Single dopants in semiconductor nanostructures have been studied in great detail recently as they are good candidates for quantum bits, provided they are coupled to a detector. Here we report the coupling of a single As donor atom to a single-electron transistor (SET) in a silicon nanowire field-effect transistor. Both capacitive and tunnel coupling are achieved, the latter resulting in a dramatic increase of the conductance through the SET, by up to one order of magnitude. The experimental results are well explained by the rate-equation theory developed in parallel with the experiment.

We have investigated the effect of pressure on electrical transport and magnetic properties of ferromagnetic Fe${}_{1-x}$Co${}_x$Si alloys for $x=0.1$ ($T_{\mathrm{C}}~11\hspace{0.5em}\mathrm{K}$) and $x=0.2$ ($T_{\mathrm{C}}~32\hspace{0.5em}\mathrm{K}$) using electrical resistivity measurements up to about 30 GPa. We have also studied the magnetic properties of these samples ($x=0.1$ and $x=0.2$) and a sample with $x=0.3$ ($T_{\mathrm{C}}~43\hspace{0.5em}\mathrm{K}$) at ambient pressure using ${}^{57}\mathrm{Fe}$ Mössbauer-effect (ME) spectroscopy. The ME results indicate that the effective magnetic hyperfine field $B_{\mathrm{eff}}$ at the ${}^{57}\mathrm{Fe}$ nucleus exhibits the same concentration dependence as the macroscopic magnetic moment and confirm that the onset of magnetic order is above $x~0.02$. The analysis of the high-pressure results reveals in both samples a gradual suppression of the ferromagnetic state to a quantum phase transition (QPT) at pressures of $p~11\hspace{0.5em}\mathrm{GPa}$ and $p~12\hspace{0.5em}\mathrm{GPa}$ for $x=0.1$ and $x=0.2$, respectively. High-pressure x-ray diffraction measurements on the three samples indicate very similar change of the volume of the cubic unit cell with pressure and exclude that the observed QPTs are connected with a structural phase transition. We discuss the observed instability of the ferromagnetic state with increasing Co concentration in the context of increasing local crystallographic disorder, which causes a change of the distribution of the helix wave vector as well as a modification of the ferromagnetic half-metallic state. We further show that, in the pressure-induced nonmagnetic metallic state, all samples, regardless of their different local crystallographic disorder, exhibit similar non-Fermi-liquid behavior [$\rho (T)\propto T$]. Finally, we find a small but positive magnetoresistance in the high-pressure metallic state well beyond the QPT of Fe${}_{0.9}$Co${}_{0.1}$Si. This can be attributed to a slight field-induced modification of the spin majority and minority band which leads to a very small magnetic moment.

Superconductivity, magnetic order, and the high field ordered phase have been investigated in the filled skutterudite system Pr${}_{1-x}$Nd${}_x$Os${}_4$Sb${}_{12}$ as a function of composition $x$ in magnetic fields up to 9 T and at temperatures between 50 mK and 10 K. Electrical resistivity measurements indicate that the high field ordered phase, which has been identified with antiferroquadruoplar order, persists to $x$ $~$ 0.5. The superconducting critical temperature $T_c$ of PrOs${}_4$Sb${}_{12}$ is depressed linearly with Nd concentration to $x$ $~$ 0.55, whereas the Curie temperature $T_{\mathrm{FM}}$ of NdOs${}_4$Sb${}_{12}$ is depressed linearly with Pr composition to ($1-x$) $~$ 0.45. In the superconducting region, the upper critical field $H_{c2}(x,0)$ is depressed quadratically with $x$ in the range 0 $<$ $x$ $\lesssim$ 0.3, exhibits a kink at $x$ $\approx$ 0.3, and then decreases linearly with $x$ in the range 0.3 $\lesssim$ $x$ $\lesssim$ 0.6. The behavior of $H_{c2}(x,0)$ appears to be a result of pair breaking caused by the applied magnetic field and the exchange field associated with the polarization of the Nd magnetic moments in the superconducting state. From magnetic susceptibility measurements, the correlations between the Nd moments in the superconducting state appear to change from ferromagnetic in the range 0.3 $\lesssim$ $x$ $\lesssim$ 0.6 to antiferromagnetic in the range 0 $<$ $x$ $\lesssim$ 0.3. Specific-heat measurements on a sample with $x$ $=$ 0.45 indicate that magnetic order may occur in the superconducting state, as is also inferred from the depression of $H_{c2}(x,0)$ with $x$.

