Starting from a general $N$-band Hamiltonian with weak spatial and temporal variations, we derive a low-energy effective theory for transport within one or several overlapping bands. To this end, we use the Wigner representation that allows us to systematically construct the unitary transformation that brings the Hamiltonian into band-diagonal form. We address the issue of gauge invariance and discuss the necessity of using kinetic variables in order to obtain a low-energy effective description that is consistent with the original theory. Essentially, our analysis is a semiclassical one and quantum corrections appear as Berry curvatures in addition to quantities that are related to the appearance of persistent currents. We develop a transport framework, which is manifestly gauge invariant, and it is based on a quantum Boltzmann formulation along with suitable definitions of current density operators such that Liouville's theorem is satisfied. Finally, we incorporate the effects of an external electromagnetic field into our theory.

The built-in dipole moments of excitonic and multiexcitonic complexes of GaN/AlN quantum dots (charged and uncharged excitons and biexcitons) are investigated in detail, both experimentally and theoretically. The calculations are done with Hartree-Fock wave functions in the framework of an 8-band $\mathbf{k}·\mathbf{p}$ approximation. Electric dipole moments of different excitonic complexes are found to vary significantly. Hence, their Coulomb coupling to charged defects in the vicinity of a QD produces emission energy variations, known as spectral diffusion, which can be used to identify the emitting excitonic complex. This method was exemplary applied to the presented cathodoluminescence and $\mu$-photoluminescence data and opens a way to identify excitonic complexes by their response to external electric fields.

We report detailed magnetization measurements on the perovskite oxide NdNiO${}_3$. This system has a first order metal-insulator (M-I) transition at about 200 K which is associated with charge ordering. There is also a concurrent paramagnetic to antiferromagnetic spin ordering transition in the system. We show that the antiferromagnetic state of the nickel sublattice is spin canted. We also show that the concurrency of the charge ordering and spin ordering transitions is seen only while warming up the system from low temperature. The transitions are not concurrent while cooling the system through the M-I transition temperature. This is explained based on the fact that the charge ordering transition is first order while the spin ordering transition is continuous. In the magnetically ordered state the system exhibits divergence between zero field cooled and field cooled magnetizations, as well as history-dependent magnetization and aging. Our analysis rules out the possibility of spin glass or superparamagnetism and suggests that the irreversibility arises from magnetocrystalline anisotropy and domain wall pinning.

Local excitations of indistinguishable particles in a solid are quantum-mechanically superposed to give delocalized wave functions. Their interference is often so short-lived that it eludes observation and manipulation. Here we have actively controlled interference of delocalized vibrational wave functions in solid para-hydrogen produced by a pair of ultrashort laser pulses. The ultrafast evolution of their interference changes from almost completely constructive (amplification by a factor of $\sim$4) to destructive when we change the timing of those two laser pulses by only 4 fs. This active control serves as an experimental tool to investigate the spatiotemporal evolution of a wave function in a bulk solid.

We present calculated electronic structures and x-ray emission and absorption spectroscopy measurements of five copper pnictides. The x-ray emission and absorption spectra, which probe the occupied and unoccupied states, respectively, provide an empirical justification of the calculated electronic structure. We therefore verify the prediction that the copper 3$d$ states are fully occupied and buried deep in the valence band, and the Fermi surface of these materials is characterized by $s$,$p$ states. We show that generally the calculated electronic structure is in excellent agreement with our measurements. We finally examine the Fermi surfaces of these pnictides, and suggest that copper antimonides may exhibit efficient Fermi surface nesting similar to iron pnictides.

In order to explore why the multilayered cuprates have such high $T_{\text{c}}$ values, we have examined various interlayer processes. Since the interlayer one-electron hopping has little effect on the band structure, we turn to the interlayer pair hopping. The superconductivity in a double-layer Hubbard model with and without the interlayer pair hopping, as studied by solving the Eliashberg equation with the fluctuation exchange approximation, reveals that the interlayer pair hopping acts to increase the pairing interaction and the self-energy simultaneously, but that the former effect supersedes the latter and enhances the superconductivity, along with how the sign of the interlayer off-site pair hopping determines the relative configuration of $d$ waves between the adjacent layers. Study of the triple-layer case with the interlayer pair hopping further reveals that the superconductivity is further enhanced but tends to be saturated toward the triple-layer case.

We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)${}_2$PF${}_6$ and (TMTSF)${}_2$AsF${}_6$ (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF)${}_2$PF${}_6$-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)${}_2$PF${}_6$-H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of $5\times {10}^2$ MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF${}_6$ and AsF${}_6$ salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies ${\theta }_E$ $=$ 8.3 K and ${\theta }_E$ $=$ 6.7 K for the PF${}_6$-D12 and AsF${}_6$-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F…D-CD${}_2$ bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)${}_2$PF${}_6$ is dominated by the librational motion of the PF${}_6$ units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF${}_6$ librations: ${\theta }_E$ ≈ 50 K and ${\theta }_E$ $=$ 76 K for the PF${}_6$-D12 and PF${}_6$-H12 salts, respectively.

