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Results for “FERMI LEVEL”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Investigation of the electrical conduction mechanisms in P-type amorphous germanium electrical contacts for germanium detectors in searching for rare-event physics

For the first time, electrical conduction mechanisms in the disordered material system is experimentally studied for p-type amorphous germanium (a-Ge) used for high-purity Ge detector contacts. The localization length and the hopping parameters in a-Ge are determined using the surface leakage current measured from three high-purity planar Ge detectors. The temperature dependent hopping distance and hopping energy are obtained for a-Ge fabricated as the electrical contact materials for high-purity Ge planar detectors. As a result, we find that the hopping energy in a-Ge increases as temperature increases while the hopping distance in a-Ge decreases as temperature increases. The localization length of a-Ge is on the order of 2.13$^{-0.05}_{+0.07}$A° to 5.07$^{0.83}_{+2.58}$A°,depending on the density of states near the Fermi energy level within bandgap. Using these parameters, we predict that the surface leakage current from a Ge detector with a-Ge contacts can be much smaller than one yocto amp (yA) at helium temperature, suitable for rare-event physics searches.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

The impact of the charge barrier height on Germanium (Ge) detectors with amorphous-Ge contacts for light dark matter searches

Germanium (Ge) detectors with ability of measuring a single electron–hole (e–h) pair are needed in searching for light dark matter (LDM) down to the MeV scale. We investigate the feasibility of Ge detectors with amorphous-Ge (a-Ge) contacts to achieve the sensitivity of measuring a single e-h pair, which requires extremely low leakage current. Three Ge detectors with a-Ge contacts are used to study the charge barrier height for blocking electrons and holes. Using the measured bulk leakage current and the Döhler–Brodsky model, we obtain the values for charge barrier height and the density of localized energy states near the Fermi energy level for the top and bottom contacts, respectively. We predict that the bulk leakage current is extremely small and can be neglected at helium temperature ( $$\sim $$ ~ 4 K). Thus, Ge detectors with a-Ge contacts possess the potential to measure a single e–h pair for detecting LDM particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Synthesis and Observation of Emergent Phenomena in Epitaxial Heusler Compound Heterostructures

The proposal was on the synthesis and observation of emergent phenomena in epitaxial Heusler compound heterostructures. The large range of properties and number of Heusler compounds opens up a wide number of potential compounds that will exhibit emergent phenomena. The similarity, large range of relatively inexpensive, large area, high crystal quality, III-V bulk substrates, lattice parameters and the ability to tune the lattice parameters through ternary or quaternary III-V compound semiconductor epitaxial growth, makes III-V semiconductors an ideal choice for substrates for epitaxial growth of Heusler compounds. A number of half Heusler compounds have been predicted to exhibit band inversion, making them topological and are therefore expected to exhibit spin-momentum locked topological surface states with linear dispersion. Others are predicted to be semimetals with Weyl points and others semiconducting and magnetic. During the course of this grant, emphasis has been on investigating Heusler compounds with emergent phenomena and demonstrating the ability to tune their properties through alloying and strain. We have grown toplogical semimetal (PtLuSb, PtMnBi), Weyl (Co 2 MnAl, Co 2 TiGe), half metal (PtMnSb, Co 2 MnSi, Co 2 MnAl x Si 1-x , Co2FeAl), and semiconducting (CoTiSb, NiTiSn) and tuned their properties through alloying and epitaxial strain. We also investigated the closely related materials of rare-earth monopnictide, some of which have also been predicted to be topological. During the attempts to grow the PtMnBi, it was discovered that Bi, another predicted topological material when ultrathin, could be grown epitaxially on InSb, results for which are also reported here. The main focus for this effort has been on using variable photon energy and spin-dependent angle resolved photoemission (ARPES) to determine bulk band structure and surface states of pristine epitaxial films grown on III-V semiconductor and MgO substrates and correlate results with theory and transport measurements. Theory has been critical to interpretation of experimental results and has been essential in guiding experiments. The research benefited from several strong collaborations between the PIs and the beamline scientists at the Advanced Light Source at Lawrence Berkeley Laboratory, the Stanford Linear Accelerator Center (SLAC) at Stanford and at the Max Lab at Lund University in Sweden. The strong experiment - theory collaboration between the PI’s groups, the Palmstrøm group at UCSB and the Janotti group at the University of Delaware, has been critical for interpreting the experimental ARPES and magnetotransport measurements results and making predictions to guide experiments. Weekly interactive Zoom meetings made this work well. A collaboration between the Palmstrøm group and Dr. Alexei Fedorov at the Advanced Light Source (ALS) resulted in significant modifications to his end chamber to accommodate the vacuum suitcase that was designed and constructed in the Palmstrøm group at UCSB. In collaboration with beamline scientists, Drs. Makoto Hashimoto and Donghui Lu at SLAC, Palmstrøm made modifications to the vacuum suitcase and developed special sample holders that allowed samples to be grown in the Palmstrøm MBE systems at UCSB and transported in the UHV vacuum suitcase to SLAC for ARPES measurements. The development of the vacuum suitcase was essential for this grant as it has allowed variable photon energies to be used to identify surface versus bulk states on samples that could not be capped and decapped using As- or Sb-capping layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2021

