram for ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ is in agreement with the established temperature $\left(T\right)$–external pressure $\left(P\right)$ phase diagram of ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

The recent surge of interest in phase-change materials GeTe, ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$, and related compounds motivated us to revisit the structural phase transition in GeTe in more detail than was done before. The rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied using high-resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. The results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral-to-cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe, we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.

In this paper we investigate the acoustic-to-optical up-conversion phonon processes in a multicomponent system. These processes take place during heat transport and limit the efficiency of heat flow. By combining time-resolved optical and heat capacity experiments we quantify the thermal coupling constant to be $g\sim 0.4{10}^{17} \text{W}/{\text{Km}}^3$. The method is based on selective excitation of a part of a multicomponent system, and the measurement of the thermalization dynamics by probing the linear birefringence of the sample with femtosecond resolution. In particular, we study a layered multiferroic organic-inorganic hybrid, in the vicinity of the ferroelectric phase transition. A diverging term of the heat capacity is associated to soft-mode dynamics, in agreement with previous spectroscopy measurements.

We have investigated epitaxial structures and physical properties of double-perovskite ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ films grown by pulsed-laser deposition. The films showed magnetic hysteresis at low temperatures regardless of the degree of Mn/Fe order and the saturation magnetization became smaller for the higher Mn/Fe-order films. The x-ray absorption and x-ray magnetic circular dichroism measurements revealed antiferromagnetic coupling between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Fe}}^{3+}$ ions, resulting in ferrimagnetism of double-perovskite ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$. A band structure was established based on the synchrotron radiation photoemission and optical spectra. The optical gap ranged 1.1 eV above the valence state dominated by the Mn $3d e_g$ orbitals, which is similar to the charge-transfer insulating state in the end member compound of ${\mathrm{LaMnO}}_3$.

We theoretically study the axial current ${\mathit{j}}_5$ (defined as the difference between the charge current with opposite helicity) in the magnetic insulator/doped Dirac semimetal using microscopic theory. In the Dirac semimetal, the axial current is induced by the magnetization dynamics, which is produced from the proximity effect of the magnetization of the magnetic insulator. We find that the induced axial current can be detected by using ferromagnetic resonance or the inverse spin Hall effect and can be converted into charge current with no accompanying energy loss. These properties make the Dirac semimetal advantageous for application to low-consumption electronics with new functionality.

The dynamical properties of hybrid normal metal/superconductor structures have recently come into research focus both experimentally and theoretically. Recent experimental studies of the coherent admittance $Y\left(\omega \right)$ of SNS rings as a function of the phase difference ${\varphi }_0$ are still not fully understood. Here we concentrate on the linear response regime, calculating $Y\left(\omega \right)$ by solving the Usadel equations, linearized in an electric field. Partially reproducing previously known results, we find qualitatively different behavior in the collisionless regime of ${\tau }_{\text{in}}^{-1}\ll \omega \lesssim E_{\text{Th}}$ and high temperature $T\gg E_{\text{Th}}$ and low temperature $T\lesssim E_{\text{Th}}$ near the minigap closing ${\varphi }_0\sim \pi$. We find that the dissipative part $\text{Re}Y\left(\omega \right)$ peaks when the minigap closes (at a phase difference of $\pi$) even at high temperatures, when the equilibrium supercurrent is fully suppressed.

We propose a simple theoretical description of the metal-insulator transition of rare-earth nickelates. The theory involves only two orbitals per nickel site, corresponding to the low-energy antibonding $e_g$ states. In the monoclinic insulating state, bond-length disproportionation splits the manifold of $e_g$ bands, corresponding to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion $U$ and Hund's coupling $J$, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when $U-3J$ is small or negative, a spontaneous instability to bond disproportionation takes place for large enough $J$. This minimal theory emphasizes that a small or negative charge-transfer energy, a large Hund's coupling, and a strong coupling to bond disproportionation are the key factors underlying the transition. Experimental consequences of this theoretical picture are discussed.

The hyperkagome antiferromagnet ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ represents the first genuine candidate for the realization of a three-dimensional quantum spin liquid. It can also be doped towards a metallic state, thus offering a rare opportunity to explore the nature of the metal-insulator transition in correlated, frustrated magnets. Here, we report thermodynamic and transport measurements in both metallic and weakly insulating single crystals down to 150 mK. While in the metallic sample the phonon thermal conductivity $\left({\kappa }^{\mathrm{ph}}\right)$ is almost in the boundary scattering regime, in the insulating sample, we find a large reduction ${\kappa }^{\mathrm{ph}}$ over a very wide temperature range. This result can be ascribed to the scattering of phonons off the gapless magnetic excitations that are seen in the low-temperature specific heat. This works highlights the peculiarity of the metal-insulator transition in ${\mathrm{Na}}_{3+x}{\mathrm{Ir}}_3{\mathrm{O}}_8$ and demonstrates the importance of the coupling between lattice and spin degrees of freedom in the presence of strong spin-orbit coupling.

