Variable range hopping in Si:P at very low temperature

The resistivity of doped semiconductor Si: P in the intermidiate concentration range was measured in low temperature region down to 10 mK. In the lowest temperatures Mott's formula for the variable range hopping is shown to be not applicable and T^-n-like temperature dependence of resistivity is confirmed. The Importance of the level correlation in the random system is discussed.     

Pressure dependence of resistivity and magnetoresistivity in InSb below 1 K

It is shown that the hydrostatic pressure leads to the strong increase of InSb (n=2·10¹⁴ cm^-3) crystal resistivity below 100 mK. Increase of the electron effective mass due to applied pressure does not influence the behavior of magnetoresistivity for temperatures higher than 100 mK whereas it causes a strong increase in positive magnetoresistivity at T < 100 mK. The observed decrease in the hopping conductivity reflects the pressure and magnetic field induced reduction of the electron localization length.     

Disappearance of metal-like behavior in GaAs two-dimensional holes below 30 mK

The zero-field temperature dependence of the resistivity of two-dimensional holes is observed to exhibit two qualitatively different characteristics for a fixed carrier density for which only the metallic behavior of the so-called metal-insulator transition is anticipated. As T is lowered from 150 to 0.5 mK, the sign of the derivative of the resistivity with respect to T changes from being positive to negative when the temperature is lowered below approximately 30 mK and the resistivity continuously rises with cooling down to 0.5 mK, suggesting a crossover from being metal-like to insulatorlike.     

Thermal conductivity in the vicinity of the quantum critical end point in Sr3Ru2O7

Thermal conductivity of Sr3Ru2O7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at Hc=7.85 T. A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T-->0 K, we find that the Wiedemann-Franz law is satisfied to within 5% at all fields, implying that there is no breakdown of the electron despite the destruction of the Fermi liquid state at quantum criticality. A significant change in disorder [from rho0(H=0 T)=2.1 to 0.5 microOmega cm] does not influence our conclusions. At finite temperatures, the temperature dependence of the Lorenz number is consistent with ferromagnetic fluctuations causing the non-Fermi liquid behavior as one would expect at a metamagnetic quantum critical end point.     

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p > or approximately4x10;{9} cm;{-2}), the nonmonotonic temperature dependence ( approximately 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of T=50 mK, the conductivity versus density data indicate an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8x10;{9} cm;{-2}.     

Direct measurements of electron thermalization in Coulomb blockade nanothermometers at millikelvin temperatures

We investigate electron thermalization of tunnel junction arrays installed in a powerful dilution refrigerator whose mixing chamber can produce lattice temperatures down to 3 mK. The on-chip Coulomb blockade thermometers (CBT) against other thermometers at the mixing chamber provide direct information on the thermal equilibrium between the electronic system and the refrigerator. We can detect and discriminate between the heat load delivered through the wiring and that produced by the bias current of the CBT-measurement. The basic heat leak limits the minimum of the electronic temperature to slightly below 20 mK.     

Nuclear spin ferromagnetic phase transition in an interacting two dimensional electron gas

Electrons in a two-dimensional semiconducting heterostructure interact with nuclear spins via the hyperfine interaction. Using a a Kondo lattice formulation of the electron-nuclear-spin interaction, we show that the nuclear-spin system within an interacting two-dimensional electron gas undergoes a ferromagnetic phase transition at finite temperatures. We find that electron-electron interactions and non-Fermi liquid behavior substantially enhance the nuclear-spin Curie temperature into the mK range with decreasing electron density.     

Crossover from dilute to majority spin freezing in two leg ladder system Sr(Cu,Zn)2O3

The two-dimensional electron gas (2DEG) in moderate magnetic fields in ultraclean AlAs-GaAs heterojunctions exhibits transport anomalies suggestive of a compressible anisotropic metallic state. Using scaling arguments and Monte Carlo simulations, we develop an order parameter theory of an electron nematic phase. The observed temperature dependence of the resistivity anisotropy behaves like the orientational order parameter if the transition to the nematic state occurs at a finite temperature T(c) approximately 65 mK, and is slightly rounded by a small background microscopic anisotropy. We propose a light scattering experiment to measure the critical susceptibility.     

Cold-electron bolometer arrays

To improve electron cooling, expand the dynamic range, and match the input and output impedances, series/parallel arrays of cold-electron bolometers have been developed, manufactured, and measured at temperatures from 50 to 350 mK. The bolometers are integrated into cross-slot antennas to analyze the polarization of cosmic microwave background radiation at a frequency of 345 GHz. The maximum temperature response is 6.5 μV/mK. The noise-equivalent electric power at a temperature of 300 mK is 1.2 × 10^?17 W/Hz^1/2.     

Unexpected behaviour of YbCu4.5 at very high pressures

We report resistivity measurements of the heavy fermion compound YbCu_4.5 for pressures up to 23.5 GPa. Although the temperature dependence of the resistivity in general does not change compared to previous results at lower pressures, surprisingly the temperature of the Kondo resistivity maximum increases with pressure at pressures exceeding 12.5 GPa. At the highest pressure of 23.5 GPa the resistivity was measured down to 50 mK. No trace of magnetic order, i.e. any anomaly in the resistivity behaviour has been found at this highest pressure reached.     

Low-temperature collapse of electron localization in two dimensions

We report direct experimental evidence that the insulating phase of a disordered, yet strongly interacting two-dimensional electron system becomes unstable at low temperatures. As the temperature decreases, a transition from insulating to metal-like transport behavior is observed, which persists even when the resistivity of the system greatly exceeds the quantum of resistivity h/e2. The results have been achieved by measuring transport on a mesoscopic length scale while systematically varying the strength of disorder.     

Thermal conductivity of thermoelectric Al‐substituted ZnO thin films

ZnO:Al thin films with a low electrical resistivity were grown by magnetron sputtering on sapphire substrates. The cross‐plane thermal conductivity (κ = 4.5 ± 1.3 W/mK) at room temperature is almost one order of magnitude lower than for bulk materials. The thermoelectric figure of merit ZT at elevated temperatures was estimated from in‐plane power factor and the cross‐plane thermal conductivity at room temperature. It is expected that the thermal conductivity drops with increasing temperature and is lower in‐plane than cross‐plane. Consequently, the thin film ZT is at least three times higher than for bulk samples at intermediate temperatures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantum critical behaviour in the CePd2Si2/CeNi2Ge2 system

We have explored the vicinity of the antiferromagnetic quantum critical point in the related heavy fermion metals CePd₂Si₂ and CeNi₂Ge₂ as a function of hydrostatic pressure. The normal state resistivity of the antiferromagnet CePd2Si2 near the critical pressure, at which magnetic order disappears, varies as ρ ～ T^χ(1:1 < χ < 1:4) over nearly two orders of magnitude in temperature up to about 30 K. This anomalous form for the resistivity appears to defy not only Fermi-liquid theory, but also simple phenomenological models for the effect of spin fluctuations close to a quantum critical point. An analogous unconventional behaviour is observed in the ambient pressure resistivity of the electronically and structurally equivalent, non-magnetic metal CeNi₂Ge₂. At pressures above 15 kbar, a new and unexpected superconducting transition appears in CeNi₂Ge₂ below 220 mK, which rises to higher temperatures with increasing pressure, reaching 400 mK at 26 kbar.     

High-pressure study of non-Fermi liquid and spin-glass-like behavior in CeRhSn

We present measurements of the temperature dependence of the electrical resistivity ρ(T) of CeRhSn up to ~27 kbar. At low temperatures, ρ(T) varies linearly with T for all pressures, indicating non-Fermi liquid behavior. Below Tf ~ 6 K, ρ(T) deviates from a linear dependence. We found that the low-T feature centered at T = Tf shows a pressure dependence ?Tf/?P ≈ 30 mK kbar?1 which is typical of canonical spin glasses. This interplay between spin-glass-like and non-Fermi liquid behavior was observed in both CeRhSn and a Ce0.9La0.1RhSn alloy.     

High temperature paramagnon modifications in nearly magnetic metals

The standard paramagnon structure in nearly magnetic metals is shown to be drastically modified at high temperatures. Qualitative consequences for the electron paramagnon temperature dependence of the electrical resistivity are considered. Behavior like that exhibited in αPu resistivity might be expected.     

Evidence of a first-order phase transition between Wigner-crystal and bubble phases of 2D electrons in higher Landau Levels

For filling factors nu in the range between 4.16 and 4.28, we simultaneously detect two resonances in the real diagonal microwave conductivity of a two-dimensional electron system (2DES) at low temperature T approximately 35 mK. We attribute the resonance to Wigner-crystal and Bubble phases of the 2DES in higher Landau Levels. For nu below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first-order phase transition. We estimate the transition point as nu=4.22.     

Collapse of coherent quasiparticle states in theta-(BEDT-TTF)2I3 observed by optical spectroscopy

Optical conductivity measurements on the organic metal theta-(BEDT-TTF)2I3 revealed that the system crosses over rapidly from a coherent quasiparticle state to an incoherent state with temperature. Despite the metallic temperature dependence of resistivity, a well-defined Drude peak at low temperatures turns into a far-infrared peak with temperature. The peak energy shifts to higher frequencies and, simultaneously, the spectral weight is transferred to high frequencies beyond the electron band width. These characteristics imply that theta-(BEDT-TTF)2I3, so far believed to be a typical metal, is in fact a strongly correlated electron system with "bad-metal" character.     

石墨烯纳米带量子点中的量子混沌现象

在20 mK的极低温下测量了石墨烯纳米带量子点的电子输运性质,观测到清晰的库仑阻塞菱形块和对应量子点激发态的电导峰.对库仑阻塞近邻电导峰间距和峰值进行了统计分析,发现其统计分布分别满足无规矩阵理论描述的Wigner-Dyson分布和Porter-Thomas分布,说明石墨烯纳米带量子点在低温下出现了量子混沌现象.还讨论了这种长方形量子点中量子混沌的可能成因. 关键词： 石墨烯纳米带 量子点 库仑阻塞 量子混沌     

High Precision Characterisation of Semiconductor Bolometers

We describe techniques for testing and characterising semiconductor bolometers, using the bolometer model presented in Sudiwala et. al. [1]. The procedures are illustrated with results from a prototype bolometer for the high frequency instrument (HFI) in the Planck Surveyor cosmic microwave background mission. This is a bolometer using spider-web geometry and a neutron transmutation doped (NTD) germanium thermistor, designed for operation at 100 mK. Details are given of the laboratory facility used to take data at temperatures from 70 mK to 350 mK. This employs an adiabatic demagnetisation refrigerator to cool the detector and optics. The spatial and spectral properties of the optical system are controlled using feedhorns and edge filters. To characterise the bolometer, blanked and optically loaded load curves were measured over a range of temperatures, and the response to modulated radiation was measured as a function of modulation frequency, temperature and bias current. Results for the prototype bolometer show that its behaviour is well represented by an ideal thermal detector down to a temperature of approximately 100 mK. Below this, non-thermal effects such as electron-phonon decoupling or electric field dependent resistance appear to lead to departure from ideal behaviour. The performance was in good agreement with the design goals for the bolometer.     

Evidence for a quantum phase transition in Pr2-xCe(x)CuO4-delta from transport measurements

The doping and temperature dependences of the Hall coefficient, R(H), and ab-plane resistivity in the normal state down to 350 mK is reported for oriented films of the electron-doped high-T(c) superconductor Pr(2-x)Ce(x)CuO(4-delta). The doping dependences of beta (rho=rho(0)+ATbeta) and R(H) (at 350 mK) suggest a quantum phase transition at a critical doping near x=0.165.