T. A. Olheiser

University of Illinois, Urbana-Champaign, Urbana, IL, United States

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Publications (10)14.07 Total impact

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    ABSTRACT: Penetration depth measurements were performed on fully deuterated kappa-(ET)2Cu[N(CN)2]Br, an organic superconductor. A novel aluminum plating technique was used to determine the absolute penetration depth lambda(T). As the cooling rate is varied from 30 mK/min to 180 K/min we observe systematic changes in TC, superconducting fraction and lambda(T). The data is analyzed using a model of superconducting domains embedded in an antiferromagnetic background.
    03/2008;
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    ABSTRACT: We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths (lambda(a) and lambda(c), respectively) in 2H-NbSe2. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast with some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.
    Physical Review Letters 03/2007; 98(5):057003. · 7.73 Impact Factor
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    ABSTRACT: It is well known that the Tc of kappa-(ET)2Cu[N(CN)2]Br is dependent on the rate it is cooled in the temperature range 80-60 K. One interpretation of this effect is that rapid cooling introduces disorder which suppresses Tc because of its unconventional nature. Here we present a specific heat study of this effect in both hydrogenated and deuterated samples. We find that not only does Tc depend on the cooling rate, but that the normal-state Sommerfeld coefficient, gamma, is strongly suppressed (by up to a factor 2) with rapid cooling. The data indicate that rapid cooling induces macroscopic phase separation between an insulating and metallic / superconducting phase at low temperature. The field dependence of gamma for the deuterated sample is highly unusual. As the field is increased it initially increases in a conventional way then suddenly collapses to a small value. We interpret this as evidence for a field induced superconductor-insulator transition.
    03/2007;
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    ABSTRACT: NbSe2 shows coexistence of a charge density wave (TCDW ∼ 32 K) with a superconducting state below T = 7.2 K. Recent ARPES measurements revealed different values of the superconducting gap on the main sheets of the Fermi surface. These results suggest a multigap superconductivity such as in MgB2. The temperature dependence of the magnetic penetration depth (λ(T)) down to Tc/16 has been measured on high quality single crystals in the Meissner state. A strong increase of the in-plane penetration depth is observed, signaling the presence of low lying excitations. Given the relative contributions of each Fermi surface sheet, these measurements indicate that a reduced gap is not necessarily only found on the small Se sheet as suggested by the ARPES measurements. These results are discussed in a framework of multigap superconductivity.
    Physica C Superconductivity 01/2007; · 0.72 Impact Factor
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    ABSTRACT: In- and out-of-plane London penetration depths were measured in single crystals CaAlSi (T_{c}=6.2 K and 7.3 K) using a tunnel-diode resonator. A full 3D BCS analysis of the superfluid density is consistent with a prolate spheroidal gap, with a weak-coupling BCS value in the ab-plane and stronger coupling along the c-axis. The gap anisotropy was found to significantly decrease for higher T_{c} samples.
    Physical review. B, Condensed matter 03/2006; · 3.77 Impact Factor
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    ABSTRACT: We report measurements of the interplane penetration depth lambda(T) in the organic superconductor kappa- (ET)2Cu[N(CN)2]Br (TC = 11.9 K). At low temperatures, the superfluid density rho= [lambda(0)/ lambda(T)]^2 1-AT^N with N = 1.3 -- 1.5, close to the exponent measured for the in-plane superfluid density. This result adds support to a d-wave picture, but with transport between planes more coherent than is observed in similarly anisotropic copper oxide superconductors.
    03/2006;
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    ABSTRACT: We report measurements of the London penetration depth, lambda, for different magnetic field and crystal orientations in fully deuterated kappa (ET)2Cu[N(CN)2]Br, an organic superconductor with TC = 11.9 K. lambda increases dramatically with deuteration and develops a strong magnetic field dependence. The superfluid density exhibits a power law temperature dependence indicative of a nodal order parameter. We discuss possible connections to nanoscale antiferromagnetic domains.
    03/2006;
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    ABSTRACT: In- and out- of plane London penetration depths were measured in single crystals of CaAlSi by using radio-frequency resonant technique. The obtained temperature variation of the anisotropic superfluid density can be explained by an ellipsoidal s-wave gap with a weak-coupling BCS value in the ab-plane and larger gap amplitude in the c-direction. In a mixed state, similar measurements yield Campbell length, which was used to study the peak effect (PE) in the critical current. The position of the PE line on an H-T phase diagram, the influence of anisotropy and sample parameters are discussed.
    01/2006;
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    ABSTRACT: Conductance measurements are reported for an 0.4 mu m wide GaAs/AlGaAs quantum wire with 7 cross-channel gates. The device exhibited integral conductance steps, magnetoconductance plateaus in agreement with the multi probe formula and a conductance feature at 0.65 (2e(2)/h). Differential conductance measurements down to 50 mK revealed a zero bias conductance peak that vanished with an in-plane field of I T. The width of this peak was comparable to that reported in high mobility quantum point contacts (Phys. Rev. Lett. 88 (2002) 2268051). At low conductances this device also exhibited single electron charging characteristic of a multiple quantum dot. (c) 2004 Elsevier B.V. All rights reserved.
    Physica E Low-dimensional Systems and Nanostructures 03/2005; · 1.86 Impact Factor
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    Tyson A. Olheiser
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    ABSTRACT: The temperature dependence of the magnetic penetration depth (λ) has been measured in single crystals of fully deuterated κ-(ET)2Cu[N(CN)2]Br. This material separates into superconducting (SC) and antiferromagnetic (AFI) phases. The relative amounts of SC and AFI phases has been shown to depend on the cooling rate of these materials in the vicinity of 80 K. In varying the cooling rate from 0.0025-180 K/min, the Meissner fraction appears to vary logarithmically with the cooling time, suggesting that these organics act like a glassy system near 80 K where SC nucleation grains are formed. In addition, the penetration depth of deuterated κ-(ET)2Cu[N(CN)2]Br consistently displays a power law temperature dependence with an exponent of n = 1.6-1.8 for Meissner fractions ranging from 0.03-0.60. This suggests that these samples are robust dirty d-wave superconductors like their undeuterated counterpart. With abnormally large penetration depths resulting from traditional analysis, an aluminum plating technique was used to measure the absolute penetration depth of deuterated κ-(ET)2Cu[N(CN)2]Br. The large values of λ(0) were confirmed with this technique. Measurements yielded values that increased from 29-320 μm with increasing cooling rate. This finding motivated the development of a SC spherical grain model where the deuterated sample was assumed to be a collection of independent spheres of undeuterated κ-(ET)2Cu[N(CN)2]Br. The model yielded grain sizes on the order of 5-40 μm, which agrees well with literature. Also, high and low temperature data was checked for self consistency. In addition to this work, the temperature dependence of the interplane penetration depth of undeuterated κ-(ET)2Cu[N(CN)2]Br was performed. With higher order corrections to the traditional analysis accounted for, a value of λ⊥ = 130 μm was measured. Also, the temperature dependence of the superfluid density was similar to in-plane behavior, suggesting that interplane transport in these materials is coherent despite being highly anisotropic and definitely 2D in nature.