We propose a 14-band $\mathit{k}·\mathit{p}$ model to calculate spin-orbit splittings of the valence subbands in semiconductor quantum wells. The reduced symmetry of quantum well interfaces is incorporated by means of additional terms in the boundary conditions which mix the ${\Gamma }_{15}$ conduction and valence Bloch functions at the interfaces. It is demonstrated that the interface-induced effect makes the dominating contribution to the heavy-hole spin splitting. A simple analytical expression for the interface contribution is derived. In contrast to the $4\times 4$ effective Hamiltonian model, where the problem of treating the $V_z{k}_z^3$ term seems to be unsolvable, the 14-band model naturally avoids and overcomes this problem. Our results are in agreement with the recent atomistic calculations [Luo et al., Phys. Rev. Lett. 104, 066405 (2010)].

We propose a remarkably simple electronic refrigerator based on the Coulomb barrier for single-electron tunneling. A fully normal single-electron transistor is voltage $V$ biased at a gate position such that tunneling through one of the junctions costs an energy of about $k_{B}T\ll eV,E_C$, where $T$ is the temperature and $E_C$ is the transistor charging energy. The tunneling in the junction with positive energy cost cools both leads attached to it. Immediate practical realizations of such a refrigerator make use of Andreev mirrors which suppress heat current while maintaining full electric contact.

We studied the single-layered iridate ${\text{Sr}}_2$${\text{IrO}}_4$ with a scanning tunneling microscope. The finite low temperature conductance enables the electronic structure of this antiferromagnetic Mott insulator to be measured by tunneling spectroscopy. We imaged the topography of freshly cleaved surfaces and measured differential tunneling conductance at cryogenic temperatures. We found the Mott gap in the tunneling density of states to be $2\Delta =615$ meV. Within the Mott gap, additional shoulders are observed which are interpreted as inelastic loss features due to magnons.

By making use of the Kubo formula, we calculate the conductivity of Dirac and Weyl semimetals in a magnetic field. We find that the longitudinal (along the direction of the magnetic field) magnetoresistivity is negative at sufficiently large magnetic fields for both Dirac and Weyl semimetals. The physical reason of this phenomenon is intimately connected with the dimensional spatial reduction $3\to 1$ in the dynamics of the lowest Landau level. The off-diagonal component of the transverse (with respect to the direction of the magnetic field) conductivity in Weyl semimetals contains an anomalous contribution directly proportional to the momentum-space separation between the Weyl nodes. This contribution comes exclusively from the lowest Landau level and, as expected, is independent of the temperature, chemical potential, and magnetic field. The other part of the off-diagonal conductivity is the same as in Dirac semimetals and is connected with a nonzero density of charge carriers. The signatures for experimental distinguishing Weyl semimetals from Dirac ones through the measurements of conductivity are discussed.

In a system where selective Mott localization is realized, some electrons show a gap to charge excitations while others do not. A hybridization between these two kind of electrons will lead to a smoothening of this sharp difference and can even bring the system back to a complete delocalization. We show here that there is a large region of parameters at finite hybridization where the selective localization persists and the system shows a partial filling of the selective gap with incoherent states, giving rise to a pseudogap. This result is illustrated here in a two-orbital Hubbard model with Hund's coupling, but is based on quite general assumptions and should hold for a larger class of systems, and possibly be a paradigm for the pseudogap mechanism in cuprates.

We performed process tomography of a coherent state transfer from light polarization into electron spin polarization in a semiconductor quantum well. The density matrix of the generated electron spin state was reconstructed by a three-dimensional tomographic Kerr rotation measurement so that we could quantitatively evaluate the transfer process. We found that the process fidelity of the state transfer was 81% and clarified that the phase-flip error caused by mixing with another Zeeman sublevel at the hole state was the major contributor to fidelity degradation.