We consider methods to identify the classical ground state for an exchange-coupled Heisenberg antiferromagnet on a non-Bravais lattice with interactions $J_{ij}$ to several neighbor distances. Here, we apply this to the unusual “octahedral” lattice in which spins sit on the edge midpoints of a simple cubic lattice. Our approach is informed by the eigenvectors of $J_{ij}$, taken as a matrix, having the largest eigenvalues. We discovered two families of noncoplanar states: (i) two kinds of commensurate states with cubic symmetry, each having twelve sublattices with spins pointing in (1,1,0) directions in spin space (modulo a global rotation) and (ii) varieties of incommensurate conic spiral. The latter family is addressed by projecting the three-dimensional lattice to a one-dimensional chain, with a basis of two (or more) sites per unit cell.

We report a study of single quantum dots inside photonic crystal cavities with a low-temperature scanning near-field optical microscope. The spatial maps of single excitonic lines from the quantum dot show the clear signature of coupling to the cavity modes for small detunings. We also show that the near-field tip can be used to control the exciton-photon coupling at the nanoscale. A general framework for the interpretation of near-field maps of single emitters in cavities is proposed.

We present a first-principles study based on density functional theory of thermodynamic and electronic properties of the most important intrinsic defects in the semiconductor alloy ${\mathrm{Cd}}_{1-x}{\mathrm{Zn}}_x\mathrm{Te}$ with $x<0.13$. The alloy is represented by a set of supercells with disorder on the Cd/Zn sublattice. Defect formation energies as well as electronic and optical transition levels are analyzed as a function of composition. We show that defect formation energies increase with Zn content with the exception of the neutral Te vacancy. This behavior is qualitatively similar to but quantitatively rather different from the effect of volumetric strain on defect properties in pure CdTe. Finally, the relative carrier scattering strengths of point defects, alloy disorder, and phonons are obtained. It is demonstrated that for realistic defect concentrations, carrier mobilities are limited by phonon scattering for temperatures above approximately 150 K.

The low-frequency noise in magnetic tunnel junctions and giant magnetoresistance devices in various magnetic states has been investigated. The noise measurements in both types of devices show a $1/f$ spectrum in either the parallel or antiparallel magnetic state. The noise as the magnetization switches from parallel to antiparallel and vice versa in the transition regions (characterized by a large $dR$/$dH$), in which the magnetic aftereffect is present and the system is out of equilibrium, also exhibits a $1/f$ spectrum of a magnitude consistent with that in the parallel and antiparallel states where the system relaxes to the equilibrium state. Included is how even small magnetic aftereffect signals can replicate the results of many previous studies. Two additional experiments are described which set an upper limit for the magnetic noise in the devices investigated.

The charge and spin correlations in La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ (LBCO 1/8) are studied using Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS). The static charge order (CO) is observed at a wave vector of $(0.24,0)$ and its charge nature confirmed by measuring the dependence of this peak on the incident x-ray polarization. The damped spin excitation or “paramagnon” in LBCO 1/8 is then measured as it disperses through the CO wave vector. Within the experimental uncertainty no changes are observed in the paramagnon at that wave vector and the paramagnon seems to be similar to that measured in other cuprates, which have no static CO. Given that the stripe correlation modulates both the charge and spin degrees of freedom, it is likely that subtle changes do occur in the paramagnon due to CO. Consequently, we propose that future RIXS measurements, realized with higher-energy resolution and sensitivity, should be performed to test for these effects.

A charge modulation has recently been reported in (Y,Nd)Ba${}_2$Cu${}_3$O${}_{6+x}$ [G. Ghiringhelli et al., Science 337, 821 (2012)]. Here we report Cu $L_3$ edge soft x-ray scattering studies comparing the lattice modulation associated with the charge modulation in YBa${}_2$Cu${}_3$O${}_{6.6}$ with that associated with the well-known charge and spin stripe order in La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$. We find that the correlation length in the CuO${}_2$ plane is isotropic in both cases, and is $259\pm 9$ Å for La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ and $55\pm 15$ Å for YBa${}_2$Cu${}_3$O${}_{6.6}$. Assuming weak interplanar correlations of the charge ordering in both compounds, we conclude that the order parameters of the lattice modulations in La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ and YBa${}_2$Cu${}_3$O${}_{6.6}$ are of the same order of magnitude.

We report numerical results of quantum oscillations of the specific heat in the vortex state of a $d_{x^2-y^2}$-wave superconductor in the presence of loop current order [C. M. Varma, Phys. Rev. B 55, 14554 (1997); Phys. Rev. Lett. 83, 3538 (1999)], which gives rise to the Fermi pockets coexisting with nodal $d_{x^2-y^2}$-wave superconductivity. Within a lattice tight-binding model, we find that in an intermediate temperature range, the oscillations seem to approximately follow the Onsager relation with an effective charge comparable to the electric charge. However, the quasiparticle spectrum does not resemble the Landau levels. In order to understand the origin of the oscillations, we also perform the Franz-Tesanovic transformation in the presence of the loop order and find that in addition to scalar and the Berry potentials [M. Franz and Z. Tešanović, Phys. Rev. Lett. 84, 554 (2000)], one component of the gauge invariant superfluid velocity couples to the low-lying Dirac particles as a component of a vector potential. The magnetic field associated with this vector potential vanishes on average but is highly nonuniform in the magnetic unit cell. We attribute the quantum oscillations to this field. We also compare the results with the model without the loop order but with the Zeeman-like coupling, which also induces the Fermi pockets in the superconducting state.

