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Results for “Fermionic condensates”

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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Influence of hydrostatic pressure on hidden order, the Kondo lattice, and magnetism in URu 2 Si 2-x P x

Within the chemical substitution series URu 2 Si 2-x P x , there is an evolution in the ground-state behavior from hidden ordered (HO) for x ≲ 0.03, to Kondo lattice behavior with no ordering (NO) for 0.03 ≲ x ≲ 0.26, to antiferromagnetism (AFM-2) for 0.26 ≲ x ≲ 0.5 [A. Gallagher et al., Nat. Commun. 7, 10712 (2016); A. Gallagher et al., J. Phys.: Condens. Matter 29, 024004 (2016)]. To better understand what factors control this behavior, temperature-dependent electrical resistivity measurements are performed for this series under applied pressures P up to 20.5 kbar. Specimens in the HO x region show similarities to the parent compound, where HO transforms into antiferromagnetism (AFM-1) at a critical pressure (P c ). P c decreases with increasing x and collapses towards P = 0 near x ≈ 0.03, suggesting that AFM-1 occurs at ambient pressure for this concentration. No pressure-induced phase transitions are observed in the NO x region and the AFM-2 state is only weakly suppressed by P. Measurements further reveal that AFM-1 and AFM-2 are distinct from each other. Calculations of the wave functions using the tight-binding Hartree-Fock approximation are performed and show (i) that the radial probability distributions for the phosphorus ions are more tightly bound than those for the silicon and (ii) that the energy difference between the orbitals decreases with increasing x. The cumulative effect of these two factors is that Si → P substitution decreases the hybridization strength, which correlates with the weakening of HO. At large x, additional effects such as electrical charge tuning also play an important role in determining the ground-state

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Pressure-induced changes of valence fluctuation in β - YbAlB 4 probed by x-ray absorption spectroscopy

Here we report detailed valence fluctuation phenomena in the unique quantum-critical compound β-YbAlB 4 under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature (~250K at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in β-YbAlB 4 . Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

36 MATERIALS SCIENCE↗
DOE OSTI2022

c -axis transport in UTe 2 : Evidence of three-dimensional conductivity component

Here we study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe 2 . We focus particularly on an accurate determination of the resistivity along the c axis (ρ c ) by using a generalized Montgomery technique that allows extraction of crystallographic resistivity components from a single sample. In contrast to expectations from the observed highly anisotropic band structure, our measurement of the absolute values of resistivities in all current directions reveals a surprisingly nearly isotropic transport behavior at temperatures above Kondo coherence, with ρ c ~ ρ b ~ 2 ρ a , that evolves to reveal qualitatively distinct behaviors on cooling. The temperature dependence of ρ c exhibits a peak at a temperature much lower than the onset of Kondo coherence observed in ρ a and ρ b , consistent with features in magnetotransport and magnetization that point to a magnetic origin. A comparison to the temperature-dependent evolution of the scattering rate observed in angle-resolved photoemission spectroscopy experiments provides important insights into the underlying electronic structure necessary for building a microscopic model of superconductivity in UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Dynamical Cooper pairing in nonequilibrium electron-phonon systems

In this paper, we analyze Cooper pairing instabilities in strongly driven electron-phonon systems. The light-induced nonequilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We demonstrate that the competition between these effects leads to an enhanced superconducting transition temperature in a broad range of parameters. Finally, our results may explain the observed transient enhancement of superconductivity in several classes of materials upon irradiation with high intensity pulses of terahertz light, and may pave new ways for engineering high-temperature light-induced superconducting states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Tuning the magnetic ground state of Ce 1 - x Yb x RhIn 5 by Yb valence fluctuations

We characterize the properties of Ce 1–x Yb x RhIn 5 single crystals with 0 ≤ x ≤ 1 using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. Here, the Yb valence v Yb , calculated from the magnetic susceptibility and measured using XANES, decreases from 3+ at x = 0 to ~2.1+ at x act = 0.2 , where x act is the measured Yb concentration. A transition from incommensurate to commensurate antiferromagnetism is observed in neutron diffraction measurements along Q = (0.5, 0.5, l) between 0.2 ≤ x act ≤ 0.27; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature T I (T I < T N ) for the same concentration range. Magnetic susceptibility measurements also reveal features which provide additional evidence of magneticordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of Ce 1–x Yb x RhIn 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Hierarchical Majoranas in a programmable nanowire network

