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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Exactly Solvable Model for Strongly Interacting Electrons in a Magnetic Field

States of strongly interacting particles are of fundamental interest in physics and can produce exotic emergent phenomena and topological structures. We consider here two-dimensional electrons in a magnetic field, and, departing from the standard practice of restricting to the lowest LL, introduce a model short-range interaction that is infinitely strong compared to the cyclotron energy. Here, we demonstrate that this model lends itself to an exact solution for the ground as well as excited states at arbitrary filling factors v<1/2p and produces a fractional quantum Hall effect at fractions of the form v=n/(2pn+1), where n and p are integers. The fractional quantum Hall states of our model share many topological properties with the corresponding Coulomb ground states in the lowest Landau level, such as the edge physics and the fractional charge of the excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

SWAP Gate between a Majorana Qubit and a Parity-Protected Superconducting Qubit

High fidelity quantum information processing requires a combination of fast gates and long-lived quantum memories. Here, in this Letter, we propose a hybrid architecture, where a parity-protected superconducting qubit is directly coupled to a Majorana qubit, which plays the role of a quantum memory. The superconducting qubit is based upon a π-periodic Josephson junction realized with gate-tunable semiconducting wires, where the tunneling of individual Cooper pairs is suppressed. One of the wires additionally contains four Majorana zero modes that define a qubit. We demonstrate that this enables the implementation of a SWAP gate, allowing for the transduction of quantum information between the topological and conventional qubit. This architecture combines fast gates, which can be realized with the superconducting qubit, with a topologically protected Majorana memory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Orbital ice: An exact Coulomb phase on the diamond lattice

We demonstrate the existence of an orbital Coulomb phase as the exact ground state of a p-orbital exchange Hamiltonian on the diamond lattice. The Coulomb phase is an emergent state characterized by algebraic dipolar correlations and a gauge structure resulting from local constraints (ice rules) of the underlying lattice models. For most ice models on the pyrochlore lattice, these local constraints are a direct consequence of minimizing the energy of each individual tetrahedron. On the contrary, the orbital ice rules are emergent phenomena resulting from the quantum orbital dynamics. We show that the orbital ice model exhibits an emergent geometrical frustration by mapping the degenerate quantum orbital ground states to the spin-ice states obeying the 2-in-2-out constraints on the pyrochlore lattice. We also discuss possible realization of the orbital ice model in optical lattices with p-band fermionic cold atoms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Unconventional superconductivity in heavy-fermion compounds

Over the past 35 years, research on unconventional superconductivity in heavy-fermion systems has evolved from the surprising observations of unprecedented superconducting properties in compounds that convention dictated should not superconduct at all to performing explorations of rich phase spaces in which the delicate interplay between competing ground states appears to support emergent superconducting states. In this article, we review the current understanding of superconductivity in heavy-fermion com- pounds and identify a set of characteristics that is common to their unconventional superconducting states. These core properties are compared with those of other classes of unconventional superconductors such as the cuprates and iron-based superconductors. Lastly, we conclude by speculating on the prospects for future research in this field and how new advances might contribute towards resolving the long-standing mystery of how unconventional superconductivity works.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Quantum Hall effect in HgTe quantum wells at nitrogen temperatures

We report on the observation of quantized Hall plateaus in a system of two-dimensional Dirac fermions, implemented in a 6.6 nm HgTe quantum well at magnetic fields up to 34 T at nitrogen temperatures. The activation energies determined from the temperature dependence of the longitudinal resistivity are found to be almost equal for the filling factors ν of 1 and 2. This indicates that the large values of the g-factor (about 30–40) remain unchanged at very strong magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Cubic hastatic order in the two-channel Kondo-Heisenberg model

Materials with non-Kramers doublet ground states naturally manifest the two-channel Kondo effect, as the valence fluctuations are from a non-Kramers doublet ground state to an excited Kramers doublet. Here, the development of a heavy Fermi liquid requires a channel-symmetry-breaking spinorial hybridization that breaks both single and double time-reversal symmetry, and is known as hastatic order. Motivated by cubic Pr-based materials with $Γ$ 3 non-Kramers ground state doublets, this paper provides a survey of cubic hastatic order using the simple two-channel Kondo-Heisenberg model. Hastatic order necessarily breaks time-reversal symmetry, but the spatial arrangement of the hybridization spinor can be either uniform (ferrohastatic) or break additional lattice symmetries (antiferrohastatic). The experimental signatures of both orders are presented in detail, and include tiny conduction electron magnetic moments. Interestingly, there can be several distinct antiferrohastatic orders with the same moment pattern that break different lattice symmetries, revealing a potential experimental route to detect the spinorial nature of the hybridization. We employ an SU( N ) fermionic mean-field treatment on square and simple cubic lattices, and examine how the nature and stability of hastatic order varies as we vary the Heisenberg coupling, conduction electron density, band degeneracies, and apply both channel and spin symmetry breaking fields. In conclusion, we find that both ferrohastatic and several types of antiferrohastatic orders are stabilized in different regions of the mean-field phase diagram, and evolve differently in strain and magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Topological Circular Dichroism in Chiral Multifold Semimetals

Uncovering the physical contents of the nontrivial topology of quantum states is a critical problem in condensed matter physics. Here, we study the topological circular dichroism in chiral semimetals using linear response theory and first-principles calculations. We show that, when the low-energy spectrum respects emergent SO(3) rotational symmetry, topological circular dichroism is forbidden for Weyl fermions, and thus is unique to chiral multifold fermions. This is a result of the selection rule that is imposed by the emergent symmetry under the combination of particle-hole conjugation and spatial inversion. Using first-principles calculations, we predict that topological circular dichroism occurs in CoSi for photon energy below about 0.2 eV. In conclusion, our Letter demonstrates the existence of a response property of unconventional fermions that is fundamentally different from the response of Dirac and Weyl fermions, motivating further study to uncover other unique responses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Integrating superconducting phase and topological crystalline quantum spin Hall effect in hafnium intercalated gallium film

Motivated by the growth of superconducting atomic hexagonal Ga layers on GaN surface we have calculated the electronic properties of Hf intercalated honeycomb Ga layers using first-principles theory. In contrast to the hexagonal Ga layers where substrate is necessary for their stability, we find the above structure to be dynamically stable in its freestanding form with small formation energy. In particular, six Dirac cones composed of Hf-d{sub xy}/d{sub x2-y2} orbitals are observed in the first Brillouin zone, slightly below the Fermi energy. Spin-orbit coupling opens a large band gap of 177 meV on these Dirac cones. By calculating its mirror Chern number, we demonstrate that this band gap is topologically nontrivial and protected by mirror symmetry. Such mirror symmetry protected band gaps are rare in hexagonal lattice. A large topological crystalline quantum spin Hall conductance σ{sub SH} ∼ −4 e{sup 2}/h is also revealed. Moreover, electron-phonon coupling calculations reveal that this material is superconducting with a transition temperature T{sub c} = 2.4 K, mainly contributed by Ga out-of-plane vibrations. Our results provide a route toward manipulating quantum spin Hall and superconducting behaviors in a single material which helps to realize Majorana fermions and topological superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Orbital order of spinless fermions near an optical Feshbach resonance

We study the quantum phases of a three-color Hubbard model that arises in the dynamics of the p-band orbitals of spinless fermions in an optical lattice. Strong, color-dependent interactions are induced by an optical Feshbach resonance. Starting from the microscopic scattering properties of ultracold atoms, we derive the orbital exchange constants at 1/3 filling on the cubic optical lattice. Using this, we compute the phase diagram in a Gutzwiller ansatz. We find phases with ''axial orbital order'' in which p{sub z} and p{sub x}+ip{sub y} (or p{sub x}-ip{sub y}) orbitals alternate.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Topological Phases Emerging from Spin-Orbital Physics

We study the evolution of spin-orbital correlations in an inhomogeneous quantum system with an impurity replacing a doublon by a holon orbital degree of freedom. Spin-orbital entanglement is large when spin correlations are antiferromagnetic, while for a ferromagnetic host we obtain a chain with only orbital interactions. In this regime, the orbital model can be mapped on spinless fermions and we uncover topological phases with zero energy modes at the edge or at the domain between magnetically inequivalent regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Engineering topologically protected superconducting states

Efforts to develop systems which support non-Abelian excitations are motivated by both scientific curiosity and a practical desire to built a fault tolerant quantum computer to solve problems fundamentally intractable for classical computations. Experimentally, signatures of simplest non-Abelian excitations have been seen in spectroscopic measurements, yet current experiments fall shot of demonstrating non-Abelian exchange statistics. With DOE support we explore several systems where non-Abelian excitations are expected to appear, with special attention paid to the ability to demonstrate braiding. In nanowire semiconductor/superconductor hybrid system we are developing devices where position of Majorana particles, the simplest non-Abelian excitations, can be varied by magnetic field. The system will allow braiding of Majorana fermions and study non-Abelian statistics. In another system we focus on the development of a new platform where higher order non-Abelian excitations - parafermions - can be realized. Recently we demonstrated formation of heliacal channels in the fractional quantum Hall regime, precursors of topological superconductors that support parafermion excitations. Investigation of these channels and induced superconductivity will be an important part of the proposed research. Finally, investigation of induced superconductivity in topological insulators and discovery of highly unharmonic current-phase relation in Josephson junctions is the basis for the proposed realization of a topologically protected qubits with abelian anions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase Transitions in Quasi-One-Dimensional System with Unconventional Superconductivity

The paper is devoted to a study of superconducting properties of population-imbalanced fermionic mixtures in quasi-one-dimensional optical lattices. The system can be effectively described by the attractive Hubbard model with the Zeeman magnetic field term. We investigated the ground-state phase diagram of the model as a function of the chemical potential and the magnetic field. The ground state of the system exhibits the conventional BCS-type superconductivity as well as the unconventional so-called Fulde-Ferrell-Larkin-Ovchinnikov state, in which the total momentum of Cooper pairs is non-zero. We determine the orders of transitions as well as the behavior of order parameters with a change of the model parameters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Engineering Weyl Superfluid in Ultracold Fermionic Gases by One-Dimensional Optical Superlattices

In this paper, we theoretically demonstrate by using one-dimensional superlattices to couple two-dimensional time-reversal-breaking gapped topological superfluid models, an anomalous Weyl superfluid (WS) can be obtained. This new phase features its unique Fermi arc states (FAS) on the surfaces. In the conventional WS, FAS exist only for a part of the line connecting the projections of Weyl points and extending to the border and/or center of surface Brillouin zone. But for the anomalous WS, FAS exist for the whole line. As a proof of principle, we self-consistently at the mean-field level claim the achievement of the anomalous WS in the model with a dichromatic superlattice. In addition, inversion symmetry and band inversion in this model are analyzed to provide the unique features of identifying the anomalous WS experimentally by the momentum-resolved radio-frequency spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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

Possible coexistence of antiferromagnetic and ferromagnetic spin fluctuations in the spin-triplet superconductor UTe 2 revealed by 125 Te NMR under pressure

We report a spin-triplet superconducting state mediated by ferromagnetic (FM) spin fluctuations has been suggested to occur in the newly discovered heavy-fermion superconductor UTe 2 . However, the recent neutron scattering measurements revealed the presence of antiferromagnetic (AFM) spin fluctuations in UTe 2 . Here, we report the 125 Te nuclear magnetic resonance studies of a single-crystal UTe 2 , suggesting the coexistence of FM and AFM spin fluctuations in UTe 2 . Owing to the two different Te sites in the compound, we conclude that the FM spin fluctuations are dominant within ladders and the AFM spin fluctuations originate from the interladder magnetic coupling. Although AFM spin fluctuations exist in the system, the FM spin fluctuations in the ladders may play an important role in the appearance of the spin-triplet superconducting state of UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Composite anyons on a torus

An adiabatic approach put forward by Greiter and Wilczek interpolates between the integer quantum Hall effects of electrons and composite fermions by varying the statistical flux bound to electrons continuously from zero to an even integer number of flux quanta, such that the intermediate states represent anyons in an external magnetic field with the same “effective” integer filling factor. We consider such anyons on a torus, and construct representative wave functions for their ground as well as excited states. These wave functions involve higher Landau levels in general, but can be explicitly projected into the lowest Landau level for many parameters. We calculate the variational energy gap between the first excited state and ground state and find that it remains open as the statistical phase is varied. Lastly, we obtain from these wave functions, both analytically and numerically, various topological quantities, such as ground-state degeneracy, the Chern number, and the Hall viscosity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin orbit splitting of the photon induced Fano resonance in an oscillating graphene electrostatic barrier

We investigate theoretically the effect of a time dependent oscillating potential on the transport property of the Dirac Fermion through a monolayer graphene electrostatic barrier under the influence of the Rashba spin orbit interaction. The time dependent problem is solved in the frame work of the non-perturbative Floquet approach. It is noted that the dynamic condition of the barrier may be controlled by tuning the Rashba parameter. Introduction of the spin orbit interaction causes splitting of the Fano resonance (FR), a characteristic feature in photon assisted tunneling. The separation between the spin split FR's gives an indirect measure of the fine structure of the quasi-hole bound state inside the barrier. The present findings on the Rashba splitting of the FR and its external control by tuning the oscillating field parameters might have potential for applications in spintronic devices, especially in the spin field effect transistors. The spin polarization of different Floquet sidebands is found to be quite sensitive to the spin-pseudospin interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Fully gapped d -wave superconductivity in CeCu 2 Si 2

The nature of the pairing symmetry of the first heavy fermion superconductor CeCu 2 Si 2 has recently become the subject of controversy. While CeCu 2 Si 2 was generally believed to be a d-wave superconductor, recent low-temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here, we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped d + d band-mixing pairing state for CeCu 2 Si 2 , which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