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M. H. W. Chan

Publications and source records attributed to M. H. W. Chan.

18 recordsLinked to original sources
arXiv2012

$^4$He Crystal Quality and Rotational Response in a Transparent Torsional Oscillator

We have studied natural purity $^{4}$He single crystals and polycrystals between 10 and 600 mK using a torsional oscillator with a 2 cm$^{3}$ rigid cell made of sapphire with a smooth geometry. As the temperature was lowered, we observed sample dependent but reproducible resonant frequency shifts that could be attributed to a supersolid fraction of order 0.1%. However, these shifts were observed with single crystals, not with polycrystals. Our results indicate that, in our case, the rotational anomaly of solid helium is more likely due to a change in stiffness than to supersolidity. This interpretation would presumably require gliding of dislocations in more directions than previously thought.

cond-mat.other↗
arXiv2011

Interplay between topological insulators and superconductors

Topological insulators are insulating in the bulk but possess metallic surface states protected by time-reversal symmetry. Here, we report a detailed electronic transport study in high quality Bi2Se3 topological insulator thin films contacted by superconducting (In, Al and W) electrodes. The resistance of the film shows an abrupt and significant upturn when the electrodes become superconducting. In turn, the Bi2Se3 film strongly weakens the superconductivity of the electrodes, significantly reducing both their transition temperatures and critical fields. A possible interpretation of these results is that the superconducting electrodes are accessing the surface states and the experimental results are the consequence of the interplay between the Cooper pairs of the electrodes and the spin polarized current of the surface states in Bi2Se3.

cond-mat.mtrl-sci↗
arXiv2011

Current induced anisotropic magnetoresistance in topological insulator films

Topological insulators are insulating in the bulk but possess spin-momentum locked metallic surface states protected by time-reversal symmetry. The existence of these surface states has been confirmed by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Detecting these surface states by transport measurement, which might at first appear to be the most direct avenue, was shown to be much more challenging than expected. Here, we report a detailed electronic transport study in high quality Bi2Se3 topological insulator thin films. Measurements under in-plane magnetic field, along and perpendicular to the bias current show opposite magnetoresistance. We argue that this contrasting behavior is related to the locking of the spin and current direction providing evidence for helical spin structure of the topological surface states.

cond-mat.mtrl-sci↗
arXiv2009

Influence of Ortho-H2 Clusters on the Mechanical Properties of Solid Para-H2

The quantum diffusion of ortho-H2 impurities in solid para-H2 has been investigated with a torsional oscillator technique. The unclustering dynamics of agglomerated impurities with increasing temperature are found to greatly affect the resonant period of oscillation. Based on the observed trends in both experimental data and complementary finite element calculations, we find that the oscillation period predominantly reflects changes in the solid's elastic response to the minute inertial stresses imposed on it. It is likely that the small but abrupt increase in the resonant period observed above 100 mK in high purity H2 samples is due to the interplay between remnant ortho-H2 impurities and dislocations, and not a signature of a supersolid to normal solid transition.

cond-mat.other↗
arXiv2009

Supersolid behavior in confined geometry

We have carried out torsional oscillator (TO) and heat capacity (HC) measurements on solid 4He samples grown within a geometry which restricts the helium to thin (150 um) cylindrical discs. In contrast to previously reported values from Rittner and Reppy of 20% non-classical rotational inertia (NCRI) for similar confining dimensions, 0.9% NCRI (consistent with that found in bulk samples and samples imbedded in porous media) was observed in our TO cell. In this confined geometry the heat capacity peak is consistent with that found in bulk solid samples of high crystalline quality.

cond-mat.other↗
arXiv2009

Proximity-induced superconductivity in nanowires: Mini-gap state and differential magnetoresistance oscillations

We study proximity-induced superconductivity in gold nanowires as a function of the length of the nanowire, magnetic field, and excitation current. Short nanowires exhibit a sharp superconducting transition, whereas long nanowires show nonzero resistance. At intermediate lengths, however, we observe two sharp transitions; the normal and superconducting regions are separated by what we call the mini-gap phase. Additionally, we detect periodic oscillations in the differential magnetoresistance. We provide a theoretical model for the mini-gap phase as well as the periodic oscillations in terms of the coexistence of proximity-induced superconductivity with a normal region near the center of the wire, created either by temperature or application of a magnetic field.

cond-mat.supr-con↗
arXiv2009

Heat capacity peak in solid 4He: Effects of disorder and 3He impurities

Heat capacity measurements with significantly improved resolution find the presence of a peak in a solid 4He sample in coexistence with liquid. With improved crystallinity, the peak decreases in height and moves to lower temperature. A hysteretic heat capacity signature consistent with 3He-4He phase separation, not detected in an earlier work is clearly observed.

cond-mat.other↗
arXiv2008

X-ray Studies of Structure and Defects in Solid 4He from 50 mK to Melting

Recent measurements have found non-classical rotational inertia (NCRI) in solid 4He starting at T ~ 200 mK, leading to speculation that a supersolid state may exist in these materials. Differences in the NCRI fraction due to the growth method and annealing history imply that defects play an important role in the effect. Using x-ray synchrotron radiation, we have studied the nature of the crystals and the properties of the defects in solid 4He at temperatures down to 50 mK. Measurements of peak intensities and lattice parameters do not show indications of the supersolid transition. Using growth methods similar to those of groups measuring the NCRI we find that large crystals form. Scanning with a small (down to 10 x 10 um2) beam, we resolve a mosaic structure within these crystals consistent with numerous small angle grain boundaries. The mosaic shows significant shifts over time even at temperatures far from melting. We discuss the relevance of these defects to the NCRI observations.

