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At least 865 records · Page 48
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DOE OSTI2017

Uniaxial pressure effect on the magnetic ordered moment and transition temperatures in BaFe 2 - x T x As 2 ( T = Co , Ni )

In this paper, we use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in BaFe 2 As 2 and its Co- and Ni-substituted members near optimal superconductivity. In the low-temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11% increase under 40 MPa for BaFe 1.9 Co 0.1 As 2 , and a 15% increase for BaFe 1.915 Ni 0.085 As 2 . We also observe an increase of the AF ordering temperature (T N ) of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. Finally, the doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Observation of the magnetic C 4 phase in Ca 1 - x Na x Fe 2 As 2 and its universality in the hole-doped 122 superconductors

Since its discovery in 2014, the magnetic tetragonal C 4 phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the C 4 phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the C 4 instability. Here, we report the discovery of the C 4 phase in a fourth member of the hole-doped 122 materials Ca 1-x Na x Fe 2 As 2 (0.20 ≤ x ≤ 0.50) as determined from neutron and x-ray powder diffraction studies. The maximum of the C 4 dome is observed at x = 0.44 with a reentrant temperature T r = 52 K and an extent of Δx ~ 0.07 in composition. It is observed that for a range of compositions within the C 4 dome (0.40 ≤ x ≤ 0.42), there is a second reentrance (Tr 2 < Tr) where the antiferromagnetic C 2 phase is recovered—a feature previously only seen in Ba 1-x K x Fe 2 As 2 . A phase diagram is presented for Ca 1-x Na x Fe 2 As 2 and compared to the other Na-doped 122's—A 1-x Na x Fe 2 As 2 with A = Ba, Sr, and Ca. Lastly, the structural parameters for these three systems are compared and the importance of the “chemical pressure” due to changing the A-site ion (A = Ba, Sr, Ca) is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Unusual nodal behaviors of the superconducting gap in the iron-based superconductor Ba ( F e 0.65 R u 0.35 ) 2 A s 2 : Effects of spin-orbit coupling

Here we have investigated the superconducting (SC) gap on hole Fermi surfaces (FSs) of optimally substituted Ba (Fe 0.65 Ru 0.35 ) 2 As 2 by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that, whereas the gap is isotropic in the k x - k y plane, the gap magnitudes of two resolved hole FSs show similar k z dependences and decrease as k z approaches ~ 2 π/c (i.e., around the Z point), unlike the other Fe-based superconductors reported so far, where the SC gap of only one hole FS shows a strong k z dependence. This unique gap structure can be understood in the scenario that the d z ₂ orbital character is mixed into both hole FSs due to finite spin-orbit coupling (SOC) and is reproduced by calculation within the random phase approximation including the SOC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Magnetic phase diagram and electronic structure of UPt 2 Si 2 at high magnetic fields: A possible field-induced Lifshitz transition

We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt2Si2 in magnetic fields up to 60T at temperatures down to 50mK, this way firmly establishing the phase diagram for magnetic fields Bka and c axes. Moreover, for Bkc axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5f electrons, we propose that these represent field induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the proposed Lifshitz type transition is of a discontinuous nature, as predicted for interacting electron systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Intertwined order in a frustrated four-leg t - J cylinder

Here, we report a density-matrix renormalization group study of the t–J model with nearest (t 1 and J 1 ) and next-nearest (t 2 and J 2 ) interactions on a four-leg cylinder with concentration δ=1/8 of doped holes. We observe an astonishingly complex interplay between uniform d-wave superconductivity (SC) and strong spin and charge-density wave ordering tendencies (SDW and CDW). Depending on parameters, the CDWs can be commensurate with period 4 or 8. By comparing the charge ordering vectors with 2k F , we rule out Fermi surface nesting-induced density wave order in our model. Magnetic frustration (i.e., J 2 /J 1 ~1/2) significantly quenches SDW correlations with little effect on the CDW. Typically, the SC order is strongly modulated at the CDW ordering vector and exhibits d-wave symmetry around the cylinder. There is no evidence of a near-degenerate tendency to pair-density wave (PDW) ordering, charge 4e SC, or orbital current order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Three-dimensionality of the bulk electronic structure in WTe 2

