Momentum Relaxation Processes Research Articles

Giant Angular Nernst Effect in the Organic Metal α-(BEDT-TTF)2KHg(SCN)4

We have detected a large Nernst effect in the charge density wave state of the multiband organic metal α-(BEDT-TTF)2KHg(SCN)4. We find that apart from the phonon drag effect, the energy relaxation processes that govern the electron–phonon interactions and the momentum relaxation processes that determine the mobility of the q1D charge carriers have a significant role in observing the large Nernst signal in the CDW state in this organic metal. The emphasised momentum relaxation dynamics in the low field CDW state (CDW0) is a clear indicator of the presence of a significant carrier mobility that might be the main source for observation of the largest Nernst signal. The momentum relaxation is absent with increasing angle and magnetic field, i.e., in the high-field CDW state (CDWx) as evident from the much smaller Nernst effect amplitude in this state. In this case, only the phonon drag effect and electron–phonon interactions are contributing to the transverse thermoelectric signal. Our findings advance and change previous observations on the complex properties of this organic metal.

Proposal for a solid-state magnetoresistive Larmor quantum clock

We propose a solid-state implementation of the Larmor clock that exploits tunnel magnetoresistance to distill information on how long itinerant spins take to traverse a barrier embedded in it. Keeping in mind that the tunnelling time innately involves pristine pre-selection and post-selection, our proposal takes into account the detrimental aspects of multiple reflections by incorporating multiple contacts, multiple current measurements and suitably defined magnetoresistance signals. Our analysis provides a direct mapping between the magnetoresistance signals and the tunneling times and aligns well with the interpretation in terms of generalized quantum measurements and quantum weak values. By means of an engineered pre-selection in one of the ferromagnetic contacts, we also elucidate how one can make the measurement "weak" by minimizing the back-action, while keeping the tunneling time unchanged. We then analyze the resulting interpretations of the tunneling time and the measurement back action in the presence of phase breaking effects that are intrinsic to solid state systems. We unravel that while the time-keeping aspect of the Larmor clock is reasonably undeterred due to momentum and phase relaxation processes, it degrades significantly in the presence of spin-dephasing. We believe that the ideas presented here also open up a fructuous solid state platform to encompass emerging ideas in quantum technology such as quantum weak values and its applications, that are currently exclusive to quantum optics and cold atoms.

Theoretical Analysis of Terahertz Frequency Multiplier Based on Semiconductor Superlattices.

We propose a terahertz frequency multiplier based on high order harmonic generation in a GaAs-based miniband superlattice driven by an electric field. The performance of the frequency multiplier is analyzed using the balance equation approach, which incorporates momentum and energy relaxation processes at different lattice temperatures. It is found that the generated high-order harmonic power is sensitive to temperature changes. The peak power appears around resonance between driving terahertz frequency and intrinsic Bloch frequency. In the presence of the magnetic field, the peak power shifts towards a stronger static electric field region. The simulated results about the dependence of the second and third harmonic powers on a DC electric field are in qualitative consistence with the experiments. The proposed terahertz frequency multiplier based on semiconductor superlattice, being compact and efficient, is provided as a good candidate for terahertz wave generation.

Thermoelectric figure of merit enhancement in dissipative superlattice structures

Utilizing the non-coherent quantum transport formalism, we investigate thermoelectric (TE) performance across dissipative superlattice (SL) configurations in the linear regime of operation. Using the dissipative non-equilibrium Green’s function formalism coupled self-consistently with the Poisson’s equation, we report an enhanced figure of merit zT in the multi-barrier device designs. The proposed enhancement, we show, is a result of a drastic reduction in the electronic thermal conductance triggered via non-coherent transport. We show that a maximum zT value of 18 can be achieved via the inclusion of momentum relaxation processes. There is also a reasonable enhancement in the Seebeck coefficient, with a maximum of 1000 µV K−1, which we attribute to an enhancement in electronic filtering arising from the non-coherent transport. Distinctly the thermal conduction is drastically reduced as the length of the SL scales up, although the power factor shows an overall degradation. While the presence of interfaces is known to kill phonon thermal conduction, our analysis shows that momentum relaxation processes in SL structures can effectively kill electronic thermal conduction also. We believe that the analysis presented here could set the stage to understand better the interplay between relaxation processes and coherent quantum processes in the electronic engineering of heterostructure TE devices.

