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Scaling of entanglement entropy and superselection rules
Particle number conservation in fermionic systems restricts the allowed local operations on bi-partite systems. We show how this restriction is related to measurement entropy of particle fluctuations and compute it for several regimes of practical relevance. The accessible entanglement entropy restricted by particle number conservation is equal, to leading order, to the full entanglement entropy. The correction is bounded by the log of the variance of particle number fluctuations. The results are applied to generic Fermi systems in dimension d>1 as well as to several critical systems in d=1.
On the Possibility of an Electronic-structure Modulation Transistor
We present a novel electronic-structure modulation transistor (EMT), which can possibly be used for post-CMOS logic applications. The device principle is based on the bandwidth modulation of a midgap or near-midgap localized state in the channel by a gate voltage. A single-band tight-binding method coupled with non-equilibrium Green's function formalism for quantum transport is employed to predict the IV characteristics. Our objective is to confirm if an EMT has a self gain and if it can overcome the 2.3kT/decade thermal limit with low supply voltage. The ON current depends on the bandwidth of the state and is limited by the quantum of conductance for a single band. The OFF current is set by the gate leakage and tunneling through the higher bands, which is expected to be small if these bands are a few eV above the energy level of the localized state.
Persistent Current in Metallic Rings and Cylinders
We explore the behavior of persistent current and low-field magnetic response in mesoscopic one-channel rings and multi-channel cylinders within the tight-binding framework. We show that the characteristic properties of persistent current strongly depend on total number of electrons $N_e$, chemical potential $μ$, randomness and total number of channels. The study of low-field magnetic response reveals that only for one-channel rings with fixed $N_e$, sign of the low-field currents can be predicted exactly, even in the presence of disorder. On the other hand, for multi-channel cylinders, sign of the low-field currents cannot be mentioned exactly, even in the perfect systems with fixed $N_e$ as it significantly depends on the choices of $N_e$, $μ$, number of channels, disordered configurations, etc.
Hybridization of Spin and Plasma Waves in Josephson Tunnel Junctions Containing a Ferromagnetic Layer
We study dynamics of tunnel Josephson junctions with a thin ferromagnetic layer F [superconductor-insulator-ferromagnet-superconductor (SIFS) junctions]. On the basis of derived equations relating the superconducting phase and magnetic moment to each other we analyze collective excitations in the system and find a new mode which is a hybrid of plasmalike and spin waves. The latter are coupled together in a broad range of parameters characterizing the system. Using the solution describing the collective modes we demonstrate that besides the Fiske steps new peaks appear on the I-V characteristics due to oscillations of the magnetic moment M in the ferromagnetic layer. Thus, by measuring the I-V curve of the SIFS junctions, one can extract information about the spectrum of spin excitations in the ferromagnet F.
Phonon Thermal Conduction in Graphene
We investigated theoretically the phonon thermal conductivity of single layer graphene. The phonon dispersion for all polarizations and crystallographic directions in graphene lattice was obtained using the valence-force field method. The three-phonon Umklapp processes were treated exactly using an accurate phonon dispersion and Brillouin zone, and accouting for all phonon relaxation channels allowed by the momentum and energy conservation laws. The uniqueness of graphene was reflected in the two-dimensional phonon density of states and restrictions on the phonon Umklapp scattering phase-space. The phonon scattering on defects and graphene edges has been also included in the model. The calculations were performed for the Gruneisen parameter, which was determined from the ab initio theory as a function of the phonon wave vector and polarization branch, and for a range of values from experiments. It was found that the near room-temperature thermal conductivity of single layer graphene, calculated with a realistic Gruneisen parameter, is in the range ~ 2000 - 5000 W/mK depending on the defect concentration and roughness of the edges. Owing to the long phonon mean free path the graphene edges produce strong effect on thermal conductivity even at room temperature. The obtained results are in good agreement with the recent measurements of the thermal conductivity of suspended graphene.
