Fermi Temperature Research Articles

Effect of nonlocal electrical conductivity on near-field radiative heat transfer between graphene sheets

Graphene's near-field radiative heat transfer is determined from its electrical conductivity, commonly modeled using the local Kubo and Drude formulas. In this letter, we analyze the non-locality of graphene's electrical conductivity using the Lindhard model combined with the Mermin relaxation time approximation. We also study how the variation of electrical conductivity with wavevector affects near-field radiative conductance between two graphene sheets separated by a vacuum gap. It is shown that the variation of electrical conductivity with wavevector, $k_{\rho}$, is appreciable for $k_{\rho}$s greater than $100k_0$, where $k_0$ is the magnitude of the wavevector in the free space. The Kubo electrical conductivity provides an accurate estimation of the spectral radiative conductance between two graphene sheets except for around the surface-plasmon-polariton frequency of graphene and at separation gaps smaller than 20 nm where there is a non-negligible contribution from modes with $k_{\rho}>100k_0$ to the radiative conductance. The Drude formula proves to be inaccurate for modeling the electrical conductivity and radiative conductance of graphene except for at temperatures much below the Fermi temperature and frequencies much smaller than $2{\mu}_c/{\hbar}$, where ${\mu}_c$ and ${\hbar}$ are the chemical potential and reduced Planck's constant, respectively. It is also shown that the electronic scattering processes should be considered in the Lindhard model properly, such that the local electron number is conserved. A substitution of ${\omega}$ by ${\omega}+i{\gamma}$ (${\omega}$, $i$, and ${\gamma}$ being the angular frequency, imaginary unit, and scattering rate, respectively) in the collisionless Lindhard model does not satisfy the conservation of the local electron number and results in significant errors in computing graphene's electrical conductivity and radiative conductance.

Inferring the energy sensitivity and band gap of electronic transport in a network of carbon nanotubes

Since the industrialization of single-phase nanomaterial-based devices is still challenging, intensive research focus has been given to complex materials consisting of multiple nanoscale entities, including networks and matrices of nanowires, nanotubes, nanoribbons, or other large molecules; among these complex materials, networks of carbon nanotubes are a typical example. Detailed knowledge of the energy sensitivity and band gap of electronic transport in such a material system is difficult to detect, despite its importance in electronic, energetic and sensing applications. Here, we propose a new methodology to obtain these quantities using the measured Seebeck coefficient at a certain temperature but different Fermi levels. We discover that the network consisting of semiconducting (11,10)-carbon nanotubes actually exhibits metallic transport at room temperature. It is also interesting to verify that intrananotube ballistic transport is dominant over diffusive scattering by long-range disorder, as well as the quantum hopping resistance at the contact points. The transport asymmetry ratio between the holes and electrons (1.75) is similar to the value observed in pristine graphene samples (1.50).

Thermoelectricity and electronic correlation enhancement in FeS by light Se doping

We report thermoelectric studies of FeS$_{1-x}$Se$_x$ ($x$ = 0, 0.06) superconducting single crystals that feature high irreversibility fields and critical current density $J_c$ comparable to materials with much higher superconducting critical temperatures ($T_c$'s). The ratio of $T_c$ to the Fermi temperature $T_F$ is very small indicating weak electronic correlations. With a slight selenium substitution on sulfur site in FeS both $T_c$/$T_F$ and the effective mass $m^*$ rise considerably, implying increase in electronic correlation of the bulk conducting states. The first-principle calculations show rise of the density of states at the Fermi level in FeS$_{0.94}$Se$_{0.06}$ when compared to FeS which is related not only to Fe but also to chalcogen-derived electronic states.

Raman Spectroscopy as a Simple yet Effective Analytical Tool for Determining Fermi Energy and Temperature Dependent Fermi Shift in Silicon.

The Fermi energy is known to be dependent on doping and temperature, but finding its value and corresponding thermal Fermi shift experimentally is not only difficult but is virtually impossible if one attempts their simultaneous determination. We report that temperature dependent Raman spectromicroscopy solves the purpose easily and proves to be a powerful technique to determine the position and temperature associated Fermi shift in an extrinsic semiconductor as demonstrated for silicon in the present study. The typical asymmetrically broadened Raman spectral line-shape from sufficiently doped n- and p-type silicon contains the information about the Fermi level position through its known association with the Fano coupling strength. Thus, Raman line-shape parameters, the terms quantify the Fano-coupling, have been used as experimental observables to reveal the value of the Fermi energy and consequent thermal Fermi shift. A simple formula has been developed based on existing established theoretical frameworks that can be used to calculate the position of the Fermi level. The proposed Raman spectroscopy-based formulation applies well for n- and p-type silicon. The calculated Fermi level position and its temperature dependent variation are consistent with the existing reports.

