Scanning tunneling microscope induced light emission from an atomic or molecular junction has been probed from the tunneling to contact regime in recent experiments. There, the measured light emission yields suggest a strong correlation with the high-frequency current/charge fluctuations. We show that this is consistent with the established theory in the tunneling regime, by writing the finite-frequency shot noise as a sum of inelastic transitions between different electronic states. Based on this, we develop a practical scheme to perform calculations on realistic structures using nonequilibrium Green's functions. The photon emission yields obtained reproduce the essential feature of the experiments.

Adding a small subdominant quadratic-in-momentum term to a dominant linear Dirac dispersion curve affects conduction and valence band differently and leads to an hourglass-like structure for energy as a function of momentum. This applies to the protected surface states in topological insulators. The energies of the conduction and valence band Landau levels are also different and this leads to the splitting of optical absorption lines produced by the magnetic field, which acquire a two-peak structure. It also changes the peaks in the imaginary part of the Hall conductivity into two distinct contributions of opposite signs. The real part of the circularly polarized optical conductivity, however, retains its single-peak structure but the peaks in right and left handedness cases are shifted in energy with respect to each other in contrast to the pure Dirac case. The magnitude of the semiclassical cyclotron frequency is significantly modified by the presence of a mass term, as is its variation with value of the chemical potential $\mu$. Its optical spectral weight is found to decrease with increasing $\mu$ rather than increase as it does in the pure Dirac limit.

Using low-temperature molecular beam epitaxy, we experimentally examine the substitutions of Fe atoms for Co ones in Co${}_{3-x}$Fe${}_x$Si ($1.0\le x\le 3.0$) Heusler-compound films grown on Si and Ge. The Co-Fe atomic substitution at $A$ and $C$ sites can be confirmed by the conversion electron Mössbauer spectroscopy measurements. The magnetic moment and the magnitude of spin signals in nonlocal spin-valve measurements are systematically changed with increasing $x$ in Co${}_{3-x}$Fe${}_x$Si. By assuming proper spin diffusion length for Co${}_{3-x}$Fe${}_x$Si on the basis of our previous work [Phys. Rev. B 85, 100404(R) (2012)], we experimentally verify that the Co-Fe substitution in Co${}_{3-x}$Fe${}_x$Si can directly affect the room-temperature spin polarization.

We have studied the superfluid response of liquid ${}^4\mathrm{He}$ confined in a one-dimensional nanometer-size channel by means of a twofold torsional oscillator at 2000 and 500 Hz. For the lower-frequency mode, both the superfluid onset and the dissipation peak shift to the low-temperature side by 40 mK under 0.13 MPa, and the shift is slightly enhanced by the application of pressure. The strong frequency dependence indicates that the superfluid response is a dynamical phenomenon. Furthermore, this dependence is consistent with the theoretical prediction based on the Tomonaga-Luttinger liquid model.

Using scanning tunneling spectroscopy, we address the problem of the superconductor-insulator phase transition in homogeneously disordered ultrathin (2–15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter $k_{F}l\ge 5.6$, manifest a conventional superconductivity: a spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a disordered vortex lattice in magnetic field. Upon thickness reduction, however, while $k_{F}l$ lowers, the STS reveals striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak height and small spatial inhomogeneities. In addition, the gap below $T_C$ develops on a spectral background, which becomes more and more “V-shaped” approaching the localization. The thinnest film (2.16 nm), while not being exactly at the superconductor-insulator transition (SIT) ($T_C\approx 0.4T_C^{\mathrm{bulk}}$), showed unconventional signatures such as the vanishing of the coherence peaks and the absence of vortices. This behavior suggests a weakening of long-range phase coherence, when approaching the SIT in this quasi-2D limit.