Motivated by the high sensitivity to Fermi surface topology and scattering mechanisms in magnetothermoelectric transport, we have measured the thermopower and Nernst effect on the (011) plane of the proposed topological Kondo insulator ${\mathrm{SmB}}_6$. These experiments, together with electrical resistivity and Hall effect measurements, suggest that the (011) plane also harbors a metallic surface with an effective mass on the order of ${10–10}^2$ $m_0$. The surface and bulk conductances are well distinguished in these measurements and are categorized into metallic and nondegenerate semiconducting regimes, respectively. Electronic correlations play an important role in enhancing scattering and also contribute to the heavy surface state.

The disordered potential landscape in an InGaAs/InAlAs two-dimensional electron gas patterned into narrow wires is investigated by means of scanning gate microscopy. It is found that scanning a negatively charged tip above particular sites of the wires produces conductance oscillations that are periodic in the tip voltage. These oscillations take the shape of concentric circles whose number and diameter increase for more negative tip voltages until full depletion occurs in the probed region. These observations cannot be explained by charging events in material traps, but are consistent with Coulomb blockade in quantum dots forming when the potential fluctuations are raised locally at the Fermi level by the gating action of the tip. This interpretation is supported by simple electrostatic simulations in the case of a disorder potential induced by ionized dopants. This work represents a local investigation of the mechanisms responsible for the disorder-induced metal-to-insulator transition observed in macroscopic two-dimensional electron systems at low enough density.

We present pulsed electrically detected magnetic resonance (EDMR) measurements at low magnetic fields using phosphorus-doped silicon with natural isotope composition as a model system. Our measurements show that pulsed EDMR experiments, well established at $X-\text{band}$ frequencies (10 GHz), such as coherent spin rotations, Hahn echoes, and measurements of parallel and antiparallel spin pair life times are also feasible at frequencies in the megahertz (MHz) regime. We find that the Rabi frequency of the coupled ${}^{31}\mathrm{P}$ electron-nuclear spin system scales with the magnetic field as predicted by the spin Hamiltonian, while the measured spin coherence and recombination times do not strongly depend on the magnetic field in the region investigated. The usefulness of pulsed low-field EDMR for measurements of small hyperfine interactions is demonstrated by electron spin echo envelope modulation measurements of the ${\mathrm{P}}_{\mathrm{b}0}$ dangling-bond state at the $\mathrm{Si}/{\mathrm{SiO}}_2$ interface. A pronounced modulation with a frequency at the free Larmor frequency of hydrogen nuclei was observed for radio frequencies between 38 and 400 MHz, attributed to the nuclear magnetic resonance of hydrogen in an adsorbed layer of water. This demonstrates the high sensitivity of low-field EDMR also for spins not directly participating in the spin-dependent transport investigated.

Vanadium dioxide $\left({\mathrm{VO}}_2\right)$ undergoes an unusual insulator-metal transition (IMT), and after decades of study, the origin of the IMT remains hotly debated. Here, by analyzing spectral-weight transfers (SWTs) of x-ray absorption spectroscopy at the $\mathrm{V}{L}_{3,2}$ and O K edges on specially designed ${\mathrm{VO}}_2$ films, we observe $d_{\left|\right|}\left(d_{x^2-y^2}\right)$ band splitting at the $\mathrm{V}{L}_{3,2}$ edges across the IMT, accompanied by anomalous SWTs as high as $\sim 12\mathrm{eV}$ at the $\mathrm{O}K$ edge, indicating strong electronic correlations. Surprisingly, a few oxygen vacancies induce dramatic SWTs at the $\mathrm{O}K$ edge, but the sample remains conducting. Supported by theoretical calculations, we find that in the metallic state, direct $\mathrm{V}\left(3d_{\parallel }\right)\text{-V}\left(3d_{\parallel }\right)$ and $\text{O}\left(2p\right)\text{-V}\left(3d_{\parallel }\right)$ hybridized orbital correlations are screened by $\text{O}\left(2p\right)\text{-V}\left(3d_{\pi }\right)$ hybridized orbitals, while in the insulating state they are strongly correlated due to changes in the oxygen orbital occupancy. Our result shows the importance of screenings and electronic correlations for IMTs in ${\mathrm{VO}}_2$.

The interplay between bulk spin-orbit coupling and electron-electron interactions produces umklapp scattering in the helical edge states of a two-dimensional topological insulator. If the chemical potential is at the Dirac point, umklapp scattering can open a gap in the edge state spectrum even if the system is time-reversal invariant. We determine the zero-energy bound states at the interfaces between a section of a helical liquid which is gapped out by the superconducting proximity effect and a section gapped out by umklapp scattering. We show that these interfaces pin charges which are multiples of $e/2$, giving rise to a Josephson current with $8\pi$ periodicity. Moreover, the bound states, which are protected by time-reversal symmetry, are fourfold degenerate and can be described as ${\mathbb{Z}}_4$ parafermions. We determine their braiding statistics and show how braiding can be implemented in topological insulator systems.