Probing the Origin of the Open Circuit Voltage in Perovskite Quantum Dot Photovoltaics

Perovskite quantum dots (PQDs) have many properties that make them attractive for optoelectronic applications, including expanded compositional tunability and crystallographic stabilization. While they have not achieved the same photovoltaic (PV) efficiencies of top-performing perovskite thin films, they do reproducibly show high open circuit voltage (VOC) in comparison. Further understanding of the VOC attainable in PQDs as a function of surface passivation, contact layers, and PQD composition will further progress the field and may lend useful lessons for non-QD perovskite solar cells. Here, we use photoluminescence-based spectroscopic techniques to understand and identify the governing physics of the VOC in CsPbI3 PQDs. In particular, we probe the effect of the ligand exchange and contact interfaces on the VOC and free charge carrier concentration. The free charge carrier concentration is orders of magnitude higher than in typical perovskite thin films and could be tunable through ligand chemistry. Tuning the PQD A-site cation composition via replacement of Cs+ with FA+ maintains the background carrier concentration but reduces the trap density by up to a factor of 40, reducing the VOC deficit. These results dictate how to improve PQD optoelectronic properties and PV device performance and explain the reduced interfacial recombination observed by coupling PQDs with thin-film perovskites for a hybrid absorber layer.

perovskite quantum dot↗
DOE OSTI2023

Electronic density of states of a U ( 1 ) quantum spin liquid with spinon Fermi surface. I. Orbital magnetic field effects

A quantum spin liquid with a spinon Fermi surface is an exotic insulator that hosts neutral Fermi surfaces inside the insulating gap. In an external magnetic field, it has been pointed out that the neutral Fermi surfaces are Landau quantized to form Landau levels due to the coupling to the induced emergent gauge magnetic field. In this paper, we calculate the electronic density of states (as observed in tunneling experiments) of the quantum spin liquid in an orbital magnetic field. Here, we find that the Landau levels from the neutral Fermi surfaces give rise to a set of steps emerging at the upper and lower Hubbard band edges. Each of the Hubbard band edge steps further develop into a band edge resonance peak when a weak gauge binding arising from the gauge field fluctuations is taken into account. Importantly, each Hubbard band edge step and its resulting resonance peak in the weak gauge binding are found to have a corresponding Landau level from the neutral Fermi surfaces, so the Hubbard band edge steps and the band edge resonance peaks provide signatures to the unique feature that characterizes the Landau quantization of the in-gap neutral Fermi surfaces in the spin liquid. We further consider the strong gauge-binding regime where the band edge resonance peaks move into the Mott gap and develop into true in-gap bound states. In the strong gauge binding regime, we solve the Landau-level spectrum of the in-gap bound states in an orbital magnetic field. For the in-gap bound state with a Mexican-hat-like band dispersion, we find that the envelop energy to have a state excited from the bound state Landau levels decreases quadratically with the magnetic field. The quadratic decrease behavior of the envelop energy is consistent with the intuition that applying magnetic field localizes the states and energetically promotes the in-gap bound states formation. Finally, we discuss the connection of our results to the electronic density of states spectra measured in the layered 1T-TaS 2 . We point out that a quantum spin liquid with a quasibound state in the upper Hubbard band can give the density of states spectra similar to the one measured in the experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1987

Optical characterization of semi-insulating GaAs - Determination of the Fermi energy, the concentraion of the midgap EL2 level and its occupancy

The key electronic characteristics of semiinsulating GaAs, i.e., the Fermi energy, concentration, and occupancy of the midgap donor EL2, and the net concentration of ionized acceptors can all be determined from high-resolution measurements of the EL2 intracenter absorption. The procedure is based on the measurement of zero-phonon line intensity before and after the complete transfer of EL2 to its metastable state followed by thermal recovery. The procedure is quantitative, involves no fitting parameters, and unlike existing methods, is applicable even when a significant part of the EL2 is ionized.