A rigorous formulation of time-dependent current density functional theory (TDCDFT) on a lattice is presented. The density-to-potential mapping and the $\mathcal{V}$-representability problems are reduced to a solution of a certain nonlinear lattice Schrödinger equation, to which the standard existence and uniqueness results for nonlinear differential equations are applicable. For two versions of the lattice TDCDFT, we prove that any continuous-in-time current density is locally $\mathcal{V}$-representable (both interacting and noninteracting), provided that in the initial state the local kinetic energy is nonzero everywhere. In most cases of physical interest, the $\mathcal{V}$-representability should also hold globally in time. These results put the application of TDCDFT to any lattice model on a firm foundation, and pave the way for studying exact properties of exchange-correlation potentials.

We theoretically predict that the series of Pb-based layered chalcogenides, Pb${}_n$Bi${}_2$Se${}_{n+3}$and Pb${}_n$Sb${}_2$Te${}_{n+3}$, are possible new candidates for topological insulators, and the topological phases are changed from a topological insulator to a band insulator with increasing $n$. Among the new topological insulators, we found a large bulk band gap of 0.40 eV in PbBi${}_2$Se${}_4$, and that of Pb${}_2$Sb${}_2$Te${}_5$ is located near the phase boundary between a trivial and a nontrivial topological insulator, which raises the possibility of tuning the topological phase by changing the external parameters.

We show that the exchange-correlation scalar and vector potentials obtained from the optimized effective potential (OEP) equations and from the Krieger-Li-Iafrate (KLI) approximation for the current-density functional theory (CDFT) change under a gauge transformation such that the energy functional remains invariant. This alone does not assure, however, the theory’s compliance with the continuity equation. Using the model of a quantum ring with a broken angular symmetry which is penetrated by a magnetic flux we demonstrate that the physical current density calculated with the exact-exchange CDFT in the KLI approximation violates the continuity condition. In contrast, the current found from a solution of the full OEP equations satisfies this condition. We argue that the continuity violation stems from the fact that the KLI potentials are not (in general) the exact functional derivatives of a gauge-invariant exchange-correlation functional.

Interlayer coupling in rotationally faulted graphene multilayers breaks the local sublattice symmetry of the individual layers. We present a theory of this mechanism, which reduces to an effective Dirac model with space-dependent mass in an important limit. It thus makes a wealth of existing knowledge available for the study of rotationally faulted graphene multilayers. We demonstrate quantitative agreement between our theory and a recent experiment.

We present a study of the Seebeck and Nernst coefficients of ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ extended up to 28 T. The large magnitude of the Seebeck coefficient in the optimally doped sample tracks a remarkably low normalized Fermi temperature, which like other correlated superconductors, is only one order of magnitude larger than $T_c$. We combine our data with other experimentally measured coefficients of the system to extract a set of self-consistent parameters, which identify ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.6}{\mathrm{Se}}_{0.4}$ as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizable vortex Nernst signal in a wide temperature window.

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first penetration field, we directly observe a terraced critical state as proposed theoretically by Cooley and Grishin [Phys. Rev. Lett. 74, 2788 (1995)]. However, at higher fields, the flux front tends to disorder and the classical Bean profile is restored. This study allows us to establish a clear link between the widely used integrated response measurements in the superconducting state and the nanoscale landscape defined by individual vortices.