The spin-gap evolution upon Ca doping in Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$ was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin-gap excitation survives in this series with $x$ up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at $x$ ∼ 13 by the NMR technique. The strong intensity modulation along $Q_{\mathrm{H}}$ in $x=0$ gradually evolves into a $Q$-independent feature in $x$ > 11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing $x$. It is a surprise that the spin gap persists in the normal state of this spin-ladder system with metallic behavior, which evidences the possibility of magnetically mediated carrier pairing mechanism in a two-leg spin-ladder lattice.

We have studied the magnetization reversal of epitaxial Fe(100) films in an ac magnetic field as a function of frequency between 0.1 and 1.2 MHz by polarized neutron reflectivity. Different modes of magnetization reversal are excited depending on the ac field frequency, those with domain nucleation and propagation, and those with a coherent magnetization rotation. At low frequencies the magnetization follows the external field adiabatically via nucleation and propagation of antiparallel 180${}^{\circ }$ domains. At higher frequencies additional 90${}^{\circ }$ domain walls are being nucleated and propagate. Beyond 0.5 MHz, the magnetization reversal becomes progressively suppressed and finally comes to a stop at about 1 MHz.

We investigate the electromagnetic response of asymmetric broadside coupled split-ring resonators (ABC-SRRs) as a function of the relative in-plane displacement between the two component SRRs. The asymmetry is defined as the difference in the capacitive gap widths ($\Delta g$) between the two resonators comprising a coupled unit. We characterize the response of ABC-SRRs both numerically and experimentally via terahertz time-domain spectroscopy. As with symmetric BC-SRRs ($\Delta g=0$ μm), a large redshift in the LC resonance is observed with increasing displacement, resulting from changes in the capacitive and inductive coupling. However, for ABC-SRRs, in-plane shifting between the two resonators by more than 0.375 $L_o$ ($L_o$ $=$ SRR sidelength) results in a transition to a response with two resonant modes, associated with decoupling in the ABC-SRRs. For increasing $\Delta g$, the decoupling transition begins at the same relative shift (0.375 $L_o$), though with an increase in the oscillator strength of the new mode. This strongly contrasts with symmetric BC-SRRs, which present only one resonance for shifts up to 0.75 $L_o$. Since all BC-SRRs are effectively asymmetric when placed on a substrate, an understanding of ABC-SRR behavior is essential for a complete understanding of BC-SRR based metamaterials.

We study the energy spectrum of a vortex core in a two-dimensional semiconductor with Rashba spin-orbit interaction and proximity coupled to a conventional superconductor and a ferromagnetic insulator. We perform self-consistent calculations using the microscopic tight-binding Bogoliubov–de Gennes method on a lattice and confirm the existence of Majorana fermions in the nontrivial topological phase. We also find two different topologically trivial bulk superconducting phases, only differing in the type of vortex core structure they support and separated by a zero-energy excitation. Furthermore, we find an asymmetry in the energy spectrum with respect to both Zeeman splitting and vortex rotation direction and explain its physical origin.

We propose the use of a dynamical window to investigate the real-time evolution of quantum many-body systems in a one-dimensional lattice. In a recent paper [Phien et al., Phys. Rev. B 86, 245107 (2012)], we introduced infinite boundary conditions in order to investigate real-time evolution of an infinite system under a local perturbation. This was accomplished by restricting the update of the tensors in the matrix product state to a finite region known as a window, with left and right boundaries held at fixed positions. Here we consider instead the use of a dynamical window, where the positions of left and right boundaries are allowed to change in time. In this way, all computational efforts can be devoted to the space-time region of interest, which leads to a remarkable reduction in simulation costs. For illustrative purposes, we consider two applications in the context of the spin-1 antiferromagnetic Heisenberg model in an infinite spin chain: one is an expanding window, with boundaries that are adjusted to capture the expansion in time of a local perturbation of the system; the other is a moving window of fixed size, where the position of the window follows the front of a propagating wave.