Lagowski, J.↗
NASA NTRS1972

Quantum limit studies in single crystal and pyrolytic graphite

The occupied regions of momentum space in graphite are confined to elongated volumes near the hexagonal Brillouin zone edges. The elongation is along the hexagonal axis (c-axis), so that cross sectional areas perpendicular to that axis are very small. When magnetic fields are applied parallel to the c-axis, only the n = 0 Landau levels remain below the Fermi energy for fields above 7 tesla. The energy bands in graphite can be described by a set of seven parameters (including the Fermi energy as a parameter). In a quantizing magnetic field the Landau levels are described by a secular equation determined by diagonalizing the Hamiltonian. A major feature for the levels along the Brillouin zone edge is that the n = 0 Landau level for electrons is independent of magnetic field, while that for holes is field-dependent.

Woollam, J. A.↗
NASA NTRS1974

Quantum limit studies in single-crystal and pyrolytic graphite

The occupied regions of momentum space in graphite are confined to very elongated volumes near the hexagonal Brillouin zone edges. The elongation is along the hexagonal axis (c axis) so that cross-sectional areas perpendicular to that axis are very small. Hence, when magnetic fields are applied parallel to the c axis only the n = 0 Landau levels remain below the Fermi energy for fields above 7 T (Woollam, 1971). The energy bands in graphite can be described by a set of seven parameters (McClure, 1971) (including the Fermi energy as a parameter). In a quantizing magnetic field the Landau levels are described by a secular equation determined by diagonalizing the Hamiltonian (McClure, 1960). A major unusual feature for the levels along the Brillouin zone edge is that the n = 0 Landau level for electrons is independent of magnetic field, while that for holes is field dependent.

Woollam, J. A.↗
DOE OSTI2020

Bloch ferromagnetism of composite fermions

In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy. However, experimental realizations of this effect have been hard to implement. In this work, we report the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism. Our platform utilizes the two-dimensional Fermi sea of composite fermions near half-filling of the lowest Landau level. We measure the Fermi wavevector—which directly provides the spin polarization—and observe a sudden transition from a partially spin-polarized to a fully spin-polarized ground state as we lower the density of the composite fermions. Our theoretical calculations that take Landau level mixing into account provide a semi-quantitative account of this phenomenon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Level density within a micro-macroscopic approach

Statistical level density $ρ$($E, A$) is derived for nucleonic system with a given energy $E$, particle number $A$ and other integrals of motion in the micro-macroscopic approximation beyond the standard saddle-point method of the Fermi gas model. This level density reaches the two limits; the well-known Fermi gas grand-canonical ensemble limit for a large entropy $S$ related to large excitation energies, and the finite micro-canonical limit for a small combinatorical entropy $S$ at low excitation energies. In conclusion, the inverse level density parameter $K$ as function of the particle number $A$ in the semiclassical periodic orbit theory, taking into account the extended Thomas-Fermi and Strutinsky shell corrections, is calculated and compared with experimental data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Evolution of Urca Pairs in the Crusts of Highly Magnetized Neutron Stars

Abstract We report on the effects of strong magnetic fields on neutrino emission in the modified Urca process. We show that the effect of Landau levels on the various Urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission. For low magnetic fields, the Landau levels have almost no effect on the cooling. However, as the field strength increases, the electron chemical potential increases resulting in a lower density at which Urca pairs can exist. For intermediate field strength, there is an interesting interference between the Landau level distribution and the Fermi distribution. For high enough field strength, the entire electron energy spectrum is eventually confined to a single Landau level producing dramatic spikes in the emission spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Electronic Structure of HgBa2CaCu2O(6+delta) Epitaxial films measured by x-ray Photoemission

The electronic structure and chemical states of HgBa2CaCu20(sub 6 + delta), epitaxial films have been studied with x-ray photelectron spectroscopy. Signals from the superconducting phase dominate all the core-level spectra, and a clear Fermi edge is observed in the valence-band region. The Ba, Ca, Cu, and O core levels are similar to those of Tl2Ba2CaCu208(+)O(sub 6 + delta), but distinct differences are observed in the valence bands which are consistent with differences in the calculated densities of states.