Observation of the density dependence of the closed-channel fraction of a 6Li superfluid.

Atomic Fermi gases provide an ideal platform for studying pairing and superfluid physics, using a Feshbach resonance between closed-channel molecular states and open-channel scattering states. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction [Formula: see text] provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure [Formula: see text] as a function of interaction strength and the Fermi temperature [Formula: see text] in a trapped 6Li superfluid throughout the entire Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep-BEC regime, the fraction [Formula: see text] is sensitive to [Formula: see text]. In particular, our data show [Formula: see text] with [Formula: see text] at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and [Formula: see text] increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.

The Nonlinear Magnetosonic Waves in Magnetized Dense Plasma for Quantum Effects of Degenerate Electrons

The nonlinear magnetosonic solitons are investigated in magnetized dense plasma for quantum effects of degenerate electrons in this research work. After reviewing the basic introduction of quantum plasma, we described the nonlinear phenomenon of magnetosonic wave. The reductive perturbation technique is employed for low frequency nonlinear magnetosonic waves in magnetized quantum plasma. In this paper, we have derived the Korteweg-de Vries (KdV) equation of magnetosonic solitons in a magnetized quantum plasma with degenerate electrons having arbitrary electron temperature. It is observed that the propagation of magnetosonic solitons in a magnetized dense plasma with the quantum effects of degenerate electrons and Bohm diffraction. The quantum or degeneracy effects become relevant in plasmas when fermi temperature and thermodynamic temperatures of degenerate electrons have same order.

Investigation of strontium titanate spherical shell supported terahertz all-dielectric metamaterials

Based on the strontium titanate ( S r T i O 3 , STO) spherical shell all-dielectric metamaterials (ADMs) structure, the tunable propagation characteristics have been systematically studied, taking into account the effects of structural geometrical parameter, temperature, and graphene Fermi level. The results indicate that STO-based ADMs exhibit excellent thermally and electrically tunable performances, e.g., when the temperature rises from 77 to 400 K, the modulation depths (MDs) of resonant frequency and amplitude of the reflection dip reach 62% and 93%, and the absorption is tunable in the range of 0.7995–0.9017. Additionally, when the Fermi level of graphene changes from 0.1 to 1.0 eV, the amplitude MD of the reflection dip reaches 99.89%, and the absorption of the STO-based ADM increases from 0.8352 to 0.9953. These results provide a guide for understanding the resonant mechanism of STO-based ADMs and designing multifunctional terahertz devices.

Recent progress of terahertz detectors based onphotothermoelectric effect

<p indent=0mm>The unique physical properties of terahertz (THz) waves lead the THz technology to a broad application prospect in many fields, including security imaging, communication, nondestructive testing and biomedicine. However, as for now, the lack of high-performance, room-temperature THz detectors still hinders the practical applications of THz technology. According to the detection mechanisms, the THz detectors can be divided into two types, devices based on electronic methods and devices based on photothermal effects. The former is limited by the cut-off frequency and thus generally they can only work below <sc>1 THz,</sc> while the latter is usually limited by the slow response speed, so they cannot achieve high-speed THz detection. The photothermoelectric (PTE) detector, a new kind of THz detector, may have its own power to achieve sensitive THz detection. PTE detector is a new kind of THz photothermal detector appearing in the recent ten years. Compared with other kinds of photothermal detectors, it has many advantages, such as large bandwidth, energy conservation, high speed, and room temperature operation. In addition, thanks to the hot carrier effect, the response time of THz PTE detectors can reach several nanoseconds at room temperature, which completely makes up the shortcoming of other THz photothermal detectors. Therefore, since the researchers of the Tsinghua University first observed the THz PTE response in the carbon nanotube-metal contact in 2014, THz PTE detector has attracted much attention and become a hotspot rapidly. PTE effect, also regarded as the effect that carriers are driven by temperature gradient and thus flow from the hot region to the cold region along the channel, can result in a net current while external THz wave is illuminating on the detector. The most unique requirement of PTE detector is the asymmetric design, otherwise there will be no net current in the device. According to the design of asymmetry, there are two types of THz PTE detectors, devices with asymmetric temperature distribution and devices with asymmetric Seebeck coefficient distribution. The asymmetric temperature distribution is generally induced by the asymmetric illumination and asymmetric antenna structure. In both cases, a part of the detector is heated by THz wave while the other part remains cold. The carriers are driven by the temperature difference, leading to a photo response. The asymmetric Seebeck coefficient distribution is induced by the asymmetric electrode materials or p-n junctions. In both cases, there is a Fermi energy level difference in the detector. Under THz illumination, the carriers are driven by the temperature gradient and Fermi energy level difference, leading to a photo response. In order to evaluate different THz PTE detectors, several key figure-of-merits, including photoresponsivity, response time, NEP and detectivity are introduced. According to these factors, one can know clearly about the advantages and disadvantages of a specific detector. The recent progress of THz PTE detectors is comprehensively reviewed in this paper. The typical devices are analyzed in detail according to their working characteristics and performance. The future direction of THz PTE detectors is also prospected. PTE effect can be combined with many physical effects to further enhance the detector performance. For example, the phonon blocking effect and ballistic transport have been proved to improve thermoelectric performance. Besides, the development of new materials also brings new opportunities for THz PTE detectors. Intercalated two-dimensional materials with small organic molecules can effectively improve the thermoelectric properties and the two-dimensional-electron-gas system can also bring the large Seebeck coefficient and high carrier mobility, which are helpful to THz PTE detectors. All in all, although there is still a long way to go for practical application, THz PTE detector still has a bright prospect due to its unique advantages.

