Fermi-Dirac Research Articles

Kondo interaction in FeTe and its potential role in the magnetic order

Finding d-electron heavy fermion states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator. Yet, obtaining direct spectroscopic evidence for a d-electron heavy fermion system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy and transport property measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle and Fano-type tunneling spectra at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order.

Zero-energy states in graphene quantum dot with wedge disclination

We show that wedge disclination affects the density of states for fermions that are electrostatically confined in graphene quantum dots. By considering a magnetic flux and solving the Dirac equation, we determine the eigenspinors. These are used for large arguments to derive an approximate scattering matrix, which allows us to get the density of states. Under various conditions, it is found that the density of states exhibits several resonance peaks. In particular, it is shown that the wedge disclination is able to change the amplitude, width, and positions of resonance peaks.

Investigating the optical and electrical characteristics of As60Cu40-xSex thin films

In this work, As60Cu40-xSex thin films were synthesized, and the pulsed laser deposition method was used to study the effected partial replacement of copper with selenium. The electrical characteristics and optical characteristics, as indicated by the absorbance and transmittance as a function of wavelength were calculated. Additionally, the energy gap was computed. The electrical conductivity of the DC in the various conduction zones was calculated by measuring the current and voltage as a function of temperature. Additionally, the mathematical equations are used to compute the energy constants, electron hopping distance, tail width, pre-exponential factor, and density of the energy states in variation zones (densities of the energetic extended states N(Eext), localize N(Eloc) and at the Fermi states N(Ef)). The acquired data also demonstrated that the selenium concentration obviously had an impact on the electrical conduction mechanics, energy states, and the level of randomization.

BEHAVIOR OF q-DEFORMED QUANTUM PARTICLE STATISTICS ON THE HOLOGRAPHIC SCREEN

In this study, we propose an approach to investigate the relationship between MOND theory and the holographic principle by incorporating q-deformed theory. We first present a brief overview of Verlinde's entropic gravity assumption, which suggests that gravity can be interpreted as an entropic force arising from the statistical mechanics of quantum fields. Some thermo-statistical properties of q-deformed fermion gas model in two spatial dimensions are introduced. At the low-temperature limit, we derive the q-deformed thermal energy and analyze the impacts of fermionic q-deformation on MOND theory. Specifically, we consider the q-deformed Fermi-Dirac statistics of the bits on the holographic screen and examine MOND theory depending on q-deformed acceleration scale. Deformed Friedmann equation is studied by taking into account Friedmann–Robertson–Walker (FRW) universe. This equation shows a modified identification of the evolution of the universe that is compatible with both MOND theory and the holographic principle

Photoluminescence Study of As Doped p-type HgCdTe Absorber for Infrared Detectors Operating in the Range up to 8 µm

A HgCdTe photodiode grown by chemical vapor deposition (MOCVD) on a GaAs substrate operating in the long-wave infrared (LWIR) range was characterized using photoluminescence (PL) measurements. At high temperatures, the PL spectrum originates from a free-carrier emission and might be fitted by a theoretical expression being the product of the density of states and the Fermi–Dirac distribution. At low temperatures, the PL spectrum consists of multiple emission peaks that do not originate solely from the energy gap. Such spectra are not unambiguous to interpret due to the prominence of different optical transitions. Spectral response (SR) measurements were used to determine the energy gap (Eg) and extract the band-to-band transition from the PL spectra. PL peaks visible within the band gap were fitted by a Gaussian distribution. To identify the sources of individual emission peaks, excitation power dependence analysis was conducted. Band-to-band, free-to-bound, acceptor-bound exciton, and defect-bound exciton transitions were identified. At low temperatures, transitions are mainly impurity-related, with shallow impurity levels estimated to be 6 meV and 16 meV for the donor and acceptor, respectively, while deep-level impurities were associated with VHg. The latter transition with an energy of about 78 meV does not vary with temperature. Its relative positions with respect to the energy gap is 0.8 Eg at 18 K and 0.67 Eg at 80 K.

Unruh effect and Takagi's statistics inversion in strained graphene

We present a theoretical study of how a spatially varying quasiparticle velocity in honeycomb lattices, achievable using strained graphene or in engineered cold-atom optical lattices that have a spatial dependence to the local tunneling amplitude, can yield the Rindler Hamiltonian embodying an observer accelerating in Minkowski space-time. Within this setup, a sudden switch on of the spatially varying tunneling (or strain) yields a spontaneous production of electron-hole pairs, an analog version of the Unruh effect characterized by the Unruh temperature. We discuss how this thermal behavior, along with Takagi's statistics inversion, can manifest themselves in photoemission and scanning tunneling microscopy experiments. We also calculate the average electronic conductivity and find that it grows linearly with frequency $\ensuremath{\omega}$. Finally, we find that the total system energy at zero environment temperature looks like Planck's blackbody result for photons due to the aforementioned statistics inversion, whereas for an initial thermally excited state of fermions, the total internal energy undergoes stimulated particle reduction.