Metal-organic molecular systems are fascinating alternatives to conventional spintronic materials such as transition-metal oxides and diluted magnetic semiconductors, having the advantage of their structural diversity and chemical sensitivity. Here we demonstrate, based on advanced density functional theory calculations, that a cobalt-organic-molecular crystal exhibits particular spin-crossover phenomenon, which is extremely sensitive to external pressure as well as the surrounding water ligands’ coordination. We found keen competition between crystal-field splitting and on-site Hund's exchange interaction, resulting in the highly-sensitive spin-crossover transition. Our finding paves the way to soft spintronics, in which molecular spin representing a bit can be easily flipped by small changes in external pressure and temperature at low energy cost.

Perovskite iridium oxides Sr${}_{n+1}$Ir${}_n$O${}_{3n+1}$ exhibit fascinating phenomena due to the combined effects of spin-orbit coupling (SOC) and electronic interactions. It was suggested that electronic correlation amplified via the strong SOC leads to a spin-orbit Mott insulator for $n=1$ and 2, while three-dimensional (3D) SrIrO${}_3$ remains metallic because of the large bandwidth from the 3D structure. However, this bandwidth-controlled metal-insulator transition (MIT) is only valid when SOC is large enough to split $J_{\text{eff}}=1/2$ and 3/2 bands, while the mixing of 1/2 and 3/2 bands is conspicuous among the occupied bands. Here, we investigate the MIT as a function of $n$ using weak-coupling theory. In this approach, the magnetic instability is determined by the states near the Fermi level rather than the entire band structure. Starting from $t_{2g}$ tight-binding models for $n=1$, 2, and $\infty$, the states near the Fermi level are found to be predominantly $J_{\text{eff}}=1/2$ allowing an effective single-band model. Supplementing this effective $J_{\text{eff}}=1/2$ model with Hubbard-type interactions, transitions from a metal to magnetically ordered states are obtained. Strong-coupling spin models are derived to compare the magnetic ordering patterns obtained in the weak- and strong-coupling limits. We find that they are identical, indicating that these iridates are likely in an intermediate-coupling regime.

We study a quantum dot coupled to two edge states of a quantum spin Hall insulator through electron tunnelings in the presence of a Rashba spin-orbital interaction induced by an external electric field. We show that if the electron interactions on the edge states are repulsive, there are two possible phases, depending on the Luttinger liquid parameter $K$. For $1/2<K<1$, the low-temperature physics is controlled by a previously identified two-channel Kondo fixed point. For the edge states with even stronger repulsive interactions, i.e., $1/4<K<1/2$, the system reaches another phase at low temperatures, described by a new two-channel Kondo fixed point. This phase is separated from the original one by a continuous phase transition upon varying the value of $K$ through the external electric field. The corresponding critical point is described by a free Dirac fermion backscattered by a local potential. We investigate the low-temperature properties associated with this new fixed point and also discuss the scaling behaviors of the system at the critical point.

We propose a scheme for implementing quantum gates and entanglement between spin qubits in the outer dots of a triple-dot system with an empty central dot. The voltage applied to the central dot can be tuned to realize the gate. Our scheme exemplifies the possibility of quantum gates outside the regime where each dot has an electron, so that spin-spin exchange interaction is not the only relevant mechanism. Analytic treatment is possible by mapping the problem to a $t-J$ model. The fidelity of the entangling quantum gate between the spins is analyzed in the presence of decoherence stemming from a bath of nuclear spins, as well as from charge fluctuations. Our scheme provides an avenue for extending the scope of two-qubit gate experiments to triple dots, while requiring minimal control, namely that of the potential of a single dot, and may enhance the qubit separation to ease differential addressability.

Using first-principles electronic structure calculations, the energy barriers for diffusion mechanisms of adatoms on the tungsten (W) (001) and (110) surfaces under externally applied biaxial strain fields are determined. Adatoms move either by hopping on the surface or by an exchange process with a surface atom, which is found to be completed in one step (direct exchange), or via the formation of a surface crowdion (crowdion-mediated exchange). As a result of the compact atomic stacking, hopping is found to be the major diffusion mechanism on the W(110) surface, irrespective of the surface strain state. On the other hand, the main diffusion mechanism on the less compact W(001) surface is found to be a competition between direct exchange and crowdion-mediated exchange, depending on the magnitude of the surface biaxial strain. Results of the model reveal that, if surface crowdions form, they will be highly mobile and migrate anisotropically. A microscopic explanation is presented by analyzing the charge density associated with surface crowdions. A “mechanism diagram” for atomic surface diffusion on the W(001) indicates that the diffusion direction and its rate can both be modulated by an applied biaxial strain. Migration volumes for the three mechanisms are calculated, and the significance of the results to the understanding of surface evolution under plasma or other energetic ion bombardment is highlighted.