Thermodynamics of the uniform electron gas: Fermionic path integral Monte Carlo simulations in the restricted grand canonical ensemble

The uniform electron gas (UEG) is one of the key models for the understanding of warm dense matter—an exotic, highly compressed state of matter between solid and plasma phases. The difficulty in modelling the UEG arises from the need to simultaneously account for Coulomb correlations, quantum effects, and exchange effects, as well as finite temperature. The most accurate results so far were obtained from quantum Monte Carlo (QMC) simulations with a variety of representations. However, QMC for electrons is hampered by the fermion sign problem. Here, we present results from a novel fermionic propagator path integral Monte Carlo in the restricted grand canonical ensemble. The ab initio simulation results for the spin‐resolved pair distribution functions and static structure factor are reported for two isotherms (T in the units of the Fermi temperature). Furthermore, we combine the results from the linear response theory in the Singwi‐Tosi‐Land‐Sjölander scheme with the QMC data to remove finite‐size errors in the interaction energy. We present a new corrected parametrization for the interaction energy and the exchange–correlation free energy in the thermodynamic limit, and benchmark our results against the restricted path integral Monte Carlo by Brown et al. [Phys. Rev. Lett. 110, 146405 (2013)] and configuration path integral Monte Carlo/permutation‐blocking path integral Monte Carlo by Dornheim et al. [Phys. Rev. Lett. 117, 115701 (2016)].

A Perspective on Hydrogen Near the Liquid-Liquid Phase Transition and Metallization of Fluid H.

Metallic hydrogen has been a major issue in physical chemistry since its prediction in 1935. Its predicted density implies 100 GPa (106 bar = Mbar) pressures P are needed to make metallic H with the Fermi temperature TF = 220 000 K. Temperatures T can be several 1000 K and still be "very low" with T/TF ≪ 1. In 1996, metallic fluid H was made under dynamic compression at P = 140 GPa and calculated T ≈ 3000 K generated with a two-stage light-gas gun. Those T's place metallic H in the liquid-liquid phase transition region. The purpose of this Perspective is to place the phase curve measured in laser-heated diamond anvil cells in context with those measured electrical conductivities. That phase curve is probably caused by dissociation of H2 to H starting near 90 GPa/1600 K. Metallic H then forms in a crossover as a semiconductor up to 140 GPa/3000 K. Dynamic quasi-isentropic pressure was tuned to make metallic H by design in those conductivity experiments.

Dissipative superfluid hydrodynamics for the unitary Fermi gas

In this work we establish constraints on the temperature dependence of the shear viscosity $\eta$ in the superfluid phase of a dilute Fermi gas in the unitary limit. Our results are based on analyzing experiments that measure the aspect ratio of a deformed cloud after release from an optical trap. We discuss how to apply the two-fluid formalism to the unitary gas, and provide a suitable parametrization of the equation of state. We show that in expansion experiments the difference between the normal and superfluid velocities remains small, and can be treated as a perturbation. We find that expansion experiments favor a shear viscosity that decreases significantly in the superfluid regime. Using an exponential parametrization we find $\eta(T_c/(2T_F))< 0.37\eta (T_c/T_F)$, where $T_c$ is the critical temperature, $T_F$ is the local Fermi temperature of the gas.

