arXiv subjects

L. S. Levitov

Publications and source records attributed to L. S. Levitov.

At least 73 records · Page 4Linked to original sources

arXiv2004

Fermi-Edge Resonance and Tunneling in Nonequilibrium Electron Gas

Fermi-edge singularity changes in a dramatic way in a nonequilibrium system, acquiring features which reflect the structure of energy distribution. In particular, it splits into several components if the energy distribution exhibits multiple steps. While conventional approaches, such as bosonization, fail to describe the nonequilibrium problem, an exact solution for a generic energy distribution can be obtained with the help of the method of functional determinants. In the case of a split Fermi distribution, while the `open loop' contribution to Green's function has power law singularities, the tunneling density of states profile exhibits broadened peaks centered at Fermi sub-levels.

cond-mat.mes-hall↗

arXiv2004

Tunable Fermi-Edge Resonance in an Open Quantum Dot

Resonant tunneling in an open mesoscopic quantum dot is proposed as a vehicle to realize a tunable Fermi-edge resonance with variable coupling strength. We solve the x-ray edge problem for a generic nonseparable scatterer and apply it to describe tunneling in a quantum dot. The tunneling current power law exponent is linked to the S-matrix of the dot. The control of scattering by varying the dot shape and coupling to the leads allows to explore a wide range of exponents. Transport properties, such as weak localization, mesoscopic conductance fluctuations, and sensitivity to Wigner-Dyson ensemble type, have their replicas in the Fermi-edge singularity.

cond-mat.mes-hall↗

arXiv2004

Atom-molecule coexistence and collective dynamics near a Feshbach resonance of cold fermions

Degenerate Fermi gas interacting with molecules near Feshbach resonance is unstable with respect to formation of a mixed state in which atoms and molecules coexist as a coherent superposition. Theory of this state is developed using a mapping to the Dicke model, treating molecular field in the single mode approximation. The results are accurate in the strong coupling regime relevant for current experimental efforts. The exact solution of the Dicke model is exploited to study stability, phase diagram, and nonadiabatic dynamics of molecular field in the mixed state.

cond-mat.mes-hall↗

arXiv2004

Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, which is driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. In the spatially uniform 2d electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. We analyze the mechanism of wavelength selection, and show that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface.

cond-mat.mes-hall↗

arXiv2003

DC measurements of macroscopic quantum levels in a superconducting qubit structure with a time-ordered meter

DC measurements are made in a superconducting, persistent current qubit structure with a time-ordered meter. The persistent-current qubit has a double-well potential, with the two minima corresponding to magnetization states of opposite sign. Macroscopic resonant tunneling between the two wells is observed at values of energy bias that correspond to the positions of the calculated quantum levels. The magnetometer, a Superconducting Quantum Interference Device (SQUID), detects the state of the qubit in a time-ordered fashion, measuring one state before the other. This results in a different meter output depending on the initial state, providing different signatures of the energy levels for each tunneling direction. From these measurements, the intrawell relaxation time is found to be about 50 microseconds.

cond-mat.supr-con↗

arXiv2003

Angular distribution of photoluminescence as a probe of Bose Condensation of trapped excitons

Recent experiments on two-dimensional exciton systems have shown the excitons collect in shallow in-plane traps. We find that Bose condensation in a trap results in a dramatic change of the exciton photoluminescence (PL) angular distribution. The long-range coherence of the condensed state gives rise to a sharply focussed peak of radiation in the direction normal to the plane. By comparing the PL profile with and without Bose Condensation we provide a simple diagnostic for the existence of a Bose condensate. The PL peak has strong temperature dependence due to the thermal order parameter phase fluctuations across the system. The angular PL distribution can also be used for imaging vortices in the trapped condensate. Vortex phase spatial variation leads to destructive interference of PL radiation in certain directions, creating nodes in the PL distribution that imprint the vortex configuration.