Giant enhancement of silicon plasmonic shortwave infrared photodetection using nanoscale self-organized metallic films

Many consumer technologies rely on photodetection of infrared light, such as lidar, low visibility imaging, proximity sensors/range finders, etc. However, silicon, the standard material of the semiconductor industry, becomes transparent for wavelengths above 1.1 µm, as the photons no longer have sufficient energy to stimulate direct band-to-band absorption. We report here a Schottky photodetector design that extends silicon’s optical detection range beyond this 1.1 µm limit, by utilizing internal photoemission of hot carriers. Our design relies on an ultra-thin fractally nanostructured aluminum optical absorber and yet remarkably achieves over 50% absorption of incident light. We demonstrate 2 orders of magnitude improvements of responsivity, noise-equivalent power, and specific detectivity as compared to a reference Schottky photodetector made of bulk metal films. We attribute this to the combination of superior transport and momentum relaxation processes from the nanoscale fractal geometries. Specifically, we show a direct link between internal quantum efficiency enhancement and structural parameters such as perimeter-to-surface ratio. Finally, our devices also function as bulk refractive index sensors. Our approach uses an exceedingly simple complementary metal-oxide-semiconductor (CMOS)-compatible “bottom up” fabrication that is cheap and scalable and is a promising candidate for future cost-effective and robust shortwave infrared photodetection and sensing applications.

Enhanced spin-to-charge current conversion at metal/oxide interfaces by lowering the temperature

We investigated the temperature dependence of the spin-to-charge current conversion at nonmagnetic metal (NM: Cu or Ag)/Bi2O3 interfaces with Rashba spin-splitting. Spin pumping induced inverse Edelstein effects are measured in the temperature range from 290 K to 10 K. Estimated conversion coefficients at the Cu(Ag)/Bi2O3 interfaces are increased by 40 (17)% at 10 K compared with those at 290 K. The conversion coefficient is found to be proportional to the conductivity of the NM layer, indicating that the momentum relaxation process in the metal layer dominates the spin-to-charge current conversion process at NM/Bi2O3 interfaces.

Negative viscosity and eddy flow of the imbalanced electron-hole liquid in graphene

We present a hydrodynamic theory for electron-hole magnetotransport in graphene incorporating carrier-population imbalance, energy, and momentum relaxation processes. We focus on the electric response and find that the carrier and energy imbalance relaxation processes strongly modify the shear viscosity, so that an effective viscosity can be negative in the vicinity of charge neutrality. We predict an emergent eddy flow pattern of swirling currents and explore its manifestation in nonlocal resistivity oscillations in a strip of graphene driven by a source current.

Comparison the electron momentum and energy relaxation process in wurtzite GaN, InN and AlN by Monte Carlo method

At present, both transient and steady transport properties of III-V nitride semiconductors are researched, however, few research concerns about the momentum and energy relaxation process, which will be important during device designing. In order to clarify the fundamental physical process of the relaxation phenomenon, both momentum and energy relaxation process in wurtzite GaN, InN and AlN are studied with the help of classic three valleys Monte Carlo method. Our goal is to understand the relationship of relaxation rate with electric field, temperature and clarify the role of scattering mechanisms during momentum and energy relaxation process. The research shows that when the electric field and temperature are different, the main momentum and energy relaxation mechanisms may be different. For the lower effective mass in the lowest valley for InN, the electrons are more easily accelerated by electric field and scattered into the satellite valleys. The momentum relaxation time is much lower than that of energy relaxation time, the reason is that nearly all scattering mechanisms will relax momentum, but only polar optical phonon and inter-valley scattering relax electron energy. The research here can be used for construct the relaxation model or as a guide for device design.