Comparison of highly-compressed C2/m-SnH12 superhydride with conventional superconductors

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) predicted high-temperature superconductivity in hydrogen-rich metallic alloys, based on an idea that these compounds should exhibit high Debye frequency of the proton lattice, which boosts the superconducting transition temperature, T c. The idea has got full confirmation more than four decades later when Drozdov et al (2015 Nature 525 73) experimentally discovered near-room-temperature superconductivity in highly-compressed sulphur superhydride, H3S. To date, more than a dozen of high-temperature hydrogen-rich superconducting phases in Ba–H, Pr–H, P–H, Pt–H, Ce–H, Th–H, S–H, Y–H, La–H, and (La, Y)–H systems have been synthesized and, recently, Hong et al (2021 arXiv:2101.02846) reported on the discovery of C2/m-SnH12 phase with superconducting transition temperature of T c ∼ 70 K. Here we analyse the magnetoresistance data, R(T, B), of C2/m-SnH12 phase and report that this superhydride exhibits the ground state superconducting gap of Δ(0) = 9.2 ± 0.5 meV, the ratio of 2Δ(0)/k B T c = 3.3 ± 0.2, and 0.010 < T c/T F < 0.014 (where T F is the Fermi temperature) and, thus, C2/m-SnH12 falls into unconventional superconductors band in the Uemura plot.

Ab initio path integral Monte Carlo approach to the momentum distribution of the uniform electron gas at finite temperature without fixed nodes

We present extensive new \textit{ab intio} path integral Monte Carlo results for the momentum distribution function $n(\mathbf{k})$ of the uniform electron gas (UEG) in the warm dense matter (WDM) regime over a broad range of densities and temperatures. This allows us to study the nontrivial exchange--correlation induced increase of low-momentum states around the Fermi temperature, and to investigate its connection to the related lowering of the kinetic energy compared to the ideal Fermi gas. In addition, we investigate the impact of quantum statistics on both $n(\mathbf{k})$ and the off-diagonal density matrix in coordinate space, and find that it cannot be neglected even in the strongly coupled electron liquid regime. Our results were derived without any nodal constraints, and thus constitute a benchmark for other methods and approximations.

A generalized molecule approach capturing the Feshbach-induced pairing physics in the BEC–BCS crossover

By including the effect of a trap with characteristic energy given by the Fermi temperature T F in a two-body two-channel model for Feshbach resonances, we reproduce the measured binding energy of ultracold molecules in a 40K atomic Fermi gas. We also reproduce the experimental closed-channel fraction Z across the BEC–BCS crossover and into the BCS regime of a 6Li atomic Fermi gas. We obtain the expected behavior at unitarity, together with the recently measured proportionality constant. Our results are also in agreement with recent measurements of the Z dependency on T F on the BCS side, where a significant quantitative discrepancy between experimental data and theory’s predictions has been repeatedly reported. In order to contrast with future experiments we report the proportionality constant at unitarity between Z and predicted by our model for a 40K atomic Fermi gas.

Production of Degenerate Fermi Gases of 6Li Atoms in an Optical Dipole TrapSupported by the National Key Research and Development Program of China (Grant No. 2016YFA0301503), the National Natural Science Foundation of China (Grant Nos. 11674358, 11434015, and 11974384), the Chinese Academy of Sciences (Grant No. YJKYYQ20170025), and K. C. Wong Education Foundation (Grant No. GJTD-2019-15).

We report the experimental production of degenerate Fermi gases of 6Li atoms in an optical dipole trap. The gray-molasses technique is carried out to decrease the atomic temperature to 57 μK, which facilitates the efficient loading of cold atoms into the optical dipole trap. The Fermi degeneracy is achieved by evaporative cooling of a two-spin mixture of 6Li atoms on the Feshbach resonance. The degenerate atom number per spin is 3.5 × 104, and the reduced temperature T/T F is as low as 0.1, where T F is the Fermi temperature of the non-interacting Fermi gas. We also observe the anisotropic expansion of the atom cloud in the strongly interacting regime.

Spin Susceptibility above the Superfluid Onset in Ultracold Fermi Gases

Ultracold atomic Fermi gases can be tuned to interact strongly, which produces a display of spectroscopic signatures above the superfluid transition reminiscent of the pseudogap in cuprates. However, the extent of the analogy can be questioned since many thermodynamic quantities in the low temperature spin-imbalanced normal state can be described successfully using Fermi liquid theory. Here we present spin susceptibility measurements across the interaction strength-temperature phase diagram using a novel radio frequency technique with ultracold ^{6}Li gases. For all significant interaction strengths and at all temperatures we find the spin susceptibility is reduced compared to the equivalent value for a noninteracting Fermi gas. At unitarity, we can use the local density approximation to extract the integrated spin susceptibility for the uniform gas as a function of temperature, which at high temperatures is generally less than theoretically predicted. At low temperatures, our data lie within the range of theoretical predictions, although we can also describe the entire curve using a very simple one-parameter mean field model with monotonically increasing spin susceptibility.

