Electrical transport measurements of the heavy fermion compound YbFe2Zn20 were carried out under pressures up to 8.23 GPa and down to temperatures of nearly 0.3 K. The pressure dependence of the low temperature Fermi-liquid state was assessed by fitt
ing rho(T) = rho_0 + AT^n with n = 2 for T < T_FL. Power law analysis of the low temperature resistivities indicates n = 2 over a broad temperature range for P < 5 GPa. However, at higher pressures, the quadratic temperature dependence is only seen at the very lowest temperatures, and instead shows a wider range of n < 2 power law behavior in the low temperature resistivities. As pressure was increased, T_FL diminished from ~11 K at ambient pressure to ~0.6 K at 8.23 GPa. Over the same pressure range, the A parameter increased dramatically with a functional form of A proportional to (P-Pc)^-2 with Pc~9.8GPa being the critical pressure for a possible quantum critical point.
Single crystals of Ba(Fe_(1-x)Mn_x)_2As_2, 0<x<0.148, have been grown and characterized by structural, magnetic, electrical transport and thermopower measurements. Although growths of single crystals of Ba(Fe_(1-x)Mn_x)_2As_2 for the full 0<=x<=1 ran
ge were made, we find evidence for phase separation (associated with some form of immiscibility) starting for x>0.1-0.2. Our measurements show that whereas the structural/magnetic phase transition found in pure BaFe_2As_2 at 134 K is initially suppressed by Mn substitution, superconductivity is not observed at any substitution level. Although the effect of hydrostatic pressure up to 20 kbar in the parent BaFe_2As_2 compound is to suppress the structural/magnetic transition at the approximate rate of 0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound are not cumulative. Phase diagrams of transition temperature versus substitution concentration, x, based on electrical transport, magnetization and thermopower measurements have been constructed and compared to those of the Ba(Fe_(1-x)TM_x)_2As_2 (TM=Co and Cr) series.
The ab-plane resistivity of Ba(Fe1-xRux)2As2 (x = 0.00, 0.09, 0.16, 0.21, and 0.28) was studied under nearly hydrostatic pressures, up to 7.4 GPa, in order to explore the T-P phase diagram and to compare the combined effects of iso-electronic Ru subs
titution and pressure. The parent compound BaFe2As2 exhibits a structural/magnetic phase transition near 134 K. At ambient pressure, progressively increasing Ru concentration suppresses this phase transition to lower temperatures at the approximate rate of ~5 K/% Ru and is correlated with the emergence of superconductivity. By applying pressure to this system, a similar behavior is seen for each concentration: the structural/magnetic phase transition is further suppressed and superconductivity induced and ultimately, for larger x Ru and P, suppressed. A detailed comparison of the T-P phase diagrams for all Ru concentrations shows that 3 GPa of pressure is roughly equivalent to 10% Ru substitution. Furthermore, due to the sensitivity of Ba(Fe1-xRux)2As2 to pressure conditions, the melting of the liquid media, 4 : 6 light mineral oil : n-pentane and 1 : 1 iso-pentane : n-pentane, used in this study could be readily seen in the resistivity measurements. This feature was used to determine the freezing curves for these media and infer their room temperature, hydrostatic limits: 3.5 and 6.5 GPa, respectively.
We studied the properties of the antiferromagnetic (AFM) UNi0.5Sb2 (TN approx 161 K) compound in Sb-flux grown single crystals by means of measurements of neutron diffraction, magnetic susceptibility ({chi}), specific heat (Cp), thermopower (S), ther
mal conductivity ({kappa}), linear thermal expansion ({Delta}L/L), and electrical resistivity ({rho}) under hydrostatic pressures (P) up to 22 kbar. The neutron diffraction measurements revealed that the compound crystallizes in the tetragonal P42/nmc structure, and the value of the U-moments yielded by the histograms at 25 K is approx 1.85 pm 0.12 {mu}B/U-ion. In addition to the features in the bulk properties observed at TN, two other hysteretic features centered near 40 and 85 K were observed in the measurements of {chi}, S, {rho}, and {Delta}L/L. Hydrostatic pressure was found to raise TN at the rate of approx 0.76 K/kbar, while suppressing the two low temperature features. These features are discussed in the context of Fermi surface and hybridization effects.