Scanning tunneling microscopy and spectroscopy in a magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated SiO${}_2$ thermal oxide, which allowed for the observation of the Landau level sequences characteristic of single-layer graphene while tuning the density through the Si backgate. Upon changing the carrier density, we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, at low doping levels, a marked increase in the Fermi velocity is observed, which is consistent with the logarithmic divergence expected due to the onset of many-body effects close to the Dirac point.

We study resonant response of an underdamped nanomechanical resonator with fluctuating frequency. The fluctuations are due to diffusion of molecules or microparticles along the resonator. They lead to broadening and change of shape of the oscillator spectrum. The spectrum is found for the diffusion confined to a small part of the resonator and where it occurs along the whole nanobeam. The analysis is based on extending to the continuous limit, and appropriately modifying, the method of interfering partial spectra. We establish the conditions of applicability of the fluctuation-dissipation relations between the susceptibility and the power spectrum. We also find where the effect of frequency fluctuations can be described by a convolution of the spectra without these fluctuations and with them as the only source of the spectral broadening.

We study the influence of electron-phonon coupling on electron transport through a Luttinger liquid with an embedded weak scatterer or weak link. We derive the renormalization group (RG) equations, which indicate that the directions of RG flows can change upon varying either the relative strength of the electron-electron and electron-phonon coupling or the ratio of Fermi to sound velocities. This results in a rich phase diagram with up to three fixed points: an unstable one with a finite value of conductance and two stable ones, corresponding to an ideal metal or insulator.

Using ac susceptibility, we have determined the pressure dependence of the metamagnetic critical endpoint temperature $T^{*}$ for a field applied in the $\mathit{ab}$ plane in the itinerant metamagnet ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. We find that $T^{*}$ falls monotonically to zero as pressure increases, producing a quantum critical endpoint (QCEP) at $P_c~13.6\pm 0.2$ kbar. New features are observed near the QCEP—the slope of $T^{*}$ versus pressure changes at $~$12.8 kbar, and weak subsidiary maxima appear on either side of the main susceptibility peak at pressures near $P_c$—indicating that some new physics comes into play near the QCEP. Clear signatures of a nematic phase, however, that were seen in field-angle tuning of $T^{*}$ are not observed. As $T^{*}$ is suppressed by pressure, the metamagnetic peak in the susceptibility remains sharp as a function of an applied magnetic field. As a function of temperature, however, the peak becomes broad with only a very weak maximum, suggesting that, near the QCEP, the uniform magnetization density is not the order parameter for the metamagnetic transition.

In this paper, we study transport properties of electrons on the two-dimensional honeycomb lattice. We consider a half-filled system in the vicinity of a symmetry-breaking transition from a semimetallic phase toward an insulating phase with either charge-density- or spin-density-wave order. The effect of either order is to break the sublattice inversion symmetry, which induces a finite gap for the electronic single-particle excitations. Phenomenologically, such a scenario is described in the framework of a Gross-Neveu theory. We analyze two related formulations of the model by means of (i) a controlled renormalization-group calculation and (ii) the large-$N$ method, both of which in combination with a Boltzmann transport equation. We determine the quantum critical conductivity and also discuss crossover behavior from quantum critical behavior into the insulating and/or the semimetallic phases. We find that at asymptotically low temperatures, the quantum critical conductivity is given by a temperature-independent universal number. Over a large temperature window, the temperature-independent quantum-critical conductivity is masked by a logarithmically temperature-dependent contribution due to the marginally irrelevant long-range Coulomb interaction. We discuss possible origins of this peculiarity in the two complementary formulations of the model. Furthermore, we consider possible relations of our findings to recent experiments, with a special emphasis on the quantum-critical-to-insulator crossover. We find that our results are in remarkably good qualitative and quantitative agreement with a recent analysis of the data sets under the hypothesis of an underlying gap in the single-particle spectrum.

