arXiv subjects

Peter Foldi

Publications and source records attributed to Peter Foldi.

At least 19 recordsLinked to original sources

arXiv2019

Charges carried by wave packets that are superimposed on plane waves

We consider the superposition of plane waves and localized wave packets. This kind of wave function can result from a local excitation of a particle described by a plane wave. For charged particles, the wave packet means a current, the time integral of which results in an extra charge, additionally to the one carried by the plane wave. When the duration of the wave packet is too short for its details to be resolved experimentally, it is the extra charge that can be determined and analyzed for information regarding the nature of the interaction that created the wave packet. Assuming that the wave function is known initially, we show an analytic method for the calculation of the extra charge by the aid of Fourier transform. Our approach is verified by comparing to both the well-known case of a Gaussian wave packet and numerically obtained results. As an important physical example, finally we consider the case of local excitation by laser pulses.

quant-ph↗

arXiv2017

Ultrashort laser pulse driven currents in conductors: physical mechanisms and time scales

The response of conduction band electrons to a local, pulse-like external excitation is investigated. The charge density wave packets that emerge as a consequence of the excitation leave the interaction region with a speed close to the initial state's band velocity, but there are also oscillations with essentially the same frequency as that of the laser field. As a good estimation, the excitation can also be considered as a localized, time-dependent ponderomotive potential, leading to slowly varying current oscillations. The role of all these effects are investigated for different electron energies, carrier frequencies and sizes of the interaction area.

quant-ph↗

arXiv2017

Sturmian-Floquet approach to high-order harmonic generation

We show that the Floquet approach with a Sturmian basis means an efficient description of high-order harmonic generation with monochromatic excitation. This method, although involves numerical calculations, is close to analytic approaches with the corresponding deeper insight into the dynamics. As a first application, we investigate the role of atomic coherence during the process of HHG: as it is shown, different coherent superpositions of initial atomic states produce observably different HHG spectra. For linearly polarized excitation, we demonstrate that the question whether the constituents of the initial superpositions are dipole coupled or not, strongly influences the dynamics. By investigating time-dependent HHG signals, we also show that the preparation of the initial atomic state can be used for the control of the high-harmonic radiation.

physics.atom-ph↗

arXiv2017

Gauge invariance and the interpretation of inter- and intraband processes in high-order harmonic generation from bulk solids

A theoretical model for high-order harmonic generation (HHG) in bulk solids is considered. Our approach treats laser-induced inter- and intraband currents on an equal footing. The sum of these currents is the source of the high-order harmonic radiation, and does not depend on the particular electromagnetic gauge we choose to describe the process. On the other hand, as it is shown using analytic and numerical calculations, the distinction between intra- and interband dynamics is gauge dependent, implying that the interpretation of the process of HHG using these terms requires carefulness.

quant-ph↗

arXiv2016

Correlations between the final momenta of electrons and their initial phase-space distribution in the process of tunnel ionization

We present both full quantum mechanical and semiclassical calculations of above threshold ionization (ATI) of a hydrogen atom in the tunneling regime by a few-cycle linearly polarized infrared laser pulse. As a quantum treatment, we applied the direct integration of the time dependent Schrödinger equation (TDSE). In the semiclassical approximation (SCA), it is assumed that wavepacket propagation in the post-tunneling process can be well described within the classical framework. With these two methods, we analyze the similarities and deviations for ionization of the hydrogen atom. We found that the 3 dimensional semiclassical method can describe reasonably well the momentum correlation pattern of the ATI peaks. We also show good agreement between the results obtained by TDSE method and the semi-classical method. Furthermore, with the semiclassical approximation we clearly identify and separate the regions in momentum distributions of the ejected electrons according to initial conditions. We illustrate the corresponding regions with typical electron trajectories.

physics.atom-ph↗

arXiv2016

Atomic coherence effects in few-cycle pulse induced ionization

The interaction of a short, few-cycle light pulse and an atom which is prepared initially in a superposition of two stationary states is shown to exhibit strong signatures of atomic coherence. For a given waveform of the laser pulse, appropriate quantum mechanical relative phase between the constituents of the initial superposition can increase the ionization probability by a factor of three. A similarly strong effect can be observed when the waveform of the ionizing pulse is changed. These results allow for intuitive explanations, which are in agreement with the numerical integration of the time dependent Schrödinger equation.

physics.atom-ph↗

arXiv2015

Quantum mechanical scattering on time-dependent potentials using nonequilibrium Green's functions

Time-dependent nonequilibrium Green's functions (TDNEGF) are shown to provide a flexible, effective tool for the description of quantum mechanical single particle scattering on a spatially localized, time-dependent potential. Focusing on numerical methods, arbitrary space and time dependence of the potential can be treated, provided it is zero before an initial time instant. In this case, appropriate version of the Dyson and Keldysh equations lead to a transparent description with clear physical interpretation. The interaction of a short laser pulse and an electron propagating initially in free space is discussed as an example.

quant-ph↗

arXiv2015

Spin-orbit interaction controlled properties of two-dimensional superlattices: Spintronic crystals

The band structure of two-dimensional artificial superlattices in the presence of (Rashba-type) spin-orbit interaction (SOI) is presented. The position and shape of the energy bands in these spintronic crystals depend on the geometry as well as the strength of the SOI, which can be tuned by external gate voltages. For finite mesoscopic arrays we show that their conductance properties can be understood from these spin-dependent band diagrams.

cond-mat.mes-hall↗

arXiv2015

High temperature conductance of a two-dimensional superlattice controlled by spin-orbit interaction

Rashba-type spin-orbit interaction (SOI) controlled band structure of a two-dimensional superlattice allows for the modulation of the conductance of finite size devices by changing the strength of the SOI. We consider rectangular arrays and find that the temperature dependence of the conductance disappears for high temperatures, but the strength of the SOI still affects the conductance at these temperatures. The modulation effect can be seen even in the presence of strong dephasing, which can be important in case of practical applications.

cond-mat.mes-hall↗

arXiv2014

Nanoscale optical waveform control of strong-field photoemission

Strong-field photoemission from metal nanostructures enabled fundamental discoveries recently. Here, we deliver theoretical demonstration of the electric field control of electrons in the closest nanoscale vicinity of plasmonic nanoparticles with the help of few-cycle laser waveforms. We analyze the effect of plasmonic resonance on photoemission properties and show that it is only off-resonant nanoparticles that can provide electron control on a true sub-fs timescale.

cond-mat.mes-hall↗

arXiv2014

Oscillating spin-orbit interaction as a source of spin-polarized wave packets in two-terminal nanoscale devices

Ballistic transport through nanoscale devices with time-dependent Rashba-type spin-orbit interaction (SOI) can lead to spin-polarized wave packets that appear even for completely unpolarized input. The SOI that oscillates in a finite domain generates density and spin polarization fluctuations that leave the region as propagating waves. Particularly, spin polarization has space and time dependence even in regions without SOI. Our results are based on an analytic solution of the time-dependent Schrödinger equation. The relevant Floquet quasi-energies that are obtained appear in the energy spectrum of both the transmitted and reflected waves.

cond-mat.mes-hall↗

arXiv2013

The effect of dynamical Bloch oscillations on optical-field-induced current in a wide gap dielectric

We consider the motion of charge carriers in a bulk wide-gap dielectric interacting with a few-cycle laser pulse. A semiclassical model based on Bloch equations is applied to describe the emerging time-dependent macroscopic currents for laser intensities approaching the damage threshold. At such laser intensities, electrons can reach edges of the first Brillouin zone even for electron-phonon scattering rates as high as those known for SiO_2. We find that, whenever this happens, Brag-like reflections of electron waves, also known as Bloch oscillations, affect the dependence of the charge displaced by the laser pulse on its carrier-envelope phase.

cond-mat.mes-hall↗

arXiv2009

Quantum rings with time dependent spin-orbit coupling: Rabi oscillations, spintronic Schrodinger-cat states, and conductance properties

The strength of the (Rashba-type) spin-orbit coupling in mesoscopic semiconductor rings can be tuned with external gate voltages. Here we consider the case of a periodically changing spin-orbit interaction strength as induced by sinusoidal voltages. In a closed one dimensional quantum ring with weak spin-orbit coupling, Rabi oscillations are shown to appear. We find that the time evolution of initially localized wave packets exhibits a series of collapse and revival phenomena. Partial revivals -- that are typical in nonlinear systems -- are shown to correspond to superpositions of states localized at different spatial positions along the ring. These "spintronic Schrodinger-cat sates" appear periodically, and similarly to their counterparts in other physical systems, they are found to be sensitive to environment induced disturbances. The time dependent spin transport problem, when leads are attached to the ring, is also solved. We show that the "sideband currents" induced by the oscillating spin-orbit interaction strength can become the dominant output channel, even in the presence of moderate thermal fluctuations and random scattering events.