Electron Momentum and Energy Relaxation Times in Wurtzite GaN, InN and AlN: A Monte Carlo Study

Electron relaxation process and transport property of hot electrons in wurtzite GaN, InN and AlN are investigated by an ensemble Monte Carlo (EMC) method using the classic three valleys model. Both momentum and energy relaxation processes are studied here, taking into account the effect of temperature and electric field. It can be found that under different electric fields the relaxation mechanisms are much different for the existence of satellite valley structures in III-nitrides. The momentum and energy relaxation processes of InN are different from that of GaN and AlN for the much smaller effective electron mass in the lowest valley of InN. The polar optical phonon scattering is the main energy relaxation process when the electric field is lower, but inter-valley scattering will dominate the energy relaxation process when the electric field is high. The main momentum relaxation mechanisms are ionized impurity, acoustic phonon, and polar optical phonon scattering at lower electric field, but polar optical phonon and inter-valley scattering are the main momentum relaxation mechanisms when the electric field becomes higher. The momentum relaxation time is much lower than the energy relaxation time, the reason is that nearly all scattering mechanisms relax momentum, but only polar optical phonon and inter-valley scattering relax electron energy. The temperature can affect the momentum and energy relaxation by the role in scattering. Finally, the momentum and energy relaxation times as a function of electron temperature are given, which can be used for relaxation time modeling and device transport research.

Kinetic Density Functional Theory for Plasmonic Nanostructures: Breaking of the Plasmon Peak in the Quantum Regime and Generation of Hot Electrons

We develop a quantum kinetic theory of the dynamic response of typical noble metals. Our approach is based on the density functional theory (DFT) and incorporates new important elements as compared to the conventional time-dependent DFT formalism. The kinetic DFT is derived starting from the master equation of motion for the density matrix, which involves both momentum and energy relaxation processes. Therefore, the quantum system is described by two relaxation parameters, unlike the conventional time-dependent DFT incorporating only one relaxation parameter. This allows us to describe both the absorption of light and the generation of hot plasmonic electrons. Using our kinetic DFT theory, we also observe the transition from the multiple peaks in small size-quantized systems to the intensive plasmonic resonance in large classical systems. Unlike the standard picture of collisional broadening of the plasmon peak in small systems, we observe a very different scenario: the formation of multiple plasmonic-like peaks in small quantized systems. These peaks are the result of a hybridization of the collective plasmon mode and the single-particle transitions in a quantized electron gas. There are a few experimental observations that seem to correlate with such a scenario of the plasmonic broadening in small systems. Our approach also incorporates the interband transitions, which are important for a qualitative description of gold and silver. Although this paper gives an application of our kinetic DFT only to the slab geometry, our theory can be applied to nanocrystals of arbitrary shape. The kinetic DFT formalism developed here can be employed to model and predict a variety of metal and hybrid nanostructures for applications in photocatalysis, sensors, photodetectors, metamaterials, etc.

Transport Theory of Monolayer Transition-Metal Dichalcogenides through Symmetry

We present a theory that elucidates the major momentum and spin relaxation processes for electrons, holes, and hot excitons in monolayer transition-metal dichalcogenides. We expand on spin flips induced by flexural phonons and show that the spin relaxation is ultrafast for electrons in free-standing membranes while being mitigated in supported membranes. This behavior due to interaction with flexural phonons is universal in two-dimensional membranes that respect mirror symmetry, and it leads to a counterintuitive inverse relation between mobility and spin relaxation.

Time-resolved x-ray magnetic circular dichroism study of ultrafast demagnetization in a CoPd ferromagnetic film excited by circularly polarized laser pulse

The magnetization dynamics of CoPd films excited by circularly polarized ultrashort laser pulses is studied by time-resolved x-ray magnetic circular dichroism. In those films the ultrafast dynamics measured at the Co-${L}_{3}$ edge is strongly sensitive to the orbital magnetic moment $L$${}_{z}$. The amount of angular momentum transferred by the circularly polarized ultrashort laser pulses to the ferromagnetic films is evaluated to $\ifmmode\pm\else\textpm\fi{}$0.1 $\ensuremath{\hbar}$/atom, which is above the detection limit of the experiment. Despite this, no polarization-dependent difference on the magnetization dynamics could be evidenced. These results are explained by ultrafast electronic relaxation mechanisms of the transferred angular momentum, faster than $\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}100$ fs. This experiment sets the methodology as well as an upper time limit for determination of angular momentum relaxation processes.