Our ab initio theory calculations predict a semiconducting band structure of I-Mn-V compounds. We demonstrate on LiMnAs that high-quality materials with group-I alkali metals in the crystal structure can be grown by molecular beam epitaxy. Optical measurements on the LiMnAs epilayers are consistent with the theoretical electronic structure. Our calculations also reproduce earlier reports of high antiferromagnetic ordering temperature and predict large, spin-orbit-coupling-induced magnetic anisotropy effects. We propose a strategy for employing antiferromagnetic semiconductors in high-temperature semiconductor spintronics.

We report the magnetic-field-induced Bose glass–Bose-Einstein condensate (BEC) transition of triplons in Tl${}_{1-x}$K${}_x$CuCl${}_3$ and its critical behavior, which were investigated through specific heat and electron spin resonance (ESR) measurements. The field dependence of the BEC transition temperature $T_N$ can be described by the power law $[H-H_c]\propto T_N^{\varphi }$ near the quantum critical point $H_c~3$ T. The critical exponent $\varphi$ tends to reach a value smaller than 1/2 with decreasing fitting window in contrast to $\varphi \to 3/2$ for the standard BEC in a pure system. At sufficiently low temperatures, the ESR line shape for $H\simeq H_c$ is intermediate between Gaussian and Lorentzian functions. This implies the localization of triplons for $H<H_c$ at $T=0$.

The optical properties of single Ag particles on an MgO thin film have been investigated with fluorescence spectroscopy performed in a scanning-tunneling-microscope setup. Depending on the point of excitation within the particle (top or side facet), two emission maxima are detected that are compatible with the in-plane and out-of-plane plasmons of the Ag deposits. The different excitation cross sections and spectral properties of the two modes give rise to characteristic intensity patterns in photon maps taken from individual particles.

We studied spatial distribution of dynamic nuclear polarization (DNP) in a Hall-bar device in a breakdown regime of the quantum Hall effect (QHE). We detected nuclear magnetic resonance (NMR) signals from the polarized nuclear spins by measuring the Hall voltage $V_{\mathit{xy}}$ using three pairs of voltage probes attached to the conducting channel of the Hall bar. We find that the amplitude of the NMR signal depends on the position of the Hall voltage probes and that the largest NMR signal is obtained from the pair of probes farthest from the electron-injecting electrode. Combined with results on pump-and-probe measurements, we conclude that the DNP induced by QHE breakdown develops along the electron-drift direction.

We investigate the electronic correlation effects on the Sn/Si(111) $\sqrt{3}\times \sqrt{3}R{30}^{°}$ surface by combining the ab initio density-functional approach with the dynamical mean-field theory, the variational cluster approach, and the dual fermion (DF) method. A metal-insulator transition with first or second order at finite or zero temperature is predicted at a critical on-site Hubbard $U_c$ $~0.65$ eV and the system is proven to be short-range correlated. The electron-electron interaction favors a row-wise antiferromagnetic (RW-AFM) order, although the noninteracting system does not have a pronounced nesting at the Fermi surface. The stabilization of the RW-AFM order over the ${120}^{°}$ AF one for certain interaction strengths is shown to be due to the longer-range single-particle hopping terms.

In integrable one-dimensional quantum systems an infinite set of local conserved quantities exists which can prevent a current from decaying completely. For cases like the spin current in the $\mathit{XXZ}$ model at zero magnetic field or the charge current in the attractive Hubbard model at half filling, however, the current operator does not have overlap with any of the local conserved quantities. We show that in these situations transport at finite temperatures is dominated by a diffusive contribution with the Drude weight being either small or even zero. For the $\mathit{XXZ}$ model we discuss in detail the relation between our results, the phenomenological theory of spin diffusion, and measurements of the spin-lattice relaxation rate in spin chain compounds. Furthermore, we study the Haldane-Shastry model where a conserved spin current exists.