We consider the transport properties in an applied magnetic field of the spin $S=1/2$ Kondo model with ferromagnetic exchange coupling to electronic reservoirs, a description relevant for the strong-coupling limit of underscreened spin $S=1$ Kondo impurities. Because the ferromagnetic Kondo interaction is marginally irrelevant, perturbative methods should prove accurate down to low energies. For the purpose of this study, we use a combination of Majorana diagrammatic theory with density matrix numerical renormalization group simulations. In the standard case of antiferromagnetic Kondo exchange, we first show that our simple analytical technique recovers results for the $\mathcal{T}$-matrix and spin relaxation at weak coupling (above the Kondo temperature), which were previously obtained using functional renormalization tools. Considering then the ferromagnetic case, we demonstrate how the low-energy Kondo anomaly splits for arbitrary small values of the Zeeman energy, in contrast to fully screened Kondo impurities near the strong-coupling Fermi liquid fixed point, and in agreement with recent experimental findings for spin $S=1$ molecular quantum dots.

We identify the multilayered compound GeBi${}_4$Te${}_7$ to be a topological insulator with a Dirac point slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisfies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi${}_{4-x}$Sb${}_x$Te${}_7$ we observe a transition from $n$ to $p$ type in bulk sensitive Seebeck coefficient measurements at a doping of $x=0.6$. In surface sensitive ARPES measurements a rigid band shift is observed with Sb doping, accompanied by a movement of the Dirac point towards the Fermi level. Between $x=0.8$ and $x=1$ the Fermi level crosses the band gap, changing the surface transport regime. This difference of the $n$- to $p$-type transition between the surface region and the bulk is caused by band bending effects which are also responsible for a noncoexistence of insulating phases in the bulk and in the near surface region.

We report a numerical study on the Aharonov-Bohm (AB) effect and giant magnetoresistance in rectangular rings made of graphene nanoribbons (GNRs). We show that in the low-energy regime where only the first subband of contact GNRs contributes to the transport, the transmission probability can be strongly modulated, i.e., almost fully suppressed, when tuning a perpendicular magnetic field. On this basis, strong AB oscillations with giant negative magnetoresistance can be achieved at room temperature. The magnetoresistance reaches thousands of percent in perfect GNR rings and a few hundred percent with edge-disordered GNRs. The design rules to observe such strong effects are also discussed. Our study hence provides guidelines for further investigations of the AB interference and to obtain high magnetoresistance in graphene devices.

Atomic force microscopy experiments and molecular dynamics simulations show that friction between a nanoscale tip and atomically stepped surfaces of graphite is influenced by the environment. The presence of a small amount of water increases friction at atomic steps, but does not strongly influence friction on flat terraces.

We develop a synchronization theory for the dynamics of two-dimensional electrons under a perpendicular magnetic field and microwave irradiation showing that dissipative effects can lead to the synchronization of the cyclotron phase with the driving microwave phase at certain resonant ratios between microwave and cyclotron frequencies. We demonstrate two important consequences of this effect: the stabilization of skipping orbits along the sample edges and the trapping of the electrons on localized short-ranged impurities. We then discuss how these effects influence the transport properties of ultrahigh-mobility two-dimensional electron gas and propose mechanisms by which they lead to microwave-induced zero-resistance states. Our theoretical analysis shows that the classical electron dynamics along the edge and around circular disk impurities is well described by the Chirikov standard map providing a unified formalism for those two rather different cases. We argue that this work will provide the foundations for a full quantum synchronization theory of zero-resistance states for which a fully microscopic detailed theory still should be developed.

Using large-scale dynamical cluster quantum Monte Carlo simulations, we explore the unconventional superconductivity in the hole-doped Hubbard model on the triangular lattice. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, we find a doubly degenerate singlet pairing state at an interaction strength close to the bare bandwidth. Such an unconventional superconducting state is mediated by antiferromagnetic spin fluctuations along the $\Gamma$-$K$ direction, where the Fermi surface is nested. An exact decomposition of the irreducible particle-particle vertex further confirms the dominant component of the effective pairing interaction comes from the spin channel. Our findings suggest the existence of chiral $d+id$ superconductivity in a hole-doped Hubbard triangular lattice in a strongly correlated regime, and provide insight into the superconducting phases of the water-intercalated sodium cobaltates Na${}_x$CoO${}_2·y$H${}_2$O, as well as the organic compounds $\kappa$-(ET)${}_2$X and Pd(dmit)${}_2$.