A Novel Temperature-Induced Magnetization Transition in Pt/Co/Pt Sandwiches
It is found in the Pt/Co/Pt(111) sandwiches that perpendicular magnetic anisotropy weakens associated with the dramatic magnetization enhancement as the temperature decreases below a critical temperature, T_cri, which increases with the Pt overlayer thickness t_Pt. For the sandwich with enough thick Pt overlayer, it trends to an unusual state with weak perpendicular magnetic anisotropy in the absence of obvious easy direction behavior at low temperature. It suggests that Pt is more significantly polarized in the film with thicker Pt overlayer. The results reveal the strong influence of both thick nonmagnetic coverage and temperature on the electronic structure of the Pt/Co/Pt(111) sandwich.
Anomalous quantum transport in a thin film
We present a numerical study of electron transport in a thin film of varying disorder strength with the distance from its surface. A simple tight-binding model is used to describe the system, in which the film is attached to two metallic electrodes and the coupling of this film to the electrodes is illustrated by the Newns-Anderson chemisorption theory. Quite interestingly we observe that, in the smoothly varying disordered film current amplitude increases with the increase of disorder strength in the strong disorder regime, while it decreases in the weak disorder regime. This behavior is completely opposite from a conventional disordered film, where current amplitude always decreases with the increase of disorder strength.
Giant spin rotation in the normal metal/quantum spin Hall junction
We study theoretically reflection problem in the junction between a normal metal and an insulator characterized by a parameter $M$, which is a usual insulator for $M>0$ or a quantum spin Hall system for $M<0$. The spin rotation angle $α$ at the reflection is obtained in the plane of $M$ and the incident angle $θ$ measured from the normal to the interface. The $α$ shows rich structures around the quantum critical point M=0 and $θ=0$, i.e., $α$ can be as large as $\sim π$ at an incident angle in the quatum spin Hall case $M<0$ because the helical edge modes resonantly enhance the spin rotation, which can be used to map the energy dispersion of the helical edge modes. As an experimentally relevant system, we also study spin rotation effect in quantum spin Hall/normal metal/quantum spin Hall trilayer junction.
Reply to the comment on "Influence of the Dzyaloshinskii-Moriya Exchange Interaction on Quantum Phase Interference of Spins"
First version: del Barco et al. submitted recently a comment [arXiv:0812.4070] on our latest Phys. Rev. Lett. [Phys. Rev. Lett. 101, 237204 (2008)], claiming three basic mistakes. We show here that their claims are unjustified and based on erroneous calculations and hasty conclusions. Second version: reply to the modified version of del Barco et al. submitted to Phys. Rev. Lett.
Scattering of electrons by clusters of charged impurities in graphene
It is shown that clustering of charged impurities on graphene can suppress their contribution to the resistivity by a large factor of about the number of impurities per cluster, while leaving the density dependence unchanged. If the cluster size is large in comparison with the Fermi wavelength, the scattering cross section shows sharp resonances as a function of incident angle and electron wavevector. In this regime, due to dominant contribution of scattering by small angles, the transport cross section can be much smaller than the quantum one, which can be verified experimentally by comparing the Dingle temperature and the electron mean free path.
Coherent Stimulated X-ray Raman Spectroscopy; Attosecond Extension of RIXS
Spontaneous and stimulated resonant inelastic X-ray Raman scattering signals are calculated using the Keldysh-Schwinger closed-time path loop and expressed as overlaps of Doorway and Window electron-hole wavepackets. These are recast in terms of the one-particle Green's functions and expansion coefficients of configuration interaction singles for valence excitations, which can be obtained from standard electronic structure codes. Calculation for many-body states of ground and core-excited system is avoided.
Intermode Dephasing in a Superconducting Stripline Resonator
We study superconducting stripline resonator (SSR) made of Niobium, which is integrated with a superconducting interference device (SQUID). The large nonlinear inductance of the SQUID gives rise to strong Kerr nonlinearity in the response of the SSR, which in turn results in strong coupling between different modes of the SSR. We experimentally demonstrate that such intermode coupling gives rise to dephasing of microwave photons. The dephasing rate depends periodically on the external magnetic flux applied to the SQUID, where the largest rate is obtained at half integer values (in units of the flux quantum). To account for our result we compare our findings with theory and find good agreement. Supplementary info at arXiv:0901.3133 .