cond-mat.other↗
arXiv2008

Torsional oscillator and synchrotron x-ray experiments on solid 4He in aerogel

X-ray diffraction experiments show that solid 4He grown in aerogel is highly polycrystalline, with a hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only ~3x10-4 and the onset temperature is ~ 100 mK.

cond-mat.other↗
arXiv2008

Thermal History of Solid 4He Under Oscillation

We have studied the thermal history of the resonant frequency of a torsional oscillator containing solid 4He. We find that the magnitude of the frequency shift that occurs below 100 mK is multivalued in the low temperature limit, with the exact value depending on how the state is prepared. This result can be qualitatively explained in terms of the motion and pinning of quantized vortices within the sample. Several aspects of the data are also consistent with the response of dislocation lines to oscillating stress fields imposed on the solid.

cond-mat.stat-mech↗
arXiv2007

The effect of 3He impurities on the nonclassical response to oscillation of solid 4He

We have investigated the influence of impurities on the possible supersolid transition by systematically enriching isotopically-pure 4He (< 1 ppb of 3He) with 3He. The onset of nonclassical rotational inertia is broadened and shifts monotonically to higher temperature with increasing 3He concentration, suggesting that the phenomenon is correlated to the condensation of 3He atoms onto the dislocation network in solid 4He.

cond-mat.other↗
arXiv2007

Nonclassical Rotational Inertia in Single Crystal Helium

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have carried out new torsional oscillator measurements on large helium crystals grown under constant temperature and pressure. We observe NCRI in all samples, indicating that the phenomenon cannot be explained by a superfluid film flowing along grain boundaries.

cond-mat.other↗
arXiv2006

Search for Superfluidity in Solid Hydrogen

A torsional oscillator study of solid para-hydrogen has been carried out down to 20 mK in search for evidence of superfluidity. This work was inspired by the observation of the supersolid phase in solid He-4. We found evidence of a possible phase transition, marked by an abrupt increase in the resonant period of oscillation and onset of extremely long relaxation times as the temperature was raised above 60 mK. The change in the period for para-hydrogen, in contrast to solid He-4, is not related to superfluidity as it fails a crucial test showing that it is not a consequence of irrotational flow. The long relaxation times observed suggest the effect is related to the motion of residual ortho-hydrogen molecules in the solid.

cond-mat.other↗
arXiv2006

Supersolid Helium at High Pressure

We have measured the pressure dependence of the supersolid fraction by a torsional oscillator technique. Superflow is found from 25.6 bar up to 136.9 bar. The supersolid fraction in the low temperature limit increases from 0.6 % at 25.6 bar near the melting boundary up to a maximum of 1.5% near 55 bar before showing a monotonic decrease with pressure extrapolating to zero near 170 bar.

cond-mat.other↗
arXiv2006

Critical Casimir force in $^4$He films: confirmation of finite-size scaling

We present new capacitance measurements of critical Casimir force-induced thinning of $^4$He films near the superfluid/normal transition, focused on the region below $T_λ$ where the effect is the greatest. $^4$He films of 238, 285, and 340 Åthickness are adsorbed on N-doped silicon substrates with roughness $\approx 8 Å$. The Casimir force scaling function $\vartheta $, deduced from the thinning of these three films, collapses onto a single universal curve, attaining a minimum $\vartheta = -1.30 \pm 0.03$ at $x=td^{1/ν}=-9.7\pm 0.8 Å^{1/ν}$. The collapse confirms the finite-size scaling origin of the dip in the film thickness. Separately, we also confirm the presence down to $2.13 K$ of the Goldstone/surface fluctuation force, which makes the superfluid film $\sim 2 Å$ thinner than the normal film.

cond-mat.soft↗
arXiv2005

Dissipation in Quasi One-Dimensional Superconducting Single-Crystal Sn Nanowires

Electrical transport measurements were made on single-crystal Sn nanowires to understand the intrinsic dissipation mechanisms of a one-dimensional superconductor. While the resistance of wires of diameter larger than 70 nm drops precipitately to zero at Tc near 3.7 K, a residual resistive tail extending down to low temperature is found for wires with diameters of 20 and 40 nm. As a function of temperature, the logarithm of the residual resistance appears as two linear sections, one within a few tenths of a degree below Tc and the other extending down to at least 0.47 K, the minimum temperature of the measurements. The residual resistance is found to be ohmic at all temperatures below Tc of Sn. These findings are suggestive of a thermally activated phase slip process near Tc and quantum fluctuation-induced phase slip process in the low temperature regime. When the excitation current exceeds a critical value, the voltage-current (V-I) curves show a series of discrete steps in approaching the normal state. These steps cannot be fully understood with the classical Skocpol-Beasley-Tinkham phase slip center model (PSC), but can be qualitatively accounted for partly by the PSC model modified by Michotte et al.

cond-mat.supr-con↗
arXiv2005

Suppression of superconductivity in nanowires by bulk superconductors

Transport measurements were made on a system consisting of a zinc nanowire array sandwiched between two bulk superconducting electrodes (Sn or In). It was found that the superconductivity of Zn nanowires of 40 nm diameter is suppressed either completely or partially by the superconducting electrodes. When the electrodes are driven into their normal state by a magnetic field, the nanowires switch back to their superconducting state. This phenomenon is significantly weakened when one of the two superconducting electrodes is replaced by a normal metal. The phenomenon is not seen in wires with diameters equal to and thicker than 70 nm.

cond-mat.supr-con↗