It is desirable to realize topological phases in artificial structures by engineering electronic band structures. In this paper we investigate ${\left(\mathrm{PbTe}\right)}_m$${\left(\mathrm{SnTe}\right)}_{2n-m}$ superlattices along the [001] direction and find a robust weak topological insulator phase for a large variety of layer numbers $m$ and $2n-m$. We confirm this topologically nontrivial phase by calculating $Z_2$ topological invariants and topological surface states based on the first-principles calculations. We show that the folding of Brillouin zone due to the superlattice structure plays an essential role in inducing topologically nontrivial phases in this system. This mechanism can be generalized to other systems in which band inversion occurs at multiple momenta, and gives us a brand new way to engineer topological materials in artificial structures.

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few ${10}^{11}$ cm${}^{-2}$. When the growth temperature is high ($T\approx 1500–1550{}^{\circ }$C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered ($T\approx 1450–1500{}^{\circ }$C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm${}^2/$Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.

Electrons with kinetic energies $\sim$100 keV are potentially capable of exciting atomic vibrational states from a distance of microns. Despite such a large interaction distance, our detailed calculations show that the scattering physics permits a high-energy electron beam in a scanning transmission electron microscope to locate vibrational excitations with atomic-scale spatial resolution. Attempts to realize this capability experimentally could potentially benefit numerous fields across the physical sciences.

Here we report on a study of the ternary ${\text{Np}}_2{\text{PtGa}}_3$ compound. The x-ray-powder diffraction analysis reveals that the compound crystallizes in the orthorhombic CeCu${}_2$-type crystal structure (space group $Imma$) with lattice parameters $a=0.4409\left(2\right)$ nm, $b=0.7077\left(3\right)$ nm, and $c=0.7683\left(3\right)$ nm at room temperature. The measurements of dc magnetization, specific heat, and electron transport properties in the temperature range 1.7–300 K and in magnetic fields up to 9 T imply that this intermetallic compound belongs to a class of ferromagnetic Kondo systems. The Curie temperature of $T_C\sim$ 26 K is determined from the magnetization and specific-heat data. An enhanced coefficient of the electronic specific heat $\gamma$ = 180 mJ/(mol at. Np K${}^2$) and a $-$lnT dependence of the electrical resistivity indicate the presence of a Kondo effect, which can be described in terms of the $S=1$ underscreened Kondo-lattice model. The estimated Kondo temperature $T_K\sim 24$ K, Hall mobility of $\sim$16.8 cm${}^2$/V s, and effective mass of $\sim$83$m_e$ are consistent with an assumption that the heavy-fermion state develops in ${\text{Np}}_2{\text{PtGa}}_3$ at low temperatures. We compare the observed properties of ${\text{Np}}_2{\text{PtGa}}_3$ to that found in ${\text{Np}}_2{\text{PdGa}}_3$ and discuss their difference in regard to change in the exchange interaction between the conduction and localized 5$f$ electrons. We have used the Fermi wave vector $k_F$ to evaluate the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data of the (U, Np)${}_2$(Pd, Pt)Ga${}_3$ compounds we suggest that the evolution of the magnetic ground states in these actinide compounds can be explained within the RKKY formalism.

We study quantum phase transitions in the asymmetric variation of the three-leg Heisenberg tube for half-odd-integer spin, with a modulation of one of the rung exchange couplings $J_{\perp }^{\prime }$ while the other two are kept constant $J_{\perp }$. We focus on the strong rung-coupling regime $J_{\perp }\gg J_{\parallel }$, where $J_{\parallel }$ is the leg coupling, and analyze the effective spin-orbital model with a transverse crystal field in detail. Applying the Abelian bosonization to the effective model, we find that the system is in the dimer phase for the general half-odd-integer-spin cases without the rung modulation; the phase transition between the dimer and Tomonaga-Luttinger-liquid phases induced by the rung modulation is of the SU(2)-symmetric Berezinskii-Kosterlitz-Thouless type. Moreover, we perform a level spectroscopy analysis for the effective model for spin-1/2 using exact diagonalization, to determine the precise transition point $|J_{\perp }^{\prime }-J_{\perp }|/J_{\parallel }\sim 0.283$ in the strong rung-coupling limit. The presence of the dimer phase in a small but finite region is also confirmed by a density-matrix renormalization group calculation on the original spin-tube model.

