G. M. Schmiedeshoff

Versatile and compact capacitive dilatometer

We describe the design, construction, calibration, and operation of a relatively simple differential capacitive dilatometer suitable for measurements of thermal expansion and magnetostriction from 300 to below 1K with a low-temperature resolution of about 0.05A. The design is characterized by an open architecture permitting measurements on small samples with a variety of shapes. Dilatometers of this design have operated successfully with a commercial physical property measurement system, with several types of cryogenic refrigeration systems, in vacuum, in helium exchange gas, and while immersed in liquid helium (magnetostriction only) to temperatures of 30mK and in magnetic fields to 45T.

Magnetic-field-induced lattice anomaly inside the superconducting state of CeCoIn5: anisotropic evidence of the possible Fulde-Ferrell-Larkin-Ovchinnikov state.

We report high magnetic field linear magnetostriction experiments on CeCoIn5 single crystals. Two features are remarkable: (i) a sharp discontinuity in all the crystallographic axes associated with the upper superconducting critical field B(c2) that becomes less pronounced as the temperature increases and (ii) a distinctive second orderlike feature observed only along the c axis in the high field (10 T < or approximately B< or = B(c2)) low temperature (T < or approximately 0.35 K) region. This second order transition is observed only when the magnetic field lies within 20 degrees of the ab planes and there is no signature of it above B(c2), which raises questions regarding its interpretation as a field induced magnetically ordered phase. Good agreement with previous results suggests that this anomaly is related to the transition to a possible Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.

Magnetic-field-tuned quantum criticality of the heavy-fermion system YbPtBi

In this paper, we present systematic measurements of the temperature and magnetic field dependencies of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic-field- and temperature-dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by an applied magnetic field of Hc~4 kOe. In the H-T phase diagram of YbPtBi, three regimes of its low-temperature states emerge: (I) AFM state, characterized by a spin density wave-like feature, which can be suppressed to T=0 by the relatively small magnetic field of Hc~4 kOe; (II) field-induced anomalous state in which the electrical resistivity follows Δρ(T)∝T1.5 between Hc and ~8 kOe; and (III) Fermi liquid (FL) state in which Δρ(T)∝T2 for H≥8 kOe. Regions I and II are separated at T=0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4f states of Yb, region II may be a manifestation of a spin liquid state.

High resolution miniature dilatometer based on an atomic force microscope piezocantilever.

Thermal expansion, or dilation, is closely related to the specific heat, and provides useful information regarding material properties. The accurate measurement of dilation in confined spaces coupled with other limiting experimental environments such as low temperatures and rapidly changing high magnetic fields requires a new sensitive millimeter size dilatometer that has little or no temperature and field dependence. We have designed an ultracompact dilatometer using an atomic force microscope piezoresistive cantilever as the sensing element and demonstrated its versatility by studying the charge density waves in alpha uranium to high magnetic fields (up to 31 T). The performance of this piezoresistive dilatometer was comparable to that of a titanium capacitive dilatometer.

Pinning Frequencies of the Collective Modes in alpha-Uranium

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline alpha-uranium. In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature ThetaD = 265 K. In the polycrystalline sample the heat capacity curve is thermally broadened (ThetaD = 184 K), but no excess heat capacity was observed. The excess heat capacity Cphi (taken as the difference between the single-crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency V0 for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal.

Magnetotransport and superconductivity of α-uranium

We have measured the electrical resistivity, magnetoresistance and Hall effect on several new single-crystal samples and one polycrystalline sample of α-U. The residual resistivity ratios of these samples vary from 13 to 315. Matthiessens law appears to hold above the onset of the charge-density wave phase transitions that begin near 43 K, but not below this temperature. Sharp features at all three charge-density wave transitions are observed and the effects of high magnetic fields on them are presented and discussed. The magnetoresistance is anisotropic, reaches 1000% at 2 K and 18 T and does not exhibit Kohler scaling. The Hall coefficient is positive, independent of magnetic field and slightly temperature dependent above about 40 K in agreement with earlier studies. Below 40 K the Hall coefficient changes sign as the temperature falls, varies with field and becomes much more strongly negative at the lowest temperatures than has been reported. Some of our results suggest that a spin-density wave may coexist with the charge-density wave states. Superconductivity is observed in two of our samples; we argue that it is intrinsic to α-U and suggest that it is consistent with a two-band model. Several parameters characterizing the transport and superconductivity of α-U are estimated.

Anisotropic thermal expansion and magnetostriction of YNi2B2C single crystals

We present results of anisotropic thermal expansion and low temperature magnetostriction measurements on YNi2B2C single crystals grown by high temperature flux and floating zone techniques. Quantum oscillations of magnetostriction were observed at low temperatures for starting at fields significantly below Hc2 (H<0.7Hc2). Large irreversible, longitudinal magnetostriction was seen in both in-plane and along the c axis directions of the applied magnetic field in the mixed superconducting state. Anisotropic uniaxial pressure dependences of Tc were evaluated using results of zero field, thermal expansion measurements.

Quantum criticality and the suppression of the heavy fermion state in UBe13 by high magnetic fields

We have measured the electrical resistivity of a single crystal of UBe13 below 1 K and in high magnetic fields to 33 T. The resistivity exhibits the T 2 dependence characteristic of a Fermi liquid at low temperatures. The field dependence of the coefficient of the T 2 term in the resistivity suggests that the heavy fermion state is suppressed by high magnetic fields. We discuss these results in light of a proposed field-induced quantum critical point immersed in the novel superconducting state of this compound.

Thermal expansion, heat capacity and magnetostriction of RAl3 (R = Tm, Yb, Lu) single crystals

We present thermal expansion and longitudinal magnetostriction data for cubic RAl3 (R = Tm, Yb, Lu) single crystals. The thermal expansion coefficient for YbAl3 is consistent with an intermediate valence of the Yb ion, whereas the data for TmAl3 show crystal electric field contributions and have strong magnetic field dependences. de Haas–van Alphen like oscillations were observed in the magnetostriction data for YbAl3 and LuAl3, several new extreme orbits were measured and their effective masses were estimated. Specific heat data taken at 0 and 140 kOe for both LuAl3 and TmAl3 for T≤200 K allow for the determination of a crystal electric field splitting scheme for TmAl3.

Thermal expansion and magnetostriction of a nearly saturated 3He-4He mixture

We have measured the molar volume of a nearly saturated 3He-4He mixture, at temperatures below 100 mK and in magnetic fields to 16 T, by immersing capacitive dilatometers in the dilute phase of a top-loading dilution refrigerator. In zero magnetic field the coefficient of volume thermal expansion is negative and shows the linear temperature dependence expected of a Fermi liquid. Subject to experimental assumptions, we find the fractional increase in molar volume with magnetic field at 20 mK and 16 T to be about 0.01%.