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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 847 records · Page 47
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DOE OSTI2014

Evolution of London penetration depth with scattering in single crystals of K1-xNaxFe2As2

London penetration depth, λ(T), was measured in single crystals of K1-xNaxFe2As2, x=0 and 0.07, down to temperatures of 50 mK, ~Tc/50. Isovalent substitution of Na for K significantly increases impurity scattering, with ρ(Tc) rising from 0.2 to 2.2 μΩ cm, and leads to a suppression of Tc from 3.5 to 2.8 K. At the same time, a close to T-linear Δλ(T) in pure samples changes to almost T2 in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to T2 dependence in superconductors with symmetry imposed line nodes. The superfluid density in the full temperature range follows a simple clean and dirty d-wave dependence, for pure and substituted samples, respectively. This result contradicts suggestions of multiband scenarios with strongly different gap structure on four sheets of the Fermi surface.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Crossover from spin waves to diffusive spin excitations in underdoped Ba(Fe1-xCox)2 As2

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Experimental study of the valence band of Bi 2 Se 3

The valence band of Bi 2 Se 3 is investigated with Shubnikov - de Haas measurements, galvanomagnetic and thermoelectric transport. At low hole concentration, the hole Fermi surface is closed and box-like, but at higher concentrations it develops tube-like extensions that are open. The experimentally determined density-of-states effective mass is lighter than density-functional theory calculations predict; while we cannot give a definitive explanation for this, we suspect that the theory may lack sufficient precision to compute room-temperature transport properties, such as the Seebeck coefficient, in solids in which there are Van der Waals interlayer bonds.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Complex magnetism and strong electronic correlations in Ce 2 PdGe 3

Single-crystal x-ray diffraction, magnetic susceptibility, magnetization, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound Ce 2 PdGe 3 , which forms in the space group P4 2 /mmc (No. 131)—a relative of the α₋ThSi 2 -type structure. Measurements reveal a two-part antiferromagnetic phase transition at T N,1 =10.7 K and T N,2 =9.6 K and subsequent ferromagnetlike ordering near TC≈2.25 K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat γ≈50 mJ/mol-Ce-K 2 (extrapolated to T=0 for T < T C ). Lastly, electronic structure calculations reveal a Fermi surface that includes sheets with distinct nesting vectors. We find chemical/structural disorder also plays an important role, as evidenced by results from single-crystal x-ray diffraction, the width of the peaks in the heat capacity at T N and T C , and the small residual resistivity ratio RRR=ρ300K/ρ0=1.8.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Publisher's Note: High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase of KFe 2 As 2 under high pressure [Phys. Rev. B 91 , 060508(R) (2015)]

Here, we report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe 2 As 2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low pressure regime, we show that the superconducting transition temperature T c finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum T c and reversal of pressure dependence at this pressure. Upon applying higher pressures, this T c is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe 2 As 2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-T c superconductivity in iron pnictide superconductors.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Influence of Ti doping on the incommensurate charge density wave in 1 T –TaS 2

Here, we report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in 1T–Ta 1–x Ti x S 2 . Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from 11.9° at x=0 to 16.4° at x=0.12. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by Di Salvo [F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at x ~ 0.08.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Antiferromagnetic Kondo lattice in the layered compound CePd 1 - x Bi 2 and comparison to the superconductor LaPd 1 - x Bi 2

ZrCuSi 2 -type CePd 1-x Bi 2 crystals were obtained from excess Bi flux. Magnetic susceptibility measurements reveal that CePd 1-x Bi 2 is a highly anisotropic antiferromagnet with transition temperature at 6 K, and a magnetic-field-induced metamagnetic transition at 5 T. An enhanced Sommerfeld coefficient of γ of 0.199 J-mol-Ce -1 K -2 obtained from specific heat measurements suggests a moderate Kondo effect in CePd 1-x Bi 2 . In addition to the antiferromagnetic peak the resistivity curve shows a shoulder-like behavior which could be attributed to the presence of Kondo effect and crystal-electric-field effects in this compound. Magnetoresistance and Hall effect measurements suggest an interplay between Kondo and crystal-electric-field effects which reconstructs the Fermi surface topology of CePd 1-x Bi 2 around 75 K. Electronic structure calculations reveal the Pd vacancies are important to the magnetic structure and enhance the crystal-electric-field effects which quench the orbital moment of Ce at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Isotropic in-plane quenched disorder and dilution induce a robust nematic state in electron-doped pnictides