We report on specific-heat experiments under the influence of high pressure on a strongly underdoped Co-substituted BaFe${}_2$As${}_2$ single crystal. This allows us to study the phase diagram of this iron pnictide superconductor with a bulk thermodynamic method and pressure as a clean control parameter. The data show large specific-heat anomalies at the superconducting transition temperature, which proves the bulk nature of pressure-induced superconductivity. The transitions in specific heat are sharper than those in resistivity, which demonstrates the necessity of employing bulk thermodynamic methods to explore the exact phase diagram of pressure-induced Fe-based superconductors. The $T_{\mathrm{c}}$ at optimal pressure and the superconducting condensation energy are found to be larger than those in optimally Co-doped samples at ambient pressure, which we attribute to a weak pair-breaking effect of the Co ions.

A reliable prediction of interatomic force constants in disordered alloys is an outstanding problem. This is due to the need for a proper treatment of multisite (at least pair) correlation within a random environment. The situation becomes even more challenging for systems with a large difference in atomic size and mass. We propose a systematic density functional theory (DFT) based study to predict the ab initio force constants in random alloys. The method is based on a combination of special quasirandom structures and the augmented space recursion to calculate phonon spectra, density of states (DOS), etc. The bcc TaW and fcc NiPt alloys are considered as the two distinct test cases. The Ta-Ta (W-W) bond distance in the alloy is predicted to be smaller (larger) than those in pure Ta (W), which, in turn, yields stiffer (softer) force constants for Ta (W). Pt-Pt force constants in the alloy, however, are predicted to be softer compared to Ni-Ni, due to the large bond distance of the former. Our calculated force constants, phonon spectra, and DOS are compared with experiments and other theoretical results, wherever available. A correct trend of the present results for the two alloys paves a path for future studies in more complex alloy systems.

We assess the validity of various exchange-correlation functionals for computing the structural, vibrational, dielectric, and thermodynamical properties of materials in the framework of density-functional perturbation theory (DFPT). We consider five generalized-gradient approximation (GGA) functionals (PBE, PBEsol, WC, AM05, and HTBS) as well as the local density approximation (LDA) functional. We investigate a wide variety of materials including a semiconductor (silicon), a metal (copper), and various insulators (SiO${}_2$ $\alpha$-quartz and stishovite, ZrSiO${}_4$ zircon, and MgO periclase). For the structural properties, we find that PBEsol and WC are the closest to the experiments and AM05 performs only slightly worse. All three functionals actually improve over LDA and PBE in contrast with HTBS, which is shown to fail dramatically for $\alpha$-quartz. For the vibrational and thermodynamical properties, LDA performs surprisingly very well. In the majority of the test cases, it outperforms PBE significantly and also the WC, PBEsol and AM05 functionals though by a smaller margin (and to the detriment of structural parameters). On the other hand, HTBS performs also poorly for vibrational quantities. For the dielectric properties, none of the functionals can be put forward. They all (i) fail to reproduce the electronic dielectric constant due to the well-known band gap problem and (ii) tend to overestimate the oscillator strengths (and hence the static dielectric constant).

Using pump-probe optical polarimetry we demonstrate that femtosecond laser excitation of a 2D regular lattice of magnetic nanodots effectively changes the optical anisotropy of the array. Study of the dynamics of the femtosecond laser-induced anisotropy reveals four main mechanisms occurring in the electronic, spin, and lattice subsystems. Below 1 ps, a strong Kerr-like nonlinearity causes linear birefringence, with its axis directed along the electric field of the linearly polarized femtosecond laser pump pulse. In addition, a long-living linear birefringence is also induced due to slowly relaxing excitations. Also below 1 ps, ultrafast laser-induced demagnetization of Co leads to a partial breakdown of the circular birefringence of the magnetic nanodots. On the timescale up to 300 ps, optically triggered acoustic modes of the dots drive oscillations of the linear optical birefringence. During this process, the oscillations damp while transferring their energy into acoustic modes of the substrate. On the nanosecond timescale, the signal is dominated by acoustic oscillations at the surface of the substrate.