Integer quantum Hall effect in a square lattice revisited

We investigate the phenomenon of integer quantum Hall effect in a square lattice, subjected to a perpendicular magnetic field, through Landauer–Büttiker formalism within the tight-binding framework. The oscillating nature of longitudinal resistance and near complete suppression of momentum relaxation processes are examined by studying the flow of charge current using Landauer–Keldysh prescription. Our analysis for the lattice model corroborates the finding obtained in the continuum model and provides a simple physical understanding.

Momentum relaxation due to polar optical phonons in AlGaN/GaN heterostructures

Using the dielectric continuum (DC) model, momentum relaxation rates are calculated for electrons confined in quasi-two-dimensional (quasi-2D) channels of AlGaN/GaN heterostructures. Particular attention is paid to the effects of half-space and interface modes on the momentum relaxation. The total momentum relaxation rates are compared with those evaluated by the three-dimensional phonon (3DP) model, and also with the Callen results for bulk GaN. In heterostructures with a wide channel (effective channel width >100 A), the DC and 3DP models yield very close momentum relaxation rates. Only for narrow-channel heterostructures do interface phonons become important in momentum relaxation processes, and an abrupt threshold occurs for emission of interface as well as half-space phonons. For a 30-A GaN channel, for instance, the 3DP model is found to underestimate rates just below the bulk phonon energy by 70% and overestimate rates just above the bulk phonon energy by 40% compared to the DC model. Owing to the rapid decrease in the electron-phonon interaction with the phonon wave vector, negative momentum relaxation rates are predicted for interface phonon absorption in usual GaN channels. The total rates remain positive due to the dominant half-space phonon scattering. The quasi-2D rates can have substantially higher peak values than the three-dimensional rates near the phonon emission threshold. Analytical expressions for momentum relaxation rates are obtained in the extreme quantum limits (i.e., the threshold emission and the near subband-bottom absorption). All the results are well explained in terms of electron and phonon densities of states.

Kinetic equations for classical and quantum Brownian particles and eigenfunction expansions as generalized functions

We study momentum relaxation processes of a classical and a quantum Brownian particle by considering the eigenvalue problem of the collision operators in the kinetic equations. The collision operators are anti-Hermitian with an appropriate inner product defined by an integral with a weight factor given by the inverse of the equilibrium distribution function. Owing to the weight factor, the norm of a momentum distribution function is infinite, if the distribution is characterized by a temperature higher than a threshold temperature determined by the environmental temperature. Although the eigenfunction expansion of a given distribution function with an infinite norm does not converge to a function in the Hilbert space, it has a legitimate meaning as a generalized function and defines a linear functional. We introduce an H-function through the norm which directly reflect the spectral properties of the collision operators. When the norm of the momentum distribution function diverges, the H-function reduces to a simple form characterized by only the value at the accumulation point of the spectrum of the collision operator.

Dephasing of Excitonic Polaritons Confined in GaAs Thin Films

We report the dephasing of excitonic polaritons due to propagation in GaAs thin films measured using a reflection-type pump–probe technique. The rise time of the transient signal due to the propagation of excitonic polaritons decreases with an increase in temperature. This decrease originates from the dephasing of excitonic polaritons. From the analysis based on exciton–phonon scattering, the interaction with optical phonons mainly characterizes the dephasing process of exciton polaritons. Moreover, the result of the comparison of the temperature dependence of the damping factor estimated from the rise time with that estimated from the photoluminescence width indicates that an interaction with optical phonons in the propagation process is larger than that in the momentum relaxation process.

Accumulation Point in the Spectrum of the Collision Operator for a One-Dimensional Polaron System