In this paper, we use temperature- and field-dependent resistivity measurements (Shubnikov–de Haas quantum oscillations) and ultrahigh-resolution, tunable, vacuum ultraviolet laser-based angle-resolved photoemission spectroscopy (ARPES) to study the three-dimensionality (3D) of the bulk electronic structure in WTe 2 , a type II Weyl semimetal. The bulk Fermi surface (FS) consists of two pairs of electron pockets and two pairs of hole pockets along the Χ–Γ–Χ direction as detected by using an incident photon energy of 6.7 eV, which is consistent with the previously reported data. However, if using an incident photon energy of 6.36 eV, another pair of tiny electron pockets is detected on both sides of the Γ point, which is in agreement with the small quantum oscillation frequency peak observed in the magnetoresistance. Therefore, the bulk, 3D FS consists of three pairs of electron pockets and two pairs of hole pockets in total. With the ability of fine tuning the incident photon energy, we demonstrate the strong three-dimensionality of the bulk electronic structure in WTe 2 . Finally, the combination of resistivity and ARPES measurements reveals the complete, and consistent, picture of the bulk electronic structure of this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Ladder physics in the spin fermion model

A link is established between the spin fermion (SF) model of the cuprates and the approach based on the analogy between the physics of doped Mott insulators in two dimensions and the physics of fermionic ladders. This enables one to use nonperturbative results derived for fermionic ladders to move beyond the large-N approximation in the SF model. Here, it is shown that the paramagnon exchange postulated in the SF model has exactly the right form to facilitate the emergence of the fully gapped d-Mott state in the region of the Brillouin zone at the hot spots of the Fermi surface. Hence, the SF model provides an adequate description of the pseudogap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure

The pressure dependence of the structural, magnetic, and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe ( Se 1 - x S x ) . We performed resistance measurements on single crystals with three substitution levels ( x = 0.043 , 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T and were compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe ( Se 1 - x S x ). Furthermore, on increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small domelike region in the phase diagrams. But, T s is much less suppressed by sulfur substitution, and T c of Fe ( Se 1 - x S x ) exhibits similar nonmonotonic pressure dependence with a local maximum and a local minimum present in the low-pressure range for all x . The local maximum in T c coincides with the emergence of the magnetic order above T c . At this pressure the slope of the upper critical field decreases abruptly, which may indicate a Fermi-surface reconstruction. The minimum of T c correlates with a broad maximum of the upper critical field slope normalized by T c .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Low-energy inelastic response in the superconducting phases of PrO s 4 Sb 12

Recent ac susceptibility and polar Kerr effect measurements in the skutterudite superconductor PrO s 4 Sb 12 (POS) uncovered the nature of the superconducting double transition from a high-temperature, high-field, time-reversal symmetric phase (or the A phase) to a low-temperature, low-field, time-reversal symmetry-broken phase (or the B phase). In this work, starting from a microscopic model, we derive a Ginzburg-Landau expansion relevant to POS that describes this entrance into the time-reversal symmetry-broken phase along the temperature axis. We also provide a study of the low-energy inelastic (Raman) response in both the A and B phases of POS, and seek additional signatures which could help reveal the exact form of the gap functions previously proposed in these phases. By appropriately manipulating the incoming and scattered light geometries, along with additional subtraction procedures and suitable assumptions, we show that one can access the various irreducible representations contained in the point group describing POS. We demonstrate how to use this technique on example order parameters proposed in POS. Depending on whether there exist nodes along the c axis, we find additional low-energy spectral weight within the superconducting gap in the $E_g$ geometry, a feature that could pinpoint the location of nodes on the Fermi surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Quantum oscillations in the anomalous spin density wave state of FeAs

Quantum oscillations in the binary antiferromagnetic metal FeAs are presented and compared to theoretical predictions for the electronic band structure in the anomalous spin density wave state of this material. Demonstrating a method for growing single crystals out of Bi flux, we utilize the highest quality FeAs to perform torque magnetometry experiments up to 35 T, using rotations of field angle in two planes to provide evidence for one electron and one hole band in the magnetically ordered state. Finally, the resulting picture agrees with previous experimental evidence for multiple carriers at low temperatures, but the exact Fermi surface shape differs from predictions, suggesting that correlations play a role in deviation from ab initio theory and cause up to a fourfold enhancement in the effective carrier mass.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Large magnetoresistance in the type-II Weyl semimetal WP 2

In this paper, we report a magnetotransport study on type-II Weyl semimetal WP 2 single crystals. Magnetoresistance exhibits a nonsaturating H n field dependence (14 300% at 2 K and 9 T), whereas systematic violation of Kohler's rule was observed. Quantum oscillations reveal a complex multiband electronic structure. The cyclotron effective mass close to the mass of free electron m e was observed in quantum oscillations along the b axis, while a reduced effective mass of about 0.5 m e was observed in α-axis quantum oscillations, suggesting Fermi surface anisotropy. The temperature dependence of the resistivity shows a large upturn that cannot be explained by the multiband magnetoresistance of conventional metals. Finally, even though the crystal structure of WP 2 is not layered as in transition-metal dichalcogenides, quantum oscillations suggest partial two-dimensional character.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Optical properties of highly compressed polystyrene: An ab initio study

Using all-electron density functional theory, we have performed an ab initio study on x ray absorption spectra of highly compressed polystyrene (CH). Here, we found that the K-edge shifts in strongly coupled, degenerate polystyrene cannot be explained by existing continuum-lowering models adopted in traditional plasma physics. To gain insights into the K edge shift in warm, dense CH, we have developed a model designated as “single-mixture-in-a-box” (SMIAB), which incorporates both the lowering of continuum and the rising of Fermi surface resulting from high compression. This simple SMIAB model correctly predicts the K-edge shift of carbon in highly compressed CH in good agreement with results from quantum-molecular-dynamics (QMD) calculations. Traditional opacity models failed to give the proper K-edge shifts as the CH density increased. Based on QMD calculations, we have established a first-principles opacity table (FPOT) for CH in a wide range of densities and temperatures [p = 0.1 to 100 g/cm 3 and T = 2000 to 1,000,000 K]. The FPOT gives much higher Rosseland mean opacity compared to the cold-opacity–patched astrophysics opacity table for warm, dense CH and favorably compares to the newly improved Los Alamos ATOMIC model for moderately compressed CH (pCH ≤10 g/cm 3 ) but remains a factor of 2 to 3 higher at extremely high densities (pCH ≥ 50 g/cm 3 ). We anticipate the established FPOT of CH will find important applications to reliable designs of high-energy-density experiments. Moreover, the understanding of K-edge shifting revealed in this study could provide guides for improving the traditional opacity models to properly handle the strongly coupled and degenerate conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Divergent synthesis routes and superconductivity of ternary hydride MgSiH 6 at high pressure

We predict a new ternary hydride MgSiH6 under high pressures, which is a metal with an ionic feature and takes on a simple cubic structure with space group Pm-3 above 250 GPa. Our first-principles calculations show that the cubic MgSiH 6 is a potential high-temperature superconductor with a superconducting transition temperature T c of ~ 63 K at 250 GPa. Further analysis suggests that phonon softening along mainly Γ-X and Γ-M directions induced by Fermi surface nesting plays a crucial role in the high-temperature superconductivity. Herein we propose the “triangle straight-line method” which provides a clear guide to determine the specific A + B → D type formation routes for ternary hydrides of the Mg-Si-H system and it effectively reveals two divergent paths to obtain MgSiH6 under high pressures: MgH 2 + SiH 4 → MgSiH 6 and MgSi + 3H 2 → MgSiH 6 . This method might be applicable to all ternary compounds, which will be very significant for further experimental synthesis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Superconductivity in three-dimensional spin-orbit coupled semimetals

Here, motivated by the experimental detection of superconductivity in the low-carrier density half-Heusler compound YPtBi, we study the pairing instabilities of three-dimensional strongly spin-orbit coupled semimetals with a quadratic band touching point. In these semimetals the electronic structure at the Fermi energy is described by spin j = 3/2 quasiparticles, which are fundamentally different from those in ordinary metals with spin j = 1/2. For both local and nonlocal pairing channels in j = 3/2 materials we develop a general approach to analyzing pairing instabilities, thereby providing the computational tools needed to investigate the physics of these systems beyond phenomenological considerations. Furthermore, applying our method to a generic density-density interaction, we establish that: (i) The pairing strengths in the different symmetry channels uniquely encode the j = 3/2 nature of the Fermi surface band structure—a manifestation of the fundamental difference with ordinary metals. (ii) The leading odd-parity pairing instabilities are different for electron doping and hole doping. Finally, we argue that polar phonons, i.e., Coulomb interactions mediated by the long-ranged electric polarization of the optical phonon modes, provide a coupling strength large enough to account for a Kelvin-range transition temperature in the s-wave channel, and are likely to play an important role in the overall attraction in non- s -wave channels. Furthermore, the explicit calculation of the coupling strengths allows us to conclude that the two largest non- s-wave contributions occur in nonlocal channels, in contrast with what has been commonly assumed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Quantum kinetic theory of the chiral anomaly