Lattice stability of ordered Au-Cu alloys in the warm dense matter regime

In the warm dense regime, where the electron temperature is increased to the same order of the Fermi temperature, the dynamical stability of elemental metals depends on its electronic band structure as well as its crystal structure. It has been known that phonon hardening occurs due to an enhanced internal pressure caused by electron excitations as in close-packed simple metals, whereas phonon softening occurs at a specific point in the Brillouin zone as in body-centered cubic metals. Here, we investigate the dynamical stability of binary ordered alloys (Au and Cu) in the L1$_0$ and L1$_2$ structures. By performing first principles calculations on phonon dispersions, we demonstrate that warm dense Au-Cu systems show phonon hardening behavior except the lowest frequency phonon mode at point R of AuCu in the L1$_0$ structure. We show that when a phonon mode is stabilized by long-range interatomic interactions at ambient condition, such a phonon will be destabilized by the short-range nature of the warm dense matters.

The electron-phonon coupling constant, Fermi temperature and unconventional superconductivity in the carbonaceous sulfur hydride 190 K superconductor

Recently, Snider et al (2020 Nature 586 373) reported on the observation of superconductivity in highly compressed carbonaceous sulfur hydride, H x (S,C) y . The highest critical temperature in H x (S,C) y exceeds the previous record of T c = 280 K by 5 K, as reported by Somayazulu et al (2019 Phys. Rev. Lett. 122 027001) for highly compressed LaH10. In this paper, we analyze experimental temperature-dependent magnetoresistance data, R(T,B), reported by Snider et al. The analysis shows that H x (S,C) y compound exhibited T c = 190 K (P = 210 GPa), has the electron–phonon coupling constant λ e−ph = 2.0 and the ratio of critical temperature, T c, to the Fermi temperature, T F, in the range of 0.011 ⩽ T c/T F ⩽ 0.018. These deduced values are very close to the ones reported for H3S at P = 155–165 GPa (Drozdov et al 2015 Nature 525 73). This means that in all considered scenarios the carbonaceous sulfur hydride 190 K superconductor falls into the unconventional superconductor band in the Uemura plot, where all other highly compressed super-hydride/deuterides are located. It should be noted that our analysis shows that all raw R(T,B) data sets for H x (S,C) y samples, for which Snider et al (2020 Nature 586 373) reported T c > 200 K, cannot be characterized as reliable data sources. Thus, independent experimental confirmation/disproof for high-T c values in the carbonaceous sulfur hydride are required.

Generalized Boltzmann relations in semiconductors including band tails

Boltzmann relations are widely used in semiconductor physics to express the charge-carrier densities as a function of the Fermi level and temperature. However, these simple exponential relations only apply to sharp band edges of the conduction and valence bands. In this article, we present a generalization of the Boltzmann relations accounting for exponential band tails. To this end, the required Fermi–Dirac integral is first recast as a Gauss hypergeometric function followed by a suitable transformation of that special function and a zeroth-order series expansion using the hypergeometric series. This results in simple relations for the electron and hole densities that each involve two exponentials. One exponential depends on the temperature and the other one on the band-tail parameter. The proposed relations tend to the Boltzmann relations if the band-tail parameters tend to zero. This work is timely for the modeling of semiconductor devices at cryogenic temperatures for large-scale quantum computing.

Estimation of (n,p) reaction cross sections at 14.5 [formula omitted] MeV neutron energy by using artificial neural network

The aim of this study is to develop an accurate artificial neural network algorithm for the cross-section of (n,p) reactions at 14.5 ∓0.5 MeV neutron energy which is important to developing materials for fusion reactor design. The experimental data used at artificial Neural network calculations have been taken from the Experimental Nuclear Reaction Data (EXFOR) database. Bayesian algorithm has been used at training section of artificial neural network. Regression (R) values of artificial neural network calculations have been found as 0.99363, 0.98574 and 0.99257 for training, testing and all process respectively. In addition to artificial neural network calculations, TALYS 1.95 nuclear reaction code has been used to reproduce (n,p) reactions at 14.5 ∓0.5 MeV. Two-component exciton model and Constant Temperature Fermi Gas Model have been used as pre-equilibrium and level density models respectively. Mean square errors of our calculations have been found 48.51 and 495.06 for artificial neural network and TALYS 1.95 respectively. Artificial Neural network estimations have been compared and analyzed with the TALYS 1.95 calculations and the experimental data taken from EXFOR database.