Noise and fluctuations in nanoscale gas flow

We theoretically calculate the fundamental noise that is present in gaseous (dilute fluid) flow in channels in the classical and degenerate quantum regime, where the Fermi–Dirac and Bose–Einstein distribution must be considered. Results for both regimes are analogous to their electrical counterparts. The quantum noise is calculated for a two terminal system and is a complicated function of the thermal and shot noise with the thermal noise dominating when 2k B T ρ ≫ mΔP and vice versa. The cumulant generating function for mass flow, which generates all the higher order statistics related to our mass flow distribution, is also derived and is used to find an expression for the third cumulant of flow across a fluidic channel.

Discrete global symmetries and dynamics of emergent fermions

Global symmetries that define the number of low-energy degrees of freedom have profound consequences on universal properties near topological quantum critical points and in other gapless or nearly gapless states of emergent fermions. We take a ${Z}_{2}$ global symmetry (such as time-reversal) as an example to study its effect on thermodynamic and transport properties. Although the thermal entropy density of ${Z}_{2}$ symmetric systems is simply twice that of their counterparts without any global symmetries or the ${Z}_{1}$ class, the temperature dependence of thermal conductivity $\ensuremath{\kappa}$ is distinctly and drastically different for different symmetries. For systems with dynamic exponent $z=1$, in the ${Z}_{2}$ symmetric class, we have $\ensuremath{\kappa}\ensuremath{\propto}{T}^{\ensuremath{-}(d\ensuremath{-}1)}$ in the quantum critical regime near weakly interacting fixed points, while for systems with no global symmetries (i.e., the ${Z}_{1}$ class), we have $\ensuremath{\kappa}\ensuremath{\propto}{T}^{\ensuremath{-}(d+3)}$, with $d$ being the spatial dimension. Only near strong-coupling fixed points, both cases with or without ${Z}_{2}$ global symmetries follow the same scaling function, $\ensuremath{\kappa}\ensuremath{\propto}{T}^{d\ensuremath{-}1}$. These distinct scalings of thermal conductivity can also appear in gapless surface Majorana states.

On the Fluctuation–Dissipation of the Oxide Trapped Charge in a MOSFET Operated Down to Deep Cryogenic Temperatures

In this paper, an analysis of the oxide trapped charge noise in a MOSFET operated down to deep cryogenic temperatures is proposed. To this end, a revisited derivation of the interface trap conductance [Formula: see text] and oxide trapped charge noise [Formula: see text] at the SiO2/Si MOS interface is conducted under very low temperature condition, where Fermi–Dirac statistics applies. A new relation between the oxide trapped charge noise [Formula: see text] and the interface trap conductance [Formula: see text] is established, showing the inadequacy of the Nyquist relation at very low temperature. Finally, a new formula for the oxide trapped charge 1/f noise, going beyond the classical Boltzmann expression, is developed in terms of oxide trap density and effective temperature accounting for degenerate statistics.

Inclusion statistics and particle condensation in two dimensions.

We propose a type of quantum statistics which we call inclusion statistics, in which particles tend to coalesce more than ordinary bosons. Inclusion statistics is defined in analogy with exclusion statistics, in which statistical exclusion is stronger than in Fermi statistics, but now extrapolating beyond Bose statistics, resulting in statistical inclusion. A consequence of inclusion statistics is that the lowest space dimension in which particles can condense in the absence of potentials is d=2, unlike d=3 for the usual Bose-Einstein condensation. This reduction in the dimension happens for any inclusion stronger than bosons, and the critical temperature increases with stronger inclusion. Possible physical realizations of inclusion statistics involving attractive interactions between bosons may be experimentally achievable.

Energy Spectrum of β Electrons in Neutrinoless Double-β Decay Including the Excitation of the Electron Shell of Atoms

Double-β decay is accompanied with a high probability by the excitation of the electron shell of the daughter atom; as a result, the energy carried away by β electrons decreases. The mean value and standard deviation of the excitation energy of the electron shell of the daughter atom in the double-β decay of germaniumhave been determined within the Thomas–Fermi and relativistic Dirac–Hartree–Fock methods. Using the estimates thus obtained, a two-parameter model of the energy spectrum of β electrons in the neutrinoless mode has been developed including the redistribution of the reaction energy between the decay products. The shift of the total energy of β electrons does not exceed 50 eV with a probability of 90%. However, the mean excitation energy is ~400 eV, i.e., an order of magnitude higher, whereas the standard deviation is ~2900 eV, which is apparently due to a significant contribution from inner electron levels to the energy characteristics of the process. The distortion of the shape of the peak of the 0ν2β decay should be taken into account when analyzing the data of detectors with a resolution of ~100 eV or higher.