The spectrum of Hamiltonians has been well investigated in quantum mechanics, since it plays an essential role in analyzing dynamical properties of conservative systems. For example, we know there are Hamiltonians that have a discrete spectrum, or a continuous spectrum, or a band structure, and so on, each of which leads to a characteristic behavior of the system. However, our knowledge on the spectrum of the collision operators in kinetic equations in dissipative systems still remains in a poor stage in spite of the fact that their spectral properties play a fundamental role in non-equilibrium statistical physics. In this paper we give an example of quantum systems that has a rich structure of the spectrum, such as an accumulation point and band structures, in the collision operator for a momentum relaxation process. We consider a one-dimensional (1D) polaron system in which a quantum particle is weakly interacting with a thermal reservoir consisting of an acoustic phonon field. Similar systems have been studied in different contexts, see e.g. Refs. 1)–5). As a consequence of a constraint in 1D due to the resonance condition, the momenta of the particle related successively through the collision operator form a subset separated from other momenta. Momentum relaxation occurs among those momenta in such a subset independently of other momenta. In this case the collision operator is represented by a tridiagonal matrix for each such subset of momenta. Taking advantage of the tridiagonal nature of the matrix, we found a solution of the eigenvalue problem of the collision operator in terms of continued fractions. This paper is organized as follows. In §2 we introduce a model of a quantum particle weakly coupled with a phonon field, and present a kinetic equation for the

Hofstadter's Butterfly Type of Singular Spectrum of a Collision Operator for a Model of Molecular Chains

We study the eigenvalue problem of a quantum collision operator in an irreversible kinetic equation for momentum relaxation processes in a molecular chain where a vibron is weakly coupled to a thermal phonon bath. As a consequence of a constraint due to the resonance condition represented by the conservation of the unperturbed energy, the momenta of the vibron associated through the collision operator successively with a representative momentum form a subset separated from other momenta. The number of possible values of momenta in each subset and the spectrum of the collision operator crucially depend on the rationality or irrationality of the parameter R ≡ π −1 sin −1 B ,w hereB denotes the ratio of the phonon bandwidth to the vibron bandwidth. The plot of the spectrum as a function of R shows features reminiscent of Hofstadter’s butterfly.

Effect of magnetic field on the screening due to hot electrons in the presence of shallow attractive traps

The electrostatic screening of the perturbation potential due to lattice imperfections has been investigated here in the presence of crossed electric and non-quantizing magnetic fields, under conditions where the lifetime of the carriers of the non-equilibrium ensemble in a semiconductor is controlled by shallow attractive traps. The screening characteristics are obtained in the general case of any mechanism of energy and momentum scattering of the hot carriers in the strong and weak magnetic field limits. For some definite scattering mechanisms, however, the characteristics are calculated in the presence of any finite value of the magnetic field. The results show interesting characteristic features that are quite different from what one can obtain for an ensemble of carriers that is in thermodynamic equilibrium with the lattice atoms. It has been observed that, depending upon the prevalent conditions with respect to the dominant momentum and energy relaxation processes and the strength of the applied electric field, the presence of the magnetic field sometimes brings in significant changes in both the qualitative and the quantitative aspects of the field dependence of the screening length. Apart from that, the screening length now shows a rather complex and altogether different dependence upon the lattice temperature. The numerical results featuring the interesting characteristics under different prevalent conditions are presented for Ge. It has been explained in what novel way the present problem describes the dependence of the screening length due to an ensemble of hot electrons on the external electric and magnetic fields. The limitations of the present theory, as well as its possible applicability for the characterization of semiconductors under high field conditions at low lattice temperatures, are discussed.

Relaxation of rotational angular momentum of polar diatomic molecules in simple liquids

The relaxation processes of rotational angular momentum of polar diatomic molecules diluted in simple liquids are analyzed by applying a non-Markovian relaxation theory to the study of the binary time autocorrelation function of the angular momentum. This non-Markovian theory was previously applied to the study of the infrared and Raman spectroscopy, and also to the analysis of the rotational energy relaxation processes. We have obtained non-Markovian evolution equations for the two-time $j$-level angular momentum correlation components involved in the angular momentum correlation function. In these equations, the time-dependent angular momentum transfer rates and the pure orientational angular transfer rates are given in terms of the binary time autocorrelation function of the diatomic-solvent anisotropic interaction. The non-Markovian evolution equations converge to Markovian ones in the long time limit, reaching the angular momentum transfer rates in the usual time-independent form. Alternative time scales for the angular relaxation processes, relative to the individual rotational processes as well as to the global decay correlations, are introduced and analyzed. The theory is applied to the study of the angular momentum relaxation processes of HCl diluted in liquid $\mathrm{S}{\mathrm{F}}_{6}$, a system for which rotational energy relaxation and infrared and Raman spectroscopy was previously analyzed in the scope of the same theory.