cond-mat↗

arXiv2003

Charge transport and phase transition in exciton rings

The macroscopic exciton rings observed in the photoluminescence (PL) patterns of excitons in coupled quantum wells (CQWs) are explained by a series of experiments and a theory based on the idea of carrier imbalance, transport and recombination. The rings are found to be a source of cold excitons with temperature close to that of the lattice. We explored states of excitons in the ring over a range of temperatures down to 380 mK. These studies reveal a sharp, albeit continuous, second order phase transition to a low-temperature ordered exciton state, characterized by ring fragmentation into a periodic array of aggregates. An instability at the onset of degeneracy in the cold exciton system, due to stimulated exciton formation, is proposed as the transition mechanism.

cond-mat↗

arXiv2003

Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure

Measurements of thermal activation are made in a superconducting, niobium Persistent-Current (PC) qubit structure, which has two stable classical states of equal and opposite circulating current. The magnetization signal is read out by ramping the bias current of a DC SQUID. This ramping causes time-ordered measurements of the two states, where measurement of one state occurs before the other. This time-ordering results in an effective measurement time, which can be used to probe the thermal activation rate between the two states. Fitting the magnetization signal as a function of temperature and ramp time allows one to estimate a quality factor of 10^6 for our devices, a value favorable for the observation of long quantum coherence times at lower temperatures.

cond-mat.mes-hall↗

arXiv2002

The Statistical Theory of Mesoscopic Noise

Microscopic theory of counting statistics of electrical noise is reviewed. We discuss a model of passive charge detector based on current fluctuations coupled to a spin, and its relation with the theory of photon counting in quantum optics. The statistics of tunneling current and, in particular, the properties of the third moment are studied in detail. The third moment is shown to be temperature-independent for tunneling in a generic many-body system. Then the statistics of mesoscopic transport is discussed. We consider applications of the functional determinant formula for the generating function of counting distribution to the DC and photo-assisted transport, and to mesoscopic pumping. A universal dependence of the noise in a mesoscopic pump on the pumping fields is obtained and shown to provide a method of measuring the quasiparticle charge in an open system without any fitting parameters.

cond-mat.mes-hall↗

arXiv2002

Dynamical spin-electric coupling in a quantum dot

Due to the spin-orbital coupling in a semiconductor quantum dot, a freely precessing electron spin produces a time-dependent charge density. This creates a sizeable electric field outside the dot, leading to promising applications in spintronics. The spin-electric coupling can be employed for non-invasive single spin detection by electrical methods. We also consider a spin relaxation mechanism due to long-range coupling to electrons in gates and elsewhere in the system, and find a contribution comparable to, and in some cases dominant over previously discussed mechanisms.

cond-mat.mes-hall↗

arXiv2002

Narrow gap Luttinger liquid in Carbon nanotubes

Electron interactions reinforce minigaps induced in metallic nanotubes by an external field and turn the gap field dependence into a universal power law. An exactly solvable Gross-Neveau model with an SU(4) symmetry is derived for neutral excitations near half-filling. Charge excitations, described by a sin-Gordon perturbation of Luttinger liquid theory, are composite solitons formed by the charged and neutral fields with two separate length scales. Charge compressibility at finite density, evaluated in terms of inter-soliton interaction, exhibits a crossover from overlapping to non-overlapping soliton state. Implications for the Coulomb blockade measurements are discussed.

cond-mat.mes-hall↗

arXiv2002

Electron properties of Carbon nanotubes in the field effect regime

Electron properties of Carbon nanotubes can change qualitatively in a transverse electric field. In metallic tubes the sign of Fermi velocity can be reversed in a sufficiently strong field, while in semiconducting tubes the effective mass can change sign. These changes in the spectrum manifest themselves in a breakup of the Fermi surface and in the energy gap suppression, respectively. The effect is controlled by the field inside the tube which is screened due to the polarization induced on the tube. The theory of screening links it with the chiral anomaly for 1D fermions and obtains a universal screening function determined solely by the Carbon pi electron band.

cond-mat.mes-hall↗

arXiv2001

Internal waves and synchronized precession in a cold vapor

Exchange in a Boltzmann gas of bosons with several internal states leads to collective transport of internal polarization. The internal dynamics can be understood as Larmor precession in the presence of a torque induced by atoms on each other via exchange coupling. A generalized Bloch equation that includes interatomic exchange effects as well as orbital motion in the gas is derived and used to interpret recent experiment by Lewandowski et al. as an excitation of a collective wave of internal state polarization. It is shown that exchange leads to formation of domains in which precession frequencies are synchronized.

physics.atom-ph↗

arXiv2001

Electron shot noise beyond the second moment

The form of electron counting statistics of the tunneling current noise in a generic many-body interacting electron system is obtained. The third correlator of current fluctuations (the skewness of the charge counting distribution) has a universal relation with the current I and the quasiparticle charge q. This relation C_3 = q^2 I holds in a wide bias voltage range, both at large and small eV/kT, thereby representing an advantage compared to the Schottky formula. We consider the possibility of using the counting statistics for detecting quasiparticle charge at high temperature.

cond-mat.mes-hall↗

arXiv2001

Electromechanical noise in a diffusive conductor

Electrons moving in a conductor can transfer momentum to the lattice via collisions with impurities and boundaries, giving rise to a fluctuating mechanical stress tensor. The root-mean-squared momentum transfer per scattering event in a disordered metal (of dimension L greater than the mean free path l and screening length xi) is found to be reduced below the Fermi momentum by a factor of order l/L for shear fluctuations and (xi/L)^2 for pressure fluctuations. The excitation of an elastic bending mode by the shear fluctuations is estimated to fall within current experimental sensitivity for a nanomechanical oscillator.

cond-mat.mes-hall↗

arXiv2001

Collective dynamics of internal states in a Bose gas

Theory for the Rabi and internal Josephson effects in an interacting Bose gas in the cold collision regime is presented. By using microscopic transport equation for the density matrix the problem is mapped onto a problem of precession of two coupled classical spins. In the absence of an external excitation field our results agree with the theory for the density induced frequency shifts in atomic clocks. In the presence of the external field, the internal Josephson effect takes place in a condensed Bose gas as well as in a non-condensed gas. The crossover from Rabi oscillations to the Josephson oscillations as a function of interaction strength is studied in detail.

physics.atom-ph↗

arXiv2001

Quantum spin chains and Majorana states in arrays of coupled qubits

Several designs of inter-qubit coupling are considered. It is shown that by a combination of Josephson and capacitive coupling one can realize qubit interactions of variable spin content. Qubit arrays are discussed as models of quantum spin chains. In particular, a qubit model of the 1D quantum Ising spin chain is proposed. A realization of unpaired Majorana fermion states in this system is considered. It is shown that Majorana states are represented by spin flip excitations localized on the chain ends. Using unpaired Majorana states in qubit chains for decoherence protected quantum computing is discussed.

cond-mat.mes-hall↗

arXiv2001

Effective action of a compressible QH state edge: application to tunneling

The electrodynamical response of the edge of a compressible Quantum Hall system affects tunneling into the edge. Using the composite Fermi liquid theory, we derive an effective action for the edge modes interacting with tunneling charge. This action generalizes the chiral Luttinger liquid theory of the Quantum Hall edge to compressible systems in which transport is characterized by a finite Hall angle. In addition to the standard terms, the action contains a dissipative term. The tunneling exponent is calculated as a function of the filling fraction for several models, including short-range and long-range Coulomb interaction. We find that tunneling exponents are robust and to a large extent insensitive to the particular model. We discuss recent tunneling measurements in the overgrown cleaved edge systems, and demonstrate that the profile of charge density near the edge is very sensitive to the parameters of the system. In general, the density is non-monotonic, and can deviate from the bulk value by up to 30%. Implications for the correspondence to the chiral Luttinger edge theories are discussed.

cond-mat.mes-hall↗