cond-mat.mes-hall↗

arXiv2009

Stability of spintronic devices based on quantum ring networks

Transport properties in mesoscopic networks are investigated, where the strength of the (Rashba-type) spin-orbit coupling is assumed to be tuned with external gate voltages. We analyze in detail to what extent the ideal behavior and functionality of some promising network-based devices are modified by random (spin-dependent) scattering events and by thermal fluctuations. It is found that although the functionality of these devices is obviously based on the quantum coherence of the transmitted electrons, there is a certain stability: moderate level of errors can be tolerated. For mesoscopic networks made of typical semiconductor materials, even cryogenic temperatures can smear out the desired transport properties. When the energy distribution of the input carriers is narrow enough, it turns out that the devices can operate close to their ideal limits even at relative high temperature. As an example, we present results for two different networks: one that realizes a Stern-Gerlach device and another that simulates a spin quantum walker. Finally we propose a simple network that can act as a narrow band energy filter even in the presence of random scatterers.

cond-mat.mes-hall↗

arXiv2008

Networks of quantum nanorings: programmable spintronic devices

An array of quantum rings with local (ring by ring) modulation of the spin orbit interaction (SOI) can lead to novel effects in spin state transformation of electrons. It is shown that already small (3x3, 5x5) networks are remarkably versatile from this point of view: Working in a given network geometry, the input current can be directed to any of the output ports, simply by changing the SOI strengths by external gate voltages. Additionally, the same network with different SOI strengths can be completely analogous to the Stern-Gerlach device, exhibiting spatial-spin entanglement.

cond-mat.mes-hall↗

arXiv2008

Magnetoconductance properties of rectangular arrays of spintronic quantum rings

Electron transport through multi-terminal rectangular arrays of quantum rings is studied in the presence of Rashba-type spin-orbit interaction (SOI) and of a perpendicular magnetic field. Using the analytic expressions for the transmission and reflection coefficients for single rings we obtain the conductance through such arrays as a function of the SOI strength, the magnetic flux, and of the wave vector $k$ of the incident electron. Due to destructive or constructive spin interferences caused by the SOI, the array can be totally opaque for certain ranges of $k$, while there are parameter values where it is completely transparent. Spin resolved transmission probabilities show nontrivial spin transformations at the outputs of the arrays. When point-like random scattering centers are placed between the rings, the Aharonov-Bohm peaks split, and an oscillatory behavior of the conductance emerges as a function of the SOI strength.

cond-mat.mes-hall↗

arXiv2008

Dynamics of periodic anticrossings: Decoherence, pointer states and hysteresis curves

We consider a strongly driven two-level (spin) system, with a periodic external field that induces a sequence of avoided level crossings. The spin system interacts with a bosonic reservoir which leads to decoherence. A Markovian dynamical equation is introduced without relying on the rotating wave approximation in the system-external field interaction. We show that the time evolution of the two-level system is directed towards an incoherent sum of periodic Floquet states regardless of the initial state and even the type of the coupling to the environment. Analyzing the time scale of approaching these time-dependent pointer states, information can be deduced concerning the nature and strength of the system-environment coupling. The inversion as a function of the external field is usually multi-valued, and the form of these hysteresis curves is qualitatively different for low and high temperatures. For moderate temperatures we found that the series of Landau-Zener-Stückelberg-type transitions still can be used for state preparation, regardless of the decoherence rate. Possible applications include quantum information processing and molecular nanomagnets.

quant-ph↗

arXiv2006

Interference induced preparation of spinpolarized electrons in a three-terminal quantum ring

We present an exact, analytic solution of the spin dependent quantum transport problem with spin-orbit interaction in a one-dimensional mesoscopic ring with one input and two output leads. We demonstrate that for appropriate parameters spatial interference in the ring leads to a behavior analogous to that of the Stern-Gerlach apparatus: different spin polarizations can be achieved in the two output channels from an originally totally unpolarized incoming spin state. It is shown that this requires an appropriate interference of states that carry oppositely directed currents. We find that spin polarization is possible for several geometries, including the case when the device is not symmetric with respect to the incoming lead. A clear connection is established between the Stern-Gerlach like property of the device and the relevant Aharonov-Casher phases in the loop geometry.

quant-ph↗