We present a general quantum kinetic theory of low-field magnetotransport in weakly disordered crystals that accounts fully for the interplay between electric-field-induced interband coherence, Bloch-state scattering, and an external magnetic field. The quantum kinetic equation we derive for the Bloch-state density matrix naturally incorporates the momentum-space Berry phase effects whose influence on Bloch-state wave-packet dynamics is normally incorporated into transport theory in an ad hoc manner. The Berry phase correction to the momentum-space density of states in the presence of an external magnetic field implied by semiclassical wave-packet dynamics is captured by our theory as an intrinsic density-matrix response to a magnetic field. We propose a simple and general procedure for expanding the linear response of the Bloch-state density matrix to an electric field in powers of magnetic field. As an illustration, we apply our theory to magnetotransport in Weyl semimetals. Furthermore we show that the chiral anomaly (positive magnetoconductivity quadratic in magnetic field) that appears when separate Fermi surface pockets surround distinct Weyl points survives only when intervalley scattering is very weak compared to intravalley scattering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Emergent orbitals in the cluster Mott insulator on a breathing kagome lattice

Motivated by the recent developments on cluster Mott insulating materials such as the cluster magnet LiZn 2 Mo 3 O 8 , we consider the strong plaquette charge ordered regime of the extended Hubbard model on a breathing kagome lattice and reveal the properties of the cluster Mottness. The plaquette charge order arises from the intersite charge interaction and the collective motion of three localized electrons on the hexagon plaquettes. This model leads naturally to a reduction of the local moments by 2/3, as observed in LiZn 2 Mo 3 O 8 . Furthermore, at low temperatures, each hexagon plaquette contains an extra orbital-like degree of freedom in addition to the remaining spin 1/2. We explore the consequence of this emergent orbital degree of freedom. We point out the interaction between the local moments is naturally described by a Kugel-Khomskii spin-orbital model. We develop a parton approach and suggest a spin-liquid ground state with spinon Fermi surfaces for this model. Here, we further predict an emergent orbital order when the system is under a strong magnetic field. Various experimental consequences for LiZn 2 Mo 3 O 8 are discussed, including an argument that the charge ordering must be short ranged if the charge per Mo is slightly off stoichiometric.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Breakdown of the Migdal-Eliashberg theory: A determinant quantum Monte Carlo study

The superconducting (SC) and charge-density-wave (CDW) susceptibilities of the two-dimensional Holstein model are computed using determinant quantum Monte Carlo, and compared with results computed using the Migdal-Eliashberg (ME) approach. We access temperatures as low as 25 times less than the Fermi energy, E F , which are still above the SC transition. We find that the SC susceptibility at low T agrees quantitatively with the ME theory up to a dimensionless electron-phonon coupling λ 0 ≈ 0.4 but deviates dramatically for larger λ 0 . We find that for large λ 0 and small phonon frequency ω 0 << E F CDW ordering is favored and the preferred CDW ordering vector is uncorrelated with any obvious feature of the Fermi surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anomalous Hall effect in the van der Waals bonded ferromagnet Fe 3 - x GeTe 2

Here, we report the anomalous Hall effect (AHE) in single crystals of a quasi-two-dimensional Fe 3–x GeTe 2 (x ≈ 0.36) ferromagnet grown by the flux method which induces defects on the Fe site and bad metallic resistivity. Fe K-edge x-ray absorption spectroscopy was measured to provide information on the local atomic environment in such crystals. The dc and ac magnetic susceptibility measurements indicate a second-stage transition below 119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A linear scaling behavior between the modified anomalous Hall resistivity ρxy/μ0Heff and longitudinal resistivity ρ 2 xx M/μ 0 H eff implies that the AHE in Fe 3–x GeTe 2 should be dominated by the intrinsic Karplus-Luttinger mechanism rather than the extrinsic skew-scattering and side-jump mechanisms. The observed deviation in the linear- M Hall conductivity σ A xy below 30 K is in line with its transport characteristic at low temperatures, implying the scattering of conduction electrons due to magnetic disorder and the evolution of the Fermi surface induced by a possible spin-reorientation transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