Magnetic Field Induced Superconductivity in Out-of-Equilibrium Nanowires
Four-terminal resistance measurements have been carried out on Zn nanowires formed using electron-beam lithography. When driven resistive by current, these wires re-enter the superconducting state upon application of small magnetic fields. The data are qualitatively different from those of previous experiments on superconducting nanowires, which revealed either negative magnetoresistance near T_{c} or magnetic field enhanced critical currents. We suggest that our observations are associated with the damping of phase slip processes by the enhancement of dissipation by the quasiparticle conductance channel resulting from the application of a magnetic field.
Zero-bias anomaly induced by the point defect in graphene
It is generally believed that a point defect in graphene gives rise to an impurity state at zero energy and causes a sharp peak in the local density of states near the defect site. We revisit the defect problem in graphene and find the general consensus incorrect. By both analytic and numeric methods, we show that the contribution to the local density of states from the impurity state vanishes in the thermodynamic limit. Instead, the pronounced peak of the zero-bias anomaly is a power-law singularity $1/|E|$ from infinite resonant peaks in the low-energy regime induced by the defect. Our finding shows that the peak shall be viewed as a collective phenomenon rather than a single impurity state in previous studies.
Massless and massive particle-in-a-box states in single-and bi-layer graphene
Electron transport through short, phase-coherent metal-graphene-metal devices occurs via resonant transmission through particle-in-a-box-like states defined by the atomically-sharp metal leads. we study the spectrum of particle-in-a-box states for single- and bi-layer graphene, corresponding to massless and massive two-dimensional (2d) Fermions.The density of states D as a function of particle number n shows the expected relations D(n) ~ n1/2 for massless 2d Fermions (electrons in single-layer graphene) and D(n) ~ constant for massive 2d Fermions (electrons in bi-layer graphene). The single parameters of the massless and massive dispersion relations are found, Fermi velocity vF = 1.1 x 106m/s and effective mass m* = 0.032 me, where me is the electron mass, in excellent agreement with theoretical expectations.
Universality of the conductance in quantum dot transport
We revisited the scaling behavior of the transport properties of a quantum dot system described by the spin-1/2 Anderson model using analytical methods. In the low temperature limit we show that the conductance has a universal behavior with universality between temperature and bias. We compare this result with the empirical formula used to fit the experimental data for conductance in the case of the equilibrium transport through a single channel quantum dot. In the high temperature limit the conductance obtained from the Anderson model is compared with previous results obtained from the Kondo model. The universal behavior is present also in the high temperature limit. These results are in good agreement with the Renormalization group calculations.
Excitation spectra, spin structures, and entanglement characteristics of four-electron double-quantum-dot artificial molecules
Energy spectra, spin configurations, and entanglement characteristics of a system of four electrons in lateral double quantum dots are investigated using exact diagonalization (EXD), as a function of interdot separation, applied magnetic field, and strength of interelectron repulsion. A distinctly different quantum behavior is found compared to that of circular single quantum dots. As a function of the magnetic field, the energy spectra exhibit a low-energy band consisting of a group of six states, with the number six being a consequence of the conservation of the total spin of the four electrons and the ensuing spin degeneracies. These six states appear to cross at a single value of the magnetic field, with the crossing point becoming sharper for larger interdot distances. As the strength of the Coulomb repulsion increases, the six states tend to become degenerate and a well defined energy gap separates them from the higher-in-energy excited states. The appearance of the low-energy band is a consequence of the formation of a Wigner supermolecule, with the four electrons (two in each dot) being localized at the vertices of a rectangular parallelogram. Using the spin-resolved pair-correlation functions, one can map the EXD many-body wave functions onto the spin functions associated with the four localized electrons. Analogies with nanomagnets, such as finite Heisenberg clusters, are discussed. The ability to determine associated spin functions enables investigations concerning entanglement properties of the system of four electrons. In particular, the formation of Wigner supermolecules generates EXD solutions belonging to the class of strongly entangled states referred to as N-qubit Dicke states in the quantum-information literature.