The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic in-plane quenched disorder strength and dilution by means of computational techniques applied to a three-orbital (xz,yz,xy) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. Moreover, in this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the N eel T N and the structural T S transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled in our model by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi surface produced by electron doping affect only slightly both critical temperatures. Our results also suggest that the specific material-dependent phase diagrams experimentally observed could be explained as a consequence of the variation in disorder profiles introduced by the different dopants. Finally, our findings are also compatible with neutron scattering and scanning tunneling microscopy, unveiling a patchy network of locally magnetically ordered clusters with anisotropic shapes, even though the quenched disorder is locally isotropic. Our study reveals a remarkable and unexpected degree of complexity in pnictides: the fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by quenched disorder and dilution to stabilize the robust nematic phase experimentally found in electron-doped 122 compounds.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2015

Three energy scales in the superconducting state of hole-doped cuprates detected by electronic Raman scattering

We explore by electronic Raman scattering the superconducting state of the Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) crystal by performing a fine-tuned doping study. We find three distinct energy scales in A 1g , B 1g , and B 2g symmetries which show three distinct doping dependencies. Above p=0.22, the three energies merge; below p=0.12, the A 1g scale is no longer detectable, while the B 1g and B 2g scales become constant in energy. In between, the A 1g and B 1g scales increase monotonically with underdoping, while the B 2g one exhibits a maximum at p=0.16. The three superconducting energy scales appear to be a universal feature of hole-doped cuprates. Furthermore, we propose that the nontrivial doping dependencies of the three scales originate from the Fermi-surface changes and reveal competing orders inside the superconducting dome.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin excitations in a model of FeSe with orbital ordering

We present a theoretical study of the dynamical spin susceptibility for the intriguing Fe-based superconductor FeSe, based on a tight-binding model developed to account for the temperature-dependent band structure in this system. The model allows for orbital ordering in the d xz /d yz channel below the structural transition and presents a strongly C 4 -symmetry-broken Fermi surface at low temperatures which accounts for the nematic properties of this material. The calculated spin excitations are peaked at wave vector (π,0) in the 1-Fe Brillouin zone, with a broad maximum at energies of order a few meV. In this range, the occurrence of superconductivity sharpens this peak in energy, creating a (π,0) “neutron resonance” as seen in recent experiments. With the exception of the quite low energy scale of these fluctuations, these results are roughly similar to standard behavior in Fe pnictide systems. At higher energies, however, intensity increases and shifts to wave vectors along the (π,0)–(π,π) line. Here, we compare with existing inelastic neutron experiments and NMR data, and give predictions for further studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, H c2,c , of single crystalline FeSe was studied using measurements of the interplane resistivity, ρ c , in magnetic fields parallel to tetragonal c axis. H c2,c (T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, T c , of FeSe exhibits a nonmonotonic dependence with local maximum at p 1 ≈ 0.8 GPa and local minimum at p 2 ≈ 1.2 GPa. The slope of the upper critical field at T c ,(dH c2,c /dT)T c , also exhibits a nonmonotonic pressure dependence with distinct changes at p 1 and p 2 . For p < p 1 the slope can be described within a multiband orbital model. For both p 1 < p < p 2 and p > p 2 the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov–de Haas measurements. Lastly, this finding indicates that Fermi surface changes are responsible for the local minimum of T c (p) at p 2 ≈ 1.2 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Observation of Dirac-like band dispersion in LaAgSb 2

In this paper, we present a combined angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations study of the electronic structure of LaAgSb 2 in the entire first Brillouin zone. We observe a Dirac-cone-like structure in the vicinity of the Fermi level formed by the crossing of two linear energy bands, as well as the nested segments of a Fermi surface pocket emerging from the cone. In conclusion, our ARPES results show the close relationship of the Dirac cone to the charge-density-wave ordering, providing consistent explanations for exotic behaviors in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Non-Fermi-liquid behavior from partial nesting in multiorbital superconductors

We present that partial nesting between two connected or disconnected regions of the Fermi surface leads to fractional powers of the Coulomb scattering lifetime as a function of temperature. This result is first demonstrated for a toy band structure where partial nesting occurs within a single band and between different regions of the Brillouin zone. A comparison is then made to a multiband scenario by studying the scattering rate of an effective two-orbital model that was proposed in the context of multiorbital superconductors. In the process, various model independent features affecting the temperature exponent n are identified. The logarithmically divergent contributions of the lowest order vertex correction to the multiorbital susceptibility, and the role played by nesting in suppressing these divergences, is analyzed. Lastly, the relevance of these results is discussed keeping the recently observed anomalous resistivity in the Co doped iron superconductor LiFeAs as a backdrop.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electron doping evolution of the magnetic excitations in NaFe 1-x Co x As

We use time-of-flight (TOF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe 1-x Co x As with x = 0, 0.0175, 0.0215, 0.05, and 0.11. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy (E 80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy (E > 80 meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility "(!) of NaFe 1-x Co x As reveals a total fluctuating moment of 3.6 μ2 B/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Cooverdoped nonsuperconducting NaFe0.89Co0.11As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe 2-x Ni x As 2 , confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Evolution of critical pressure with increasing Fe substitution in the heavy-fermion system URu 2 - x Fe x Si 2

Measurements of electrical resistivity, ρ(T ), were performed under quasihydrostatic pressure up to P ~ 2.2 GPa to determine the pressure dependence of the so-called hidden order (HO) and large-moment antiferromagnetic (LMAFM) phases for the URu 2-x FexSi2 system with x = 0.025, 0.05, 0.10, 0.15, and 0.20. As the Fe concentration (x) is increased, we observed that a smaller amount of external pressure, P c , is required to induce the HO → LMAFM phase transition. A critical pressure of P c ~ 1.2 GPa at x = 0.025 reduces to P c ~ 0 at x = 0.15, suggesting the URu 2-x Fe x Si 2 system is fully expressed in the LMAFM phase for x ≥ x* c = 0.15, where x * c denotes the ambient pressure critical concentration of Fe. Furthermore, when using a bulk modulus calculation to convert x to chemical pressure, P ch (x), we consistently found that the induced HO → LMAFM phase transition occurred at various combinations of x c and P c such that P ch (x c ) + P c ≈ 1.5 GPa, where xc denotes those critical concentrations of Fe that induce the HO→LMAFM phase transition for the URu 2-x Fe x Si 2 compounds under pressure. We performed exponential fits of ρ(T ) in the HO and LMAFM phases in order to determine the pressure dependence of the energy gap, , that opens over part of the Fermi surface in the transition from the paramagnetic (PM) phase to the HO/LMAFM phase at the transition temperature, T 0 . Finally, this change in the pressure variation of Δ(P) at the HO→LMAFM phase transition is consistent with the values of P c determined from the T 0 (P) phase lines at the PM→HO/LMAFM transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Interlayer electronic transport in CaMnBi 2 antiferromagnet

Here, we report interlayer electronic transport in CaMnBi 2 single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Furthermore, the small cyclotron mass, high mobility of carriers, and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similarly to SrMnBi 2 , which features an anisotropic Dirac cone, our results suggest that magnetic-field-induced changes in interlayer conduction are also present in layered bismuth-based materials with a zero-energy line in momentum space created by the staggered alkaline earth atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Optical properties of A Fe 2 As 2 ( A = Ca , Sr, and Ba) single crystals

The detailed optical properties have been determined for the iron-based materials A Fe 2 As 2 , where A = Ca , Sr, and Ba, for light polarized in the iron-arsenic (a-b) planes over a wide frequency range, above and below the magnetic and structural transitions at T N = 138 , 195, and 172 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above T N , the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below T N there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for T $\ll$ T N may be related to the Dirac conelike dispersion of the electronic bands. Below T N new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at Δ 1 ≃ 45 – 80 meV, and Δ 2 ≃ 110 – 210 meV. In conclusion, the reduction in the spectral weight associated with the free carriers is captured by the gap structure; specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.

36 MATERIALS SCIENCE↗