We propose a hierarchical architecture for building “logical” Majorana zero modes using “physical” Majorana zero modes at the Y-junctions of a hexagonal network of semiconductor nanowires. Each Y-junction contains three “physical” Majoranas, which hybridize when placed in close proximity, yielding a single effective Majorana mode near zero energy. The hybridization of effective Majorana modes on neighboring Y-junctions is controlled by applied gate voltages on the links of the honeycomb network. This gives rise to a tunable tight-binding model of effective Majorana modes. We show that selecting the gate voltages that generate a Kekule vortex pattern in the set of hybridization amplitudes yields an emergent “logical” Majorana zero mode bound to the vortex core. The position of a logical Majorana can be tuned adiabatically, without moving any of the “physical” Majoranas or closing any energy gaps, by programming the values of the gate voltages to change as functions of time. Furthermore, a nanowire network supporting multiple such “logical” Majorana zero modes provides a physical platform for performing adiabatic non-Abelian braiding operations in a fully controllable manner.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ground-state and dynamical properties of two-dimensional dipolar Fermi liquids

We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler–Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schrödinger equation for the “pair amplitude” √(g(r)), where g(r) is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow–Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree–Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition. Using the fluctuation–dissipation theorem and a static approximation for the effective inter-particle interactions, we calculate the dynamical density–density response function, and furthermore demonstrate that an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings. -- Highlights: •We have studied the ground state properties of a strongly correlated two-dimensional fluid of dipolar fermions. •We have calculated the effective inter-particle interaction and the dynamical density–density response function. •We have shown that an undamped zero sound mode exists at any value of the interaction strength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Hidden Rotation Symmetry of the Jordan–Wigner Transformation and Its Application to Measurement in Quantum Computation

Using a global rotation by 𝜃 about the z-axis in the spin sector of the Jordan–Wigner transformation rotates Pauli matrices 𝑋̂ and 𝑌̂ in the 𝑥−𝑦 -plane, while it adds a global complex phase to fermionic quantum states that have a fixed number of particles. With the right choice of angles, this relates expectation values of Pauli strings containing products of 𝑋̂ and 𝑌̂ to different products, which can be employed to reduce the number of measurements needed when simulating fermionic systems on a quantum computer. Here, we derive this symmetry and show how it can be applied to systems in Physics and Chemistry that involve Hamiltonians with only single-particle (hopping) and two-particle (interaction) terms. We also discuss the consequences of this for finding efficient measurement circuits in variational ground state preparation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Classification of (2 + 1) D invertible fermionic topological phases with symmetry

Here we provide a classification of invertible topological phases of interacting fermions with symmetry in two spatial dimensions for general fermionic symmetry groups G f and general values of the chiral central charge c - . Here G f is a central extension of a bosonic symmetry group G b by fermion parity, (-1) F , specified by a second cohomology class [ω 2 ]∈ $\mathscr{H}^2$(Gb,$\mathbb{Z}_2$). Our approach proceeds by gauging fermion parity and classifying the resulting G b symmetry-enriched topological orders while keeping track of certain additional data and constraints. We perform this analysis through two perspectives, using G-crossed braided tensor categories and Spin(2c - ) 1 Chern-Simons theory coupled to a background G gauge field. These results give a way to characterize and classify invertible fermionic topological phases in terms of a concrete set of data and consistency equations, which is more physically transparent and computationally simpler than the more abstract methods using cobordism theory and spectral sequences. Our results also generalize and provide a different approach to the recent classification of fermionic symmetry-protected topological phases by Wang and Gu, which have chiral central charge c - = 0. We show how the tenfold way classification of topological insulators and superconductors fits into our scheme, along with general nonperturbative constraints due to certain choices of c - and G f . Mathematically, our results also suggest an explicit general parametrization of deformation classes of (2 + 1)D invertible topological quantum field theories with G f symmetry.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Heavy fermion Ce{sub 3}Co{sub 4}Sn{sub 13} compound under pressure

The non-magnetic heavy fermion compound Ce{sub 3}Co{sub 4}Sn{sub 13} was studied under pressure. We report single crystalline measurements of electrical resistivity as a function of temperature ρ(T) under pressure. Some characteristic features related to a structural transition (T{sub S}), crystalline field effects (T{sub CEF}), and a low temperature maximum (T{sub max}), possibly connected simultaneously to the onset of Kondo lattice coherence and short range magnetic correlations, were identified in the ρ(T) data. A pressure-temperature phase diagram with T{sub S} and T{sub max} was constructed by mapping these features. Like for most Ce-based heavy fermion compounds, T{sub max} moves to higher temperatures with pressure, indicating that it is related to the Kondo energy scale, due to the increase of hybridization induced by pressure. On the other hand, T{sub S}, associated to a superlattice distortion and probably combined with a charge density wave transition, decreases as a function of pressure. However, differently from the Sr{sub 3−x}Ca{sub x}Ir{sub 4}Sn{sub 13} system, where a superlattice quantum phase transition is observed [L. E. Klintberg et al., Phys. Rev. Lett. 109, 237 008 (2012)], in Ce{sub 3}Co{sub 4}Sn{sub 13} T{sub S} ∼ 154 K, at ambient pressure (P = 0), seems to stabilize at around 143 K for P ≥ 19 kilobars. We also investigated ρ(T) in external magnetic fields, at P = 0. Negative magnetoresistance and increase of T{sub max} are observed, suggesting suppression of low temperature short range magnetic correlations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Non-adiabatic Dynamics in d + id-Wave Fermionic Superfluids

We consider a problem of non-adiabatic dynamics of a 2D fermionic system with d + id-wave symmetry of pairing amplitude. Under the mean-field approximation, we determine the asymptotic behavior of the pairing amplitude following a sudden change of coupling strength. We also study an extended d + id pairing system for which the long-time asymptotic states of the pairing amplitude in the collisionless regime can be determined exactly. By using numerical methods, we have identified three non-equilibrium steady states described by different long-time asymptotes of the pairing amplitude for both the non-integrable and the integrable versions of d + id-wave models. We found that despite of its lack of integrability, long-time dynamics resulting from pairing quenches in the non-integrable d + id model are essentially similar to the ones found for its exactly integrable extended d + id model. We also obtain the long-time phase diagram of the extended d + id model through the Lax construction that exploits underlying integrability showing that the dynamic phases obtained by numerics are consistent with the dynamics of the exactly integrable approach. Both models describe a topological fermionic system with a topologically non-trivial BCS phase appearing at weak coupling strength. We show that the presence of oscillating order parameter region in the chiral d + id pairing dynamics differs from the d-wave (d{sub x{sup 2}−y{sup 2}}), which may be used to probe pairing symmetries of chiral superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Fluctuations and phase transitions in Larkin-Ovchinnikov liquid-crystal states of a population-imbalanced resonant Fermi gas

Motivated by a realization of imbalanced Feshbach-resonant atomic Fermi gases, we formulate a low-energy theory of the Fulde-Ferrell and the Larkin-Ovchinnikov (LO) states and use it to analyze fluctuations, stability, and phase transitions in these enigmatic finite momentum-paired superfluids. Focusing on the unidirectional LO pair-density-wave state, which spontaneously breaks the continuous rotational and translational symmetries, we show that it is characterized by two Goldstone modes, corresponding to a superfluid phase and a smectic phonon. Because of the liquid-crystalline ''softness'' of the latter, at finite temperature the three-dimensional state is characterized by a vanishing LO order parameter, quasi-Bragg peaks in the structure and momentum distribution functions, and a ''charge''-4, paired-Cooper-pairs, off-diagonal long-range order, with a superfluid-stiffness anisotropy that diverges near a transition into a nonsuperfluid state. In addition to conventional integer vortices and dislocations, the LO superfluid smectic exhibits composite half-integer vortex-dislocation defects. A proliferation of defects leads to a rich variety of descendant states, such as the charge-4 superfluid and Fermi-liquid nematics and topologically ordered nonsuperfluid states, that generically intervene between the LO state and the conventional superfluid and the polarized Fermi liquid at low and high imbalance, respectively. The fermionic sector of the LO gapless superconductor is also quite unique, exhibiting a Fermi surface of Bogoliubov quasiparticles associated with the Andreev band of states, localized on the array of the LO domain walls.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Spectroscopic evidence for the direct involvement of local moments in the pairing process of the heavy-fermion superconductor CeCoIn 5

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, “composite pairing” based on the cooperative two-channel Kondo effect directly involving the f -electron moments, has also been proposed for some heavy-fermion compounds including CeCoIn 5 . The origin of the spin-resonance peak observed in neutron-scattering measurements on CeCoIn 5 is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. In this study, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn 5 over wide ranges of temperature and magnetic field. The pairing gap opens at T p ~ 5 K , well above the bulk T c = 2.3 K , and its directional dependence is consistent with d x 2 – y 2 symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T p . The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f -electron local moments are directly involved in the pairing process in CeCoIn 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Low-energy electrodynamics and a hidden Fermi liquid in the heavy-fermion compound CeCoIn 5