We compute analytically the distributions of concurrence $\text{C}$ and squared norm $\text{N}$ for the production of electronic entanglement in a chaotic quantum dot. The dot is connected to the external world via one ideal and one partially transparent lead, characterized by the opacity $\gamma$. The average concurrence increases with $\gamma$ while the average squared norm of the entangled state decreases, making it less likely to be detected. When a minimal detectable norm ${\text{N}}_0$ is required, the average concurrence is maximal for an optimal value of the opacity ${\gamma }^{☆}\left({\text{N}}_0\right)$ which is explicitly computed as a function of ${\text{N}}_0$. If ${\text{N}}_0$ is larger than the critical value ${\text{N}}_0^{☆}\simeq 0.3693\cdots$, the average entanglement production is maximal for the completely ideal case, a direct consequence of an interesting bifurcation effect.

We investigate the effect of spin-orbit coupling on the behavior of a magnetic impurity at the edge of a zigzag graphene ribbon by means of quantum Monte Carlo simulations. A peculiar interplay of Kane-Mele type spin-orbit and impurity-host coupling is found to greatly affect properties for the local moment. The local characters of the impurity are mainly dominated by the local density of states at the edge, such as double occupancy, magnetic moment, and spin susceptibilities. The special helical nature of the topological insulator on the boundary is found to affect nonlocal quantities, such as the two-particle and spin-spin correlation functions linking electrons on the impurity with those in the conduction band; in particular, due to the spin-orbit coupling, the symmetry of the spin rotation in the Kondo cloud around the impurity is partly broken.

Using the time-dependent density-matrix renormalization group, we study the time evolution of electronic wave packets in the one-dimensional extended Hubbard model with on-site and nearest-neighbor repulsion, $U$ and $V$, respectively. As expected, the wave packets separate into spin-only and charge-only excitations (spin-charge separation). Charge and spin velocities exhibit nonmonotonic dependence on $V$. For small and intermediate values of $V$, both velocities increase with $V$. However, the charge velocity exhibits a stronger dependence than that of the spin, leading to a more pronounced spin-charge separation. Charge fractionalization, on the other hand, is weakly affected by $V$. The results are explained in terms of Luttinger liquid theory in the weak-coupling limit and an effective model in the strong-coupling regime.

The response to an electric field (dc and ac) of electronic systems in which the Fermi “surface” consists of a number of three-dimensional (3D) Weyl points (such as some pyrochlore iridates) exhibits a peculiar combination of characteristics usually associated with insulating and conducting behavior. Generically a neutral plasma in clean materials can be described by a tight-binding model with a strong spin-orbit interaction. A system of that type has a vanishing dc conductivity; however the current response to the dc field is very slow: The current decays with time in a powerwise manner, different from an insulator. The ac conductivity, in addition to a finite real part ${\sigma }^{\prime }\left(\Omega \right)$ which is linear in frequency, exhibits an imaginary part ${\sigma }^{\prime \prime }\left(\Omega \right)$ that increases logarithmically as a function of the UV cutoff (atomic scale). This leads to a substantial dielectric response like a large dielectric constant at low frequencies. This is in contrast to a two-dimensional (2D) Weyl semimetal-like graphene at a neutrality point where the ac conductivity is purely pseudodissipative. The Coulomb interaction between electrons is long range and sufficiently strong to make a significant impact on transport. The interaction contribution to the ac conductivity is calculated within the tight-binding model. The result for the real part expressed via the renormalized (at frequency $\overline{\Omega }$) Fermi velocity $v$ is $\Delta {\sigma }^{\prime }\left(\Omega \right)=e^4\Omega /\left(9{\pi }^2\hslash v\right)[2\log (\Omega /\overline{\Omega })-5]$.