Rotationally invariant isospinning baby-skyrmions dressed by fermions

We couple Fermions to the isospinning (2+1) baby-skyrme model. We show that consistent rotationally invariant localised solutions can be found but the Fermionic solutions to the equations of motion are not in general eigenstates of the Hamiltonian. These do however become full eigenstates in a particular limit. We also find the corresponding Fermionic eigenstates of the Hamiltonian, for which our Fermion ansatz is also well suited. For localised Fermionic states, a novel constraint on the internal isospin frequency exists. Throughout our studies, we include the backreaction of the Fermions on the baby-skyrmion.

Numerical analysis of a finite volume scheme for charge transport in perovskite solar cells

Abstract In this paper, we consider a drift-diffusion charge transport model for perovskite solar cells, where electrons and holes may diffuse linearly (Boltzmann approximation) or nonlinearly (e.g., due to Fermi–Dirac statistics). To incorporate volume exclusion effects, we rely on the Fermi–Dirac integral of order $-1$ when modeling moving anionic vacancies within the perovskite layer, which is sandwiched between electron and hole transport layers. After nondimensionalization, we first prove a continuous entropy-dissipation inequality for the model. Then, we formulate a corresponding two-point flux finite volume scheme on Voronoi meshes and show an analogous discrete entropy-dissipation inequality. This inequality helps us to show the existence of a discrete solution of the nonlinear discrete system with the help of a corollary of Brouwer’s fixed point theorem and the minimization of a convex functional. Finally, we verify our theoretically proven properties numerically, simulate a realistic device setup and show exponential decay in time with respect to the $L^2$ error as well as a physically and analytically meaningful relative entropy.

Parametrization and thermodynamic scaling of pair correlation functions for the fractional quantum Hall effect

The calculation of pair correlations and density profiles of quasiholes are routine steps in the study of proposed fractional quantum Hall states. Nevertheless, the field has not adopted a standard way to present the results of such calculations in an easily reproducible form. We develop a polynomial expansion that allows for easy quantitative comparison between different candidate wavefunctions, as well as reliable scaling of correlation and quasihole profiles to the thermodynamic limit. We start from the well-known expansion introduced by Girvin [PRB, 30 (1984)] (see also [Girvin, MacDonald and Platzman, PRB, 33 (1986)]), which is physically appealing but, as we demonstrate, numerically unstable. We orthogonalize their basis set to obtain a new basis of modified Jacobi polynomials, whose coefficients can be stably calculated. We then apply our expansion to extract pair correlation expansion coefficients and quasihole profiles in the thermodynamic limit for a wide range of fractional quantum Hall wavefunctions. These include the Laughlin series, composite fermion states with both reverse and direct flux attachment, the Moore-Read Pfaffian state, and BS hierarchy states. The expansion procedure works for both abelian and non-abelian quasiholes, even when the density at the core is not zero. We find that the expansion coefficients for all quantum Hall states considered can be fit remarkably well using a cosine oscillation with exponentially decaying amplitude. The frequency and the decay length are related in an intuitive, but not elementary way to the filling fraction. Different states at the same filling fraction can have distinct values for these parameters. Finally, we also use our scaled correlation functions to calculate estimates for the magneto-roton gaps of the various states.

On melting of Boltzmann system of quantum hard spheres.

Melting of a quantum system of hard spheres has been considered in the case when the effects of Bose and Fermi statistics can be neglected. It has been found that the quantum melting line always differs from the classical line except for T = 0, P = 0, where the both lines cross. It is shown that the classical limit is not reachable at any finite temperature.

Energy Spectrum of β Electrons in Neutrinoless Double-β Decay Including the Excitation of the Electron Shell of Atoms

Double-β decay is accompanied with a high probability by the excitation of the electron shell of the daughter atom; as a result, the energy carried away by β electrons decreases. The mean value and standard deviation of the excitation energy of the electron shell of the daughter atom in the double-β decay of germanium _{{32}}^{{76}}{text{Ge}} to ;_{{34}}^{{76}}{text{Se}}text{*} + ;2{{beta }^{ - }}( + 2{{bar {nu }}_{e}}) have been determined within the Thomas–Fermi and relativistic Dirac–Hartree–Fock methods. Using the estimates thus obtained, a two-parameter model of the energy spectrum of β electrons in the neutrinoless mode has been developed including the redistribution of the reaction energy between the decay products. The shift of the total energy of β electrons does not exceed 50 eV with a probability of 90%. However, the mean excitation energy is ~400 eV, i.e., an order of magnitude higher, whereas the standard deviation is ~2900 eV, which is apparently due to a significant contribution from inner electron levels to the energy characteristics of the process. The distortion of the shape of the peak of the 0ν2β decay should be taken into account when analyzing the data of detectors with a resolution of ~100 eV or higher.