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn 5 . Below the ≈40 K Kondo coherence temperature, a narrow Drude-like peak forms, as a result of the 𝑓-orbital–conduction-electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate (1/𝜏) and effective mass (𝑚*/𝑚 𝑏 ). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the renormalized quasiparticle scattering rate (1/𝜏*=𝑚 𝑏 /𝑚*⁢𝜏) shows a 𝑇 2 dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. In conclusion, this gives evidence for a Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Relaxing Kondo-screened Kramers doublets in CeRhSi 3

${\mathrm{CeRhSi}}_{3}$ is a superconductor under pressure coexisting with a weakly antiferromagnetic phase characterized by a Bragg peak at q → 0 = (~ 0.2,0,0.5) [N. Aso et al., J. Magn. Magn. Mater. 310, 602 (2007)]. The compound is also a heavy-fermion material with a large specific heat coefficient ${\gamma}=110$ mJ ${\mathrm{mol}}^{{-}1}{\mathrm{K}}^{{-}2}$ and a high Kondo temperature of ${T}_{K}=50$ K, indicating ${\mathrm{CeRhSi}}_{3}$ is in a strongly Kondo screened state. We apply high-resolution neutron spectroscopy to investigate the magnetic fluctuations in the normal phase, at ambient pressures, and at low temperatures. We measure a commensurate dynamic response centered around the Q → = (0,0,2) position that gradually evolves to $H{\sim}0.2$ with decreasing temperature and/or energy transfers. The response is broadened both in momentum and energy and is not reminiscent of sharp spin wave excitations found in insulating magnets where the electrons are localized. We parametrize the excitation spectrum and temperature dependence using a heuristic model utilizing the random-phase approximation to couple relaxing ${\mathrm{Ce}}^{3+}$ ground-state Kramers doublets with a Kondo-like dynamic response. With a Ruderman-Kittel-Kasuya-Yosida exchange interaction within the $ab$ plane and an increasing single-site susceptibility, we can qualitatively reproduce the neutron spectroscopic results in ${\mathrm{CeRhSi}}_{3}$ and, namely, the trade-off between scattering at commensurate and incommensurate positions. We suggest that the antiferromagnetic phase in ${\mathrm{CeRhSi}}_{3}$ is driven by weakly correlated relaxing localized Kramers doublets and that ${\mathrm{CeRhSi}}_{3}$ at ambient pressures is on the border between a Rudderman-Kittel-Yosida antiferromagnetic state and a Kondo-screened phase where static magnetism is predominately absent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Orbital selective Kondo effect in heavy fermion superconductor UTe2

Abstract Heavy fermion systems emerge from the collective Kondo effect, and their superconductivity can serve as a promising platform for realizing next-generation quantum technologies. However, it has been a great challenge to explore many-body effects in heavy fermion systems with ab-initio approaches. We computed the electronic structure of UTe 2 without purposive judgements, such as intentional selection of on-site Coulomb interaction and disregarding spin-orbit coupling. We show that U-5 f electrons are highly localized in the paramagnetic normal state, giving rise to the Kondo effect. It is also found that the hybridization between U-5 f and U-6 d predominantly in the orthorhombic a b -plane is responsible for the high-temperature Kondo effect. In contrast, the hybridization between U-5 f and Te-5 p along the c -axis manifests the Kondo scattering at a much lower temperature, which could be responsible for the low-temperature upturn of the c -axis resistivity. Our results show that the electron correlation in UTe 2 is orbital selective, which naturally elucidates the recent experimental observations of anomalous temperature dependence of resistivity. Furthermore, we suggest that the Kondo effect is suppressed at high pressure owing to weak localization of magnetic moments, which results from enhanced U-5 f electron hopping. Our discovery provides significant insight toward understanding anisotropic quantum behavior including selective re-entrant superconductivity in heavy fermion UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Fluctuations and magnetoresistance oscillations near the half-filled Landau level

We study theoretically the magnetoresistance oscillations near a half-filled lowest Landau level (ν = 1/2) that result from the presence of a periodic one-dimensional electrostatic potential. In this work, we use the Dirac composite fermion theory of Son, where the ν = 1/2 state is described by a (2 + 1)-dimensional theory of quantum electrodynamics. We extend previous work that studied these oscillations in the mean-field limit by considering the effects of gauge-field fluctuations within a large flavor approximation. A self-consistent analysis of the resulting Schwinger-Dyson equations suggests that fluctuations dynamically generate a Chern-Simons term for the gauge field and a magnetic field–dependent mass for the Dirac composite fermions away from ν = 1/2. We show how this mass results in a shift of the locations of the oscillation minima that improves the comparison with experiment. The temperature-dependent amplitude of these oscillations may enable an alternative way to measure this mass. This amplitude may also help distinguish the Dirac and Halperin, Lee, and Read composite fermion theories of the half-filled Landau level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