Vasquez, R. P.↗
DOE OSTI2020

Sensitivity of EDM experiments in paramagnetic atoms and molecules to hadronic CP violation

Experiments searching for the electric dipole moment (EDM) of the electron d e utilize atomic/molecular states with one or more uncompensated electron spins, and these paramagnetic systems have recently achieved remarkable sensitivity to d e . If the source of CP violation resides entirely in the hadronic sector, the two-photon exchange processes between electrons and the nucleus induce CP-odd semileptonic interactions, parametrised by the Wilson coefficient C SP , and provide the dominant source of EDMs in paramagnetic systems instead of d e . We evaluate the C SP coefficients induced by the leading hadronic sources of CP violation, namely nucleon EDMs and CP-odd pion-nucleon couplings, by calculating the nucleon-number-enhanced CP-odd nuclear scalar polarisability, employing chiral perturbation theory at the nucleon level and the Fermi-gas model for the nucleus. This allows us to translate the ACME EDM limits from paramagnetic ThO into novel independent constraints on the QCD theta term |$\bar θ$| < 3 × 10 –8 , proton EDM |d p | < 2 × 10 –23 e cm, isoscalar CP-odd pion-nucleon coupling |$\bar g$$^{(1)}_{πNN}$| < 4 × 10 –10 , and color EDMs of quarks |$\tilde d$ u -$\tilde $d d | < 2 × 10 –24 cm. We note that further experimental progress with EDM experiments in paramagnetic systems may allow them to rival the sensitivity of EDM experiments with neutrons and diamagnetic atoms to these quantities.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Fractional Quantum Hall Effect with Unconventional Pairing in Monolayer Graphene

Motivated by the observation of even denominator fractional quantum Hall effect in the n = 3 Landau level of monolayer graphene [Kim et al., Nat. Phys. 15, 154 (2019)], we consider a Bardeen-Cooper-Schrieffer variational state for composite fermions and find that the composite-fermion Fermi sea in this Landau level is unstable to an f-wave pairing. Analogous calculation suggests the possibility of a p-wave pairing of composite fermions at half filling in the n = 2 graphene Landau level, whereas no pairing instability is found at half filling in the n = 0 and n = 1 graphene Landau levels. The relevance of these results to experiments is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1975

Line positions of H2O in the 1.33 to 1.45 micron region

Results are presented for an experimental investigation, using a vacuum IR spectrometer, of the line positions and strengths of water vapor in the spectral region from 6900 to 7500 kaysers. The line-center frequencies of the vibration-rotation transitions of the five H2O bands in this spectral interval are determined along with the rotational levels of the (101), (200), (021), (120), and (002) states. It is noted that several of the upper-state levels are perturbed by resonance effects involving either Fermi coupling or Coriolis coupling of the near-resonance levels of various states. Tables are provided which list the measured line-center frequencies, upper- and lower-state rotational quantum numbers, ground-state energy levels, line strengths, and band assignments. It is shown that the (101) and (021) bands are type A with origins at 7249.811 and 6871.51 kaysers, respectively, while the (200), (002), and (120) bands are type B with origins at 7201.540, 7445.07, and 6775.10 kaysers, respectively.

Toth, R. A.↗
DOE OSTI2022

Interpreting angle-dependent magnetoresistance in layered materials: Application to cuprates

The evolution of the low-temperature electronic structure of the cuprate metals from the overdoped to the underdoped side has recently been addressed through angle-dependant magnetoresistance (ADMR) experiments in La 1.6-x Nd 0.4 Sr x CuO 4 . Here the results show a striking difference between hole dopings p = 0.24 and p = 0.21, which lie on either side of a putative quantum critical point at intermediate p. Motivated by this, we here study the theory of ADMR in correlated layered materials, paying special attention to the role of angle-dependent quasiparticle weights Z k . Such a Z k is expected to characterize a number of popular models of the cuprate materials, particularly when underdoped. Further, in the limit of weak interlayer hopping the quasiparticle weight will affect the c-axis transport measured in ADMR experiments. We show that proper inclusion of the quasiparticle weight does not support an interpretation of the data in terms of a (π,π) spin density wave ordered state, in agreement with the lack of direct evidence for such order. We show that a simple model of Fermi surface reconfiguring across a van Hove point captures many of the striking differences seen between p = 0.21 and p = 0.24. We comment on why such a model may be appropriate for interpreting the ADMR data, despite having a large Fermi surface at p = 0.21, seemingly in contradiction with other evidence for a small Fermi surface at that doping level.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Field-angle dependence of sound velocity in the Weyl semimetal TaAs

The elastic modulus c 44 of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. Furthermore, we also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Comparison of GW band structure to semiempirical approach for an FeSe monolayer

We present the G 0 W 0 band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the M pocket near the Fermi energy, while leaving the Γ pocket roughly unchanged. We then compare the G 0 W 0 quasiparticle band energies with the band structure from a simple empirical + A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

36 MATERIALS SCIENCE↗