Experiments with phase transitions at very high pressure

Diamond cells were constructed for use to 1 Mbar. A refrigerator for cooling diamond cells was adapted for studies between 15 and 300 K. A cryostat for superconductivity studies between 1.5 to 300 K was constructed. Optical equipment was constructed for fluorescence, transmission, and reflectance studies. X-ray equipment was adapted for use with diamond cells. Experimental techniques were developed for X-ray diffraction studies using synchrotron radiation. AC susceptibility techniques were developed for detecting superconducting transitions. The following materials were studied: compressed solidified gases (Xe, Ar), semiconductors (Ge, Si, GaAs), superconductors (Nb3Ge, Nb3Si, Nb3As, CuCl), molecular crystals (I)

Magnetometer uses bismuth-selenide

Characteristics of bismuth-selenide magnetometer are described. Advantages of bismuth-selenide magnetometer over standard magnetometers are stressed. Thermal stability of bismuth-selenide magnetometer is analyzed. Linearity of output versus magnetic field over wide range of temperatures is reported

Hall effect magnetometer

A magnetometer which uses a single crystal of bismuth selenide is described. The rhombohedral crystal structure of the sensing element is analyzed. The method of construction of the magnetometer is discussed. It is stated that the sensing crystal has a positive or negative Hall coefficient and a carrier concentration of about 10 to the 18th power to 10 to the 20th power per cubic centimeter

Properties of crystalline bismuth selenide and its use as a Hall effect magnetometer

Single crystals of n-type Bi2Se3 grown by the Bridgman technique are found to make excellent Hall effect magnetometers. Plots of Hall resistivity sub yx against magnetic field B to 10 tesla are linear to within 1 percent. Furthermore, the slope of the sub yx against B curve varies by about 1 percent in the region 1.1 to 35 K and by less than 20 percent in the region 1.1 to 300 K. Analysis of galvanomagnetic measurements indicate the samples have semimetallic densities of approximately 10 to the 25th power/cu cm, with two band conduction and near carrier compensation. Reflectivity measurements suggest a band gap of approximately 0.08 eV for the samples. The temperature dependence of mobility is also measured. A series of 50 direct immersions into liquid helium and liquid nitrogen demonstrate the reliability of Bi2Se3 magnetometers for cryogenic use

Screening and interlayer coupling in multilayer graphene field-effect transistors

With the motivation of improving the performance and reliability of aggressively scaled nano-patterned graphene field-effect transistors, we present the first systematic experimental study on charge and current distribution in multilayer graphene field-effect transistors. We find a very particular thickness dependence for Ion, Ioff, and the Ion/Ioff ratio, and propose a resistor network model including screening and interlayer coupling to explain the experimental findings. In particular, our model does not invoke modification of the linear energy-band structure of graphene for the multilayer case. Noise reduction in nano-scale few-layer graphene transistors is experimentally demonstrated and can be understood within this model as well.Comment: 13 pages, 4 figures, 20 reference

Possible Fractional Quantum Hall Effect in Graphite

Measurements of basal plane longitudinal rho_b(B) and Hall rho_H(B) resistivities were performed on highly oriented pyrolytic graphite (HOPG) samples in pulsed magnetic field up to B = 50 T applied perpendicular to graphene planes, and temperatures 1.5 K 30 T and for all studied samples, we observed a sign change in rho_H(B) from electron- to hole-like. For our best quality sample, the measurements revealed the enhancement in rho_b(B) for B > 34 T (T = 1.8 K), presumably associated with the field-driven charge density wave or Wigner crystallization transition. Besides, well defined plateaus in rho_H(B) were detected in the ultra-quantum limit revealing the signatures of fractional quantum Hall effect in graphite.Comment: 15 pages, including 4 figure

Finite temperature bosonization

Finite temperature properties of a non-Fermi liquid system is one of the most challenging probelms in current understanding of strongly correlated electron systems. The paradigmatic arena for studying non-Fermi liquids is in one dimension, where the concept of a Luttinger liquid has arisen. The existence of a critical point at zero temperature in one dimensional systems, and the fact that experiments are all undertaken at finite temperature, implies a need for these one dimensional systems to be examined at finite temperature. Accordingly, we extended the well-known bosonization method of one dimensional electron systems to finite temperatures. We have used this new bosonization method to calculate finite temperature asymptotic correlation functions for linear fermions, the Tomonaga-Luttinger model, and the Hubbard model.Comment: REVTex, 48 page

Compressibility of CeMIn5Ce M In_5 and Ce2MIn8Ce_2 M In_8 (M = Rh, Ir and Co) Compounds

The lattice parameters of the tetragonal compounds Ce$M$In$_{5}$ and Ce$_{2}M$In$_{8}$($M=$Rh, Ir and Co) have been studied as a function of pressure up to 15 GPa using a diamond anvil cell under both hydrostatic and quasihydrostatic conditions at room temperature. The addition of $M$In$_{2}$ layers to the parent CeIn$_{3}$ compound is found to stiffen the lattice as the 2-layer systems (average of bulk modulus values $B_{0}$ is 70.4 GPa) have a larger $B_{0}$ than CeIn$_{3}$ (67 GPa), while the 1-layer systems with the are even stiffer (average of $B_{0}$ is 81.4 GPa). Estimating the hybridization using parameters from tight binding calculations shows that the dominant hybridization is $fp$ in nature between the Ce and In atoms. The values of $V_{pf}$ at the pressure where the superconducting transition temperature $T_{c}$ reaches a maximum is the same for all Ce$M$In$_{5}$ compounds. By plotting the maximum values of the superconducting transition temperature $T_{c}$ versus $c/a$ for the studied compounds and Pu-based superconductors, we find a universal $T_{c}$ versus $c/a$ behavior when these quantities are normalized appropriately. These results are consistent with magnetically mediated superconductivity.Comment: Updated version resubmitted to Phys. Rev.

New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al and HTSC oxides

A new DTA (Differential Thermal Analysis) device was designed and installed in a Hot Isostatic Pressure (HIP) furnace in order to perform high-pressure thermodynamic investigations up to 2 kbar and 1200C. Thermal analysis can be carried out in inert or oxidising atmosphere up to p(O2) = 400 bar. The calibration of the DTA apparatus under pressure was successfully performed using the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard calibration references. The thermal properties of these metals have been studied under pressure. The values of DV (volume variation between liquid and solid at Tm), ROsm (density of the solid at Tm) and ALPHAm (linear thermal expansion coefficient at Tm) have been extracted. A very good agreement was found with the existing literature and new data were added. This HP-DTA apparatus is very useful for studying the thermodynamics of those systems where one or more volatile elements are present, such as high TC superconducting oxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 kbar under reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading to the formation of the 2223 phase was found to occur at higher temperatures when applying pressure: the reaction DTA peak shifted by 49C at 2 kbar compared to the reaction at 1 bar. This temperature shift is due to the higher stability of the Pb-rich precursor phases under pressure, as the high isostatic pressure prevents Pb from evaporating.Comment: 6 figures, 3 tables, Thermodynamics, Thermal property, Bi-2223, fundamental valu

Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating

We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments, and find less heating for dense regions characterized by R_V=5.5 than for diffuse regions with R_V=3.1. We provide fitting functions which reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. In a separate paper we will examine the implications of these results for the thermal structure of the interstellar medium. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We find that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also find that photoelectric heating by dust can account for the observed variation of temperature with distance from the galactic midplane in the warm ionized medium.Comment: 50 pages, including 18 figures; corrected title and abstract field