Oxides RNiO3 (R = rare-earth, R # La) exhibit a metal-insulator (MI) transition at a temperature TMI and an antiferromagnetic (AF) transition at TN. Specific heat (CP) and anelastic spectroscopy measurements were performed in samples of Nd1-xEuxNiO3,
0 <= x <= 0.35. For x = 0, a peak in CP is observed upon cooling and warming at essentially the same temperature TMI = TN ~ 195 K, although the cooling peak is much smaller. For x >= 0.25, differences between cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures, that define TN, and the other one at TMI. An external magnetic field of 9 T had no significant effect on these results. The elastic compliance (s) and the reciprocal of the mechanical quality factor (Q^-1) of NdNiO3, measured upon warming, showed a very sharp peak at essentially the same temperature obtained from CP, and no peak is observed upon cooling. The elastic modulus hardens below TMI much more sharply upon warming, while the cooling and warming curves are reproducible above TMI. On the other hand, for the sample with x = 0.35, s and Q^-1 curves are very similar upon warming and cooling. The results presented here give credence to the proposition that the MI phase transition changes from first to second order with increasing Eu doping.
The effects of pressure generated in a liquid medium, clamp, pressure cell on the in-plane and c-axis resistance, temperature-dependent Hall coefficient and low temperature, magnetoresistance in CaFe2As2 are presented. The T - P phase diagram, includ
ing the observation of a complete superconducting transition in resistivity, delineated in earlier studies is found to be highly reproducible. The Hall resistivity and low temperature magnetoresistance are sensitive to different states/phases observed in CaFe2As2. Auxiliary measurements under uniaxial, c-axis, pressure are in general agreement with the liquid medium clamp cell results with some difference in critical pressure values and pressure derivatives. The data may be viewed as supporting the potential importance of non-hydrostatic components of pressure in inducing superconductivity in CaFe2As2.
We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe2As2, BaFe2As2, and (Ba0.55K0.45)Fe2As2, by means of measurements of electrical resistivi
ty (rho) in the 1.8 - 300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe2As2 and BaFe2As2 (lightly doped with Sn) display structural phase transitions near 170 K and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba0.55K0.45)Fe2As2 is superconducting for T < 30 K. The effect of pressure on BaFe2As2 is to shift the onset of the crystallographic transformation down in temperature at the rate of about -1.04 K/kbar, while shifting the whole rho(T) curves downward, whereas its effect on superconducting (Ba0.55K0.45)Fe2As2 is to shift the onset of superconductivity to lower temperatures at the rate of about -0.21 K/kbar. The effect of pressure on CaFe2As2 is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P < 5 kbar. However, higher pressures bring about another phase transformation characterized by reduced resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (Hc2) data in (Ba0.55K0.45)Fe2As2 and CaFe2As2 are discussed.
The effects of pressure up to $sim 20$ kbar, on the structural phase transition of SrFe$_2$As$_2$ and lightly Sn-doped BaFe$_2$As$_2$, as well as on the superconducting transition temperature and upper critical field of (Ba$_{0.55}$K$_{0.45}$)Fe$_2$A
s$_2$ single crystals have been studied. All the transition temperatures decrease with pressure in an almost linear fashion. Under pressure, the upper critical field curve, $H_{c2}(T)$, for (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ shifts down in temperature to follow the zero field $T_c$ with very little change in slope. Composite $P - T$ phase diagrams for three parent compounds, AFe$_2$As$_2$ (A = Ba, Sr, Ca), are constructed and appear to be remarkably similar: (i) having a structural (antiferromagnetic) phase transition line with a negative slope and (ii) showing signs of the emerging superconducting state at intermediate pressures.
The intermetallic compound LaAgSb2 displays two charge-density-wave (CDW) transitions, which were detected with measurