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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Pressure-temperature phase diagrams of CaK ( Fe 1 - x Ni x ) 4 As 4 superconductors

In this paper, the pressure dependence of the magnetic and superconducting transitions and that of the superconducting upper critical field are reported for $\mathrm{CaK}{({\mathrm{Fe}}_{1{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$, the first example of an Fe-based superconductor with spin-vortex-crystal-type magnetic ordering. Resistance measurements were performed on single crystals with two substitution levels $(x=0.033,0.050)$ under hydrostatic pressures up to 5.12 GPa and in magnetic fields up to 9 T. Our results show that, for both compositions, magnetic transition temperatures ${T}_{\mathrm{N}}$ are suppressed upon applying pressure; the superconducting transition temperatures ${T}_{\mathrm{c}}$ are suppressed by pressure as well, except for $x=0.050$ in the pressure region where ${T}_{\mathrm{N}}$ and ${T}_{\mathrm{c}}$ cross. Furthermore, the pressure associated with the crossing of the ${T}_{\mathrm{N}}$ and ${T}_{\mathrm{c}}$ lines also coincides with a minimum in the normalized slope of the superconducting upper critical field, consistent with a likely Fermi-surface reconstruction associated with the loss of magnetic ordering. In conclusion, at $p{\sim}4$ GPa, both Ni-substituted $\mathrm{CaK}{({\mathrm{Fe}}_{1{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ samples likely go through a half-collapsed-tetragonal phase transition, similar to the parent compound ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$.

4-terminal techniques↗
DOE OSTI2018

Doping evolution of charge and spin excitations in two-leg Hubbard ladders: Comparing DMRG and FLEX results [Doping evolution of charge and spin excitations in two-leg Hubbard ladders: Comparing DMRG and RPA+FLEX results]

Here, we study the magnetic and charge dynamical response of a Hubbard model in a two-leg ladder geometry using the density matrix renormalization group (DMRG) method and the random phase approximation within the fluctuation-exchange approximation (FLEX). Our calculations reveal that FLEX can capture the main features of the magnetic response from weak up to intermediate Hubbard repulsion for doped ladders, when compared with the numerically exact DMRG results. However, while at weak Hubbard repulsion both the spin and charge spectra can be understood in terms of weakly interacting electron-hole excitations across the Fermi surface, at intermediate coupling DMRG shows gapped spin excitations at large momentum transfer that remain gapless within the FLEX approximation. For the charge response, FLEX can only reproduce the main features of the DMRG spectra at weak coupling and high doping levels, while it shows an incoherent character away from this limit. Overall, our analysis shows that FLEX works surprisingly well for spin excitations at weak and intermediate Hubbard U values even in the difficult low-dimensional geometry such as a two-leg ladder. Finally, we discuss the implications of our results for neutron scattering and resonant inelastic x-ray scattering experiments on two-leg ladder cuprate compounds.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Antiferromagnetic order in CaK ( Fe 1 – x Ni x ) 4 As 4 and its interplay with superconductivity

The magnetic order in CaK(Fe 1–x Ni x ) 4 As 4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated with the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe 2 As 2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, noncollinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. Lastly, this supports the notion that both collinear and noncollinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Local quantum criticality of a one-dimensional Kondo insulator model

Here, the continuous quantum phase transition and the nature of the quantum critical point (QCP) in a modified Kondo lattice model with Ising anisotropic exchange interactions are studied within the density-matrix renormalization group algorithm. We investigate the effect of quantum fluctuations on critical Kondo destruction QCP by probing static and dynamic properties of the magnetic order and the Kondo effect. In particular, we identify that local Kondo physics itself becomes critical at the magnetic phase transition point, providing unbiased evidences for local quantum criticality between two insulators without involving the Fermi surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Robustness of the biaxial charge density wave reconstructed electron pocket against short-range spatial antiferromagnetic fluctuations

Recent experiments on underdoped high-temperature superconductors have shown that a biaxial charge density wave coexisting with antiferromagnetism is able to account for the hole-doping dependences of the Hall coefficient and the charge density wave ordering vector. Owing to the loss of long-range order over the range of hole dopings where charge density waves are observed, however, antiferromagnetism poses a serious challenge to the viability of Landau quantization and quantum oscillations. In this work, we make the surprising finding that the states along the antiferromagnetic Brillouin zone boundary, which are usually the most heavily broadened by antiferromagnetic fluctuations, become gapped once the biaxial charge density wave forms. This gapping along with the location of the electron pocket produced by Fermi surface reconstruction at a momentum-space point of high symmetry causes the single nodal electron pocket to be largely protected against antiferromagnetic fluctuations in strong magnetic fields, enabling resolved Landau levels and quantum oscillations to remain robust against antiferromagnetic correlation lengths as short as ~10 Å .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Microscopic origins of hydrodynamic transport in the type-II Weyl semimetal WP 2

A microscopic understanding of hydrodynamic transport in Dirac and Weyl semimetals has remained elusive in theoretical descriptions and experimental measurements. We investigate the structure and microscopic properties of transport in WP 2 , a type-II Weyl semimetal exhibiting hydrodynamic transport. Here, we characterize the nature of the microscopic properties of WP 2 as a function of temperature through ab initio calculations of the relevant scattering processes, including electron-phonon and electron-electron lifetimes, and provide equivalent calculations for copper as a point of reference. Additionally, we present an approach to calculate phonon drag, a mechanism invoked in recent experiments, through predictions of a phonon-mediated electron-electron lifetime. We show that the resistivity is very well described by the electron-phonon interaction alone, indicating WP 2 exhibits conventional metallic electrical behavior in which strong electron-electron correlations do not play a significant role. After establishing the zone-averaged behavior of the calculated lifetimes, we further investigate specific features of the spatial distribution of the electron-phonon lifetime across the Fermi surfaces of WP 2 to study possible scattering channels involved in hydrodynamic transport and quantify the degree of lifetime anisotropy in electron and hole pockets. This description of microscopic dynamics in WP 2 prompts additional investigation of specific scattering channels and indicates the importance of phonon interactions in understanding connections between transport in hydrodynamic materials and strongly correlated systems including unconventional metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Superconductivity at an antiferromagnetic quantum critical point: Role of energy fluctuations

Motivated by recent experiments reporting superconductivity only at very low temperature in a class of heavy fermion compounds, we study the impact of energy fluctuations with small momentum transfer on the pairing instability near an antiferromagnetic quantum critical point. While these fluctuations, formed by composite spin fluctuations, were proposed to explain the thermodynamic and transport properties near the quantum critical point of compounds such as YbRh 2 Si 2 and CeCu 6–x Au x at x ≈ 0.1, here they are found to strongly suppress T c of the d-wave pairing of the hot quasiparticles promoted by the spin fluctuations. Interestingly, if energy fluctuations are strong enough, they can induce triplet pairing involving the quasiparticles of the cold regions of the Fermi surface. Altogether, the opposing effects of energy and spin fluctuations lead to a suppression of T c .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Significant change in the electronic behavior associated with structural distortions in monocrystalline SrAg 4 As 2

Here, we report a combined study of transport and thermodynamic measurements on the layered pnictide material SrAg 4 As 2 . Upon cooling, a drop in electrical and Hall resistivity, a jump in heat capacity, and an increase in susceptibility and magnetoresistance are observed around 110 K. All suggest nonmagnetic phase transitions emerge at around 110 K, likely associated with structural distortions. In sharp contrast with the first-principles calculations based on the crystal structure at room temperature, quantum oscillations reveal small Fermi pockets with light effective masses, suggesting significant change in the Fermi surface topology caused by the low-temperature structural distortion. Furthermore, no superconductivity emerges in SrAg 4 As 2 down to 2 K under pressures up to 2.13 GPa; instead, the low-temperature structural distortion moves up linearly to high temperature at a rate of ≈ 13 K/GPa above 0.89 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Atomic and electronic structure of domains walls in a polar metal

Polar metals counterintuitively bring two well-known phenomena into coexistence, namely, bulk polar displacements, and an electronic Fermi surface giving rise to metallic conduction. Furthermore, little is known about the polar domains or domain walls in such materials. Using atomic resolution electron microscopy imaging combined with first principles density functional theory, we show that uncharged head-to-tail walls, and “charged” head-to-head and tail-to-tail walls can exist in the bulk of such crystals of polar metals Ca 3 Ru 2 O 7 , where both structural changes at the wall as well as electrostatic considerations define the wall nature. Significant built-in potentials of 30–170 meV are predicted at such walls.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Observation of superconductivity in the pressurized Weyl-semimetal candidate TaIrTe 4