Simulation of a plasmonic sensor using kinetic theory of plasma with the Vlasov equation in MATLAB

This research proposes a mathematical model for a plasmonic sensor using kinetic theory of plasma with the Vlasov equation. A nanoantenna cavity of a plasmonic material is driven by an input electromagnetic wave, which changes the charge density and current flow in the cavity, resulting in a change in the Fermi distribution function of the charged particles. The results are achieved in terms of current density and conductivity by solving the Boltzmann transport equation, Maxwell’s equations, and Taylor series expansion in terms of perturbed electric fields with linear integro differential equations. The results are simulated using MATLAB. The changes in current density and conductivity are validated by experimental analysis of graphene plasmonic material using patch antenna with the dielectric substrates SiO2 and Al2O3. By varying the applied electric fields, current changes at the output of the plasmonic antenna are analyzed using signal-processing techniques. Wavelet transforms are used to find the space-scale behavior of the output signals, such as current density variation, voltage variation, and susceptibility change with sub-band coding techniques in terms of wavelet coefficients.

Measurement-based quantum simulation of Abelian lattice gauge theories

Numerical simulation of lattice gauge theories is an indispensable tool in high energy physics, and their quantum simulation is expected to become a major application of quantum computers in the future. In this work, for an Abelian lattice gauge theory in dd spacetime dimensions, we define an entangled resource state (generalized cluster state) that reflects the spacetime structure of the gauge theory. We show that sequential single-qubit measurements with the bases adapted according to the former measurement outcomes induce a deterministic Hamiltonian quantum simulation of the gauge theory on the boundary. Our construction includes the (2+1)(2+1)-dimensional Abelian lattice gauge theory simulated on three-dimensional cluster state as an example, and generalizes to the simulation of Wegner’s lattice models M_{(d,n)}M(d,n) that involve higher-form Abelian gauge fields. We demonstrate that the generalized cluster state has a symmetry-protected topological order with respect to generalized global symmetries that are related to the symmetries of the simulated gauge theories on the boundary.Our procedure can be generalized to the simulation of Kitaev’s Majorana chain on a fermionic resource state. We also study the imaginary-time quantum simulation with two-qubit measurements and post-selections, and a classical-quantum correspondence, where the statistical partition function of the model M_{(d,n)}M(d,n) is written as the overlap between the product of two-qubit measurement bases and the wave function of the generalized cluster state.

Efficient low-temperature simulations for fermionic reservoirs with the hierarchical equations of motion method: Application to the Anderson impurity model

The hierarchical equations of motion (HEOM) approach is an accurate method to simulate open system quantum dynamics, which allows for systematic convergence to numerically exact results. To represent effects of the bath, the reservoir correlation functions are usually decomposed into summation of multiple exponential terms in the HEOM method. Since the reservoir correlation functions become highly non-Markovian at low temperatures or when the bath has complex band structures, a present challenge is to obtain accurate exponential decompositions that allow efficient simulation with the HEOM. In this work, we employ the barycentric representation to approximate the Fermi function and hybridization functions in the frequency domain. This method, by approximating these functions with optimized rational decomposition, greatly reduces the number of basis functions in decomposing the reservoir correlation functions, which further allows the HEOM method to be applied to ultralow temperature and general band structures. We demonstrate the efficiency, accuracy, and long-time stability of this decomposition scheme by applying it to the Anderson impurity model in the low-temperature regime with the Lorentzian and tight-binding hybridization functions.

Deriving density-matrix functionals for excited states

We initiate the recently proposed ww-ensemble one-particle reduced density matrix functional theory (ww-RDMFT) by deriving the first functional approximations and illustrate how excitation energies can be calculated in practice. For this endeavour, we first study the symmetric Hubbard dimer, constituting the building block of the Hubbard model, for which we execute the Levy-Lieb constrained search. Second, due to the particular suitability of ww-RDMFT for describing Bose-Einstein condensates, we demonstrate three conceptually different approaches for deriving the universal functional in a homogeneous Bose gas for arbitrary pair interaction in the Bogoliubov regime. Remarkably, in both systems the gradient of the functional is found to diverge repulsively at the boundary of the functional’s domain, extending the recently discovered Bose-Einstein condensation force to excited states. Our findings highlight the physical relevance of the generalized exclusion principle for fermionic and bosonic mixed states and the curse of universality in functional theories.