Jeffrey D. Goettee

High Field Gap Closure in the Kondo Insulator SmB6

We have measured the longitudinal magnetoresistance of SmB6 single crystals in magnetic fields up to 145 T. The insulating gap closes at ≈80–90 T and above 100 T the magnetoresistance is positive and quadratic.

Measurement instrumentation for electrical transport experiments in extreme pulsed magnetic fields generated by flux compression

We describe purpose-built instrumentation that has proved successful in making transport measurements of materials in ultrahigh magnetic fields generated by explosive-driven flux compression. The experimental arrangement minimizes severe problems of heating and pick-up associated with the microsecond pulsed field, for which dB/dt can be as large as 109 T/s. Electrical connection to multiple samples in the high field region is via long, lithographically-defined, nested coplanar transmission lines. Contactless measurements by capacitive coupling, as well as resistively coupled measurements, are made at frequencies of order 1 GHz so that pick-up from the microsecond pulse can be effectively filtered. We demonstrate our technique with data on semiconductor samples taken to 450 T using Russian MC-1 type flux compression generators combined with U.S. explosives at the Ancho Canyon firing point, Los Alamos National Laboratory. We discuss the obstacles encountered during these experiments, and outline improvement...

Complex microwave conductivity of YBa2Cu3O7 in magnetic fields up to 500T

Abstract We have measured the complex conductivity of thin films of YBa 2 Cu 3 O 7 (YBCO) superconductor down to temperatures of 4K and magnetic fields up to 500T. The highly oriented films were probed by 94 GHz radiation, with the external magnetic field app perpendicular to the c-axis. As discussed below, these measurements allowed us to assign a value of 340 ± 40 T for the upper critical field at T=0. The measurements were recently carried out at the pulsed field facility of the National High Magnetic Field Laboratory at Los Alamos using both Russian and American magnetic flux compression generators.

Experimental determination of theB−Tphase diagram ofYBa2Cu3O7−δto 150 T forB⊥c

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Studies of the correlated electron system SmB6

We have prepared high-quality, single crystals of SmB 6 under various conditions to improve sample quality. We have measured the resistivity and magnetic susceptibility from room to liquid-helium temperatures to sort samples. We have applied pulsed magnetic fields as high as 50 T at temperatures as low as 40 mK while measuring resistivity. Our samples are of higher quality than previously known. All solvent-grown, single-crystal samples should be etched to remove a surface conductivity.

Complex microwave conductivity of YBa2Cu3O7 in magnetic fields up to 500 T

Abstract The complex microwave conductivity of thin, oriented YBa 2 Cu 3 O 7 films was measured at 94 GHz in pulsed, ultrahigh-magnetic fields up to 500 T. The c -axis of the film was perpendicular to the magnetic field. We estimate the upper critical field of the film at absolute zero as B c2 (0) = 340 ± 40 T. Dynamics of the transition into a normal state and connection with previous measurements of the reversibility line are discussed.

Nonlinear Faraday effect in CdS semiconductor in an ultrahigh magnetic field

A significant nonlinearity in the angle of rotation polarization plane was observed in CdS at wavelengths of 494 in the presence of high magnetic fields (0.5-5 MG). The onset significant nonlinearity also depended on sample temperature. An absorption study with probe wavelength of {approximately} 494 nm revealed an increase in optical transmission associated with a splitting of the conduction band. Dispersion, field and temperature curves indicate a low conduction electron mass m{sub e} = 0.3 m{sub o}. A numerical calculation and interpretation of the observed effects was carried out using band theory. The optical and magnetooptical properties of semiconducting crystals of CdS were studied, reviews of which are presented in [1,2]. This article describes joint American-Russian experiments to study the optical and magnetooptical properties of CdS in ultrahigh fields to {approximately} 7 MG.

MC-1 generator performance with higher-energy explosives

The Russian designed MC-1 ultrahigh magnetic field generator was tested in 5 experiments as part of a joint US-Russian collaboration at Los Alamos National Laboratory in December of 1993. The standard Russian explosive (50/50 RDX/TNT) was replaced with higher-energy-density US explosive, either Comp-B (60/40 RDX/TNT) or PBX-9501. Generator performance with Comp-B was nominally the same as reported for experiments with the slightly lower-energy Russian explosive. The Comp-B experiment produced a measured peak field of 9.4 megagauss. Using PBX-9501, the measured peak field increased to 10.9 megagauss with an appropriate increase in the time derivative of the field. One-dimensional MHD calculations with the Lagrangian code, RAVEN are compared with the experimental results.

Zeeman-effect studies of the electronic absorption spectrum of octachlorodirhenate(2−) (ReRe) in pulsed 50-Tesla magnetic fields

Electronic absorption spectra have been measured for crystals of (Bu4N)2[Re2Cl8] at 3.8 K is pulsed magnetic fields (pulse duration ca. 10 ms; field strength to 50 T). Spectra were measured in the 530–600 nm (‘Band I’) and 435–500 nm (‘Band II’) regions, where well resolved vibrational fine structure had been noted in previous spectroscopic studies. Application of magnetic fields up to 50 T caused no measurable splittings or shifts in these two absorption bands. This indicates that both bands are attributable to spin-allowed electronic transitions, since spin-forbidden transitions would be expected to show Zeeman splitting. Possible assignments for Bands I and II include transitions to two 1Eg excited states [1(π,δ∗) and1(δ,π∗)], as previously suggested by Bursten et al. (B.E. Bursten, F.A. Cotton, P.E. Fanwick and G.G. Stanley, J. Am. Chem. Soc., 105 (1983) 3082), and to two formally doubly excited states, 1A1g [1(δδ,δ∗δ∗)] and 1Eu [1(πδ,δ∗δ∗)].

The energy gap of SmB6 at low temperatures and in high magnetic fields

We have studied the energy gap of the Kondo Insulator SmB6 by means of transverse magnetoresistance measurements at temperatures down to 40mK and in magnetic fields to 50T. At 40 mK an inflection, point in the transverse magnetoresistance is observed near 35T which may indicate the closing of the gap and the onset of closed orbits.