We report the observation of superconductivity in the pressurized type-II Weyl semimetal (WSM) candidate TaIrTe 4 by means of complementary high-pressure transport and synchrotron x-ray diffraction measurements. We find that TaIrTe 4 shows superconductivity with transition temperature (TC) of 0.57 K at the pressure of ~23.8GPa. Then, the T C value increases with pressure and reaches ~2.1K at 65.7 GPa. In situ high-pressure Hall coefficient (R H ) measurements at low temperatures demonstrate that the positive R H increases with pressure until the critical pressure of the superconducting transition is reached, but starts to decrease upon further increasing pressure. Above the critical pressure, the positive magnetoresistance effect disappears simultaneously. Our high-pressure x-ray diffraction measurements reveal that at around the critical pressure, the lattice of the TaIrTe 4 sample is distorted and its volume is reduced by ~19.2%, the value of which is predicted to result in the change of the electronic structure significantly. Furthermore, we propose that the pressure-induced distortion in TaIrTe4 is responsible for the change of topology of the Fermi surface, and such a change favors the emergence of superconductivity. Our results reveal the correlation among the lattice distortion, topological physics, and superconductivity in the WSM, a hot topic in condensed-matter physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Counter-thermal flow of holes in high-mobility LaNiO 3 thin films

Measurements of electronic structure and theoretical models indicate that the Fermi surface of LaNiO 3 (LNO) is predominantly holelike, with a small electron pocket. However, measurements of the Hall and Seebeck effects yield nominally opposite signs for the dominant charge carrier type, making charge transport in LNO puzzling. Here, we combine measurements of the Hall, Seebeck, and Nernst coefficients in high-mobility epitaxial LNO thin films, and resolve this puzzle by demonstrating that the negative Seebeck coefficient is generated by the diffusion of holes from cold to hot regions of LNO. Here, we further examine this counter-thermal flow of holes by measuring the evolution of the Nernst coefficient from the diffusive to the ballistic regime, where the suppression of energy-dependent scattering leads to a reversal of the flow of holes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quadratic to linear magnetoresistance tuning in TmB 4

The change of a material’s electrical resistance (R) in response to an external magnetic field (B) provides subtle information for the characterization of its electronic properties and has found applications in sensor and storage related technologies. In good metals, Boltzmann’s theory predicts a quadratic growth in magnetoresistance (MR) at low B, and saturation at high fields. On the other hand, a number of nonmagnetic materials with weak electronic correlation and low carrier concentration for metallicity, such as inhomogeneous conductors, semimetals, narrow gap semiconductors and topological insulators, two dimensional electron gas (2DEG) show positive, non-saturating linear magnetoresistance (LMR). However, observation of LMR in single crystals of a good metal is rare. Here we present low-temperature, angledependent magnetotransport in single crystals of the antiferromagnetic metal, TmB4. We observe large, positive and anisotropic MR(B), which can be tuned from quadratic to linear by changing the direction of the applied field. In view of the fact that isotropic, single crystalline metals with large Fermi surface (FS) are not expected to exhibit LMR, we attribute our observations to the anisotropic FS topology of TmB 4 . Furthermore, the linear MR is found to be temperature-independent, suggestive of quantum mechanical origin.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Intersublattice magnetocrystalline anisotropy using a realistic tight-binding method based on maximally localized Wannier functions

Using a realistic tight-binding Hamiltonian based on maximally localized Wannier functions, we investigate the two-ion magnetocrystalline anisotropy energy (MAE) in L 1 0 transition metal compounds. MAE contributions from throughout the Brillouin zone are obtained using magnetic force theorem calculations with and without perturbation theory. The results from both methods agree with each other, and both reflect features of the Fermi surface. The intrasublattice and intersublattice contributions to MAE are evaluated using a Green's function method. We find that the sign of the intersublattice contribution varies among compounds, and that its amplitude may be significant, suggesting MAE can not be resolved accurately in a single-ion manner. The results are further validated by scaling spin-orbit-coupling strength in density functional theory. Altogether, this realistic tight-binding method provides an effective approach to evaluate and analyze MAE while retaining the accuracy of corresponding first-principles methods.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic structure of the quadrupolar ordered heavy-fermion compound YbRu 2 Ge 2 measured by angle-resolved photoemission

We studied the electronic structure of the heavy-fermion compound Yb(Ru 1–x Rh x ) 2 Ge 2 with x = 0 and nominally x = 0.125 using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a noninteger valence close to 3+. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasiquartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru 4d character is well reproduced by our calculations. YbRu 2 Ge 2 undergoes a nonmagnetic phase transition into a ferroquadrupolar ordered state below 10.2 K and then to an antiferromagnetically ordered state below 6.5 K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu 2 Ge 2 is doped with 12.5% Rh. In conclusion, the electron doping leads to a shift of the band structure and successive Lifshitz transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phonon-mediated superconductivity in low carrier-density systems

Motivated by the observation of superconductivity in SrTiO 3 and Bi, we analyze phonon-mediated superconductivity in three-dimensional systems at low carrier density, when the chemical potential μ is comparable to or even smaller than the characteristic phonon frequency ωL. Here, we consider the attractive part of the Bardeen-Pines pairing interaction, in which the frequency-dependent electron-phonon interaction is dressed by the Coulomb potential. This dressing endows the pairing interaction with momentum dependence. We argue that the conventional Migdal-Eliashberg approximation becomes invalid when μ ≤ ω L , chiefly because the dominant contribution to pairing comes from electronic states away from the Fermi surface. We obtain the pairing onset temperature, which is equal to Tc in the absence of phase fluctuations, as a function of μ/ω L . Furthermore, we find both analytically and numerically that T c increases as the ratio μ/ωL becomes smaller. In particular, in the dilute regime, μ → 0, it holds that Tc ∝ ω L ($^{Ry}_{ωL}$)η, where Ry is the Rydberg constant and η ~ 0.2.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Coulomb problem in iron-based superconductors

We discuss the role of strong Coulomb interactions in iron-based superconductors (FeSCs). The presumed s ± character of these superconductors means that the condensate is not symmetry protected against Coulomb repulsion. Remarkably, the transition temperatures and the excitation gap are quite robust across the large family of iron-based superconductors, despite drastic changes in Fermi-surface geometry. Here, the Coulomb problem is to understand how these superconductors avoid the strong on-site Coulomb interaction of the iron atoms, while maintaining a robust transition temperature. Within the dominant space of t 2 g orbitals, on-site repulsion in the FeSCs forces two linearly independent components of the condensate to vanish. This raises the possibility that iron-based superconductors might adapt their condensate to the Coulomb constraints by rotating the pairing state within the large manifold of entangled extended s-wave gap functions with different orbital and momentum space structure. We examine this “orbital and k-space flexibility” (OKF) mechanism using both Landau theory and microscopic calculations within a multiorbital t - J model. Based on our results, we conclude that OKF necessitates a large condensate degeneracy. One interesting possibility raised by our results is that a resolution to the Coulomb problem in FeSCs might require a reconsideration of triplet pairing.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Competing phases of interacting electrons on triangular lattices in moiré heterostructures

We study the quantum many-body instabilities of interacting electrons with SU(2) × SU(2) symmetry in spin and orbital degrees of freedom on the triangular lattice near van-Hove filling. Our work is motivated by effective models for the flat bands in hexagonal moiré heterostructures like twisted bilayer boron nitride and trilayer graphene-boron nitride systems. We consider an extended Hubbard model including onsite Hubbard and Hund's couplings, as well as nearest-neighbor exchange interactions, and analyze the different ordering tendencies with the help of an unbiased functional renormalization group approach. We find three classes of instabilities controlled by the filling and bare interactions. For a nested Fermi surface at van-Hove filling, Hund-like couplings induce a weak instability towards spin or orbital density wave phases. An SU(4) exchange interaction moves the system towards a Chern insulator, which is robust with respect to perturbations from Hund-like interactions or deviations from perfect nesting. Further, in an extended range of fillings and interactions, we find topological d ± id and (spin-singlet)-(orbital-singlet) f -wave superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Backbending, seniority, and Pauli blocking of pairing correlations at high rotational frequencies in rapidly rotating nuclei

Garrett et al. systematically investigated band-crossing frequencies resulting from the rotational alignment of the first pair of $i_{13/2}$ neutrons (AB) in rare-earth nuclei. In that study, evidence was found for an odd-even neutron number dependence attributed to changes in the strength of neutron pairing correlations. The present paper carries out a similar investigation at higher rotational frequencies for the second pair of aligning $i_{13/2}$ neutrons (BC). Again, a systematic difference in band-crossing frequencies is observed between odd-$N$ and even-$N$ Er, Yb, Hf, and W nuclei, but in the BC case, it is opposite to the AB neutron-number dependence. These results are discussed in terms of a reduction of neutron pairing correlations at high rotational frequencies and of the effects of Pauli blocking on the pairing field by higher-seniority configurations. Also playing a significant role are the changes in deformation with proton and neutron numbers, the changes in location of single-particle orbitals as a function of quadrupole deformation, and the position of the Fermi surface with regard to the various Ω components of the neutron $i_{13/2}$ shell.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