Using noncollinear first-principles calculations, we perform a systematic study of the magnetic order in several families of ferropnictides. We find a fairly universal energy dependence on the magnetization order in all cases. Our results confirm that a simple Heisenberg model fails to account for the energy dependence of the magnetization in a couple of ways: first, a biquadratic term is present in all cases and, second, the magnetic moment softens depending on the orientation. We also find that hole doping substantially reduces the biquadratic contribution, although the antiferromagnetic stripe state remains stable within the whole range of doping concentrations, and thus the reported lack of the orthorhombicity in Na-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is probably due to factors other than a sign reversal of the biquadratic term. Finally, we discover that even with the biquadratic term, there is a limit to the accuracy of mapping the density functional theory energetics onto Heisenberg-type models, independent of the range of the model.

We have investigated the charge transport and low-energy dynamics originating from the antiferromagnetic (AF) domain wall (DW) in pyrochlore-type Nd${}_2$Ir${}_2$O${}_7$ with the all-in all-out type AF ground state by means of the magneto-resistance measurement and the terahertz time-domain spectroscopy. The resistivity is much lower in the multi-magnetic-domain state than in the nearly single-magnetic-domain one. Furthermore, the terahertz optical conductivity spectrum unravels a Drude-like response only in the multidomain state; the Drude-like absorption is strongly suppressed with increasing field and irreversibly disappears above 5 T. These observations suggest that the AF DW is highly conductive with a minimal damping constant of 2 meV, contrary to the fully-gapped (45 meV) AF insulating state of bulk. We discuss the nature of metallic states at AF DWs and their possible relevance to the gapless edge state inherent to the Weyl semimetal.

In frustrated magnets when magnetic ordering is suppressed down to low temperature, the formation of a quantum spin liquid becomes a possibility. How such a spin liquid manifests in the presence of conduction electrons is a question with potentially rich physical consequences, particularly when both the localized spins and conduction electrons reside on frustrated lattices. We propose a mechanism for symmetry breaking in systems where conduction electrons hybridize with a quantum spin liquid through Kondo couplings. We apply this to the pyrochlore iridate ${\text{Pr}}_2$${\text{Ir}}_2$${\text{O}}_7$, which exhibits an anomalous Hall effect without clear indications of magnetic order. We show that hybridization between the localized Pr pseudospins and Ir conduction electrons breaks some of the spatial symmetries, in addition to time-reversal symmetry, regardless of the form of the coupling. These broken symmetries result in an anomalous Hall conductivity and induce small magnetic, quadrupolar, and charge orderings. Further experimental signatures are proposed.

It has long been known that two-dimensional metals with antiferromagnetic exchange interactions have a weak-coupling instability to the superconductivity of spin-singlet, $d$-wave electron pairs. We examine additional possible instabilities in the spin-singlet particle-hole channel and study their interplay with superconductivity. We perform an unrestricted Hartree-Fock-BCS analysis of bond order parameters in a single-band model on the square lattice with nearest-neighbor exchange and repulsion, while neglecting on-site interactions. The dominant particle-hole instability is found to be an incommensurate, bidirectional, bond density wave with wave vectors along the $(1,1)$ and $(1,-1)$ directions and an internal $d$-wave symmetry. The magnitude of the ordering wave vector is close to the separation between points on the Fermi surface, which intersect the antiferromagnetic Brillouin zone boundary. The temperature dependence of the superconducting and bond order parameters demonstrates their mutual competition. We also obtain the spatial dependence of the two orders in a vortex lattice induced by an applied magnetic field: “halos” of the bond order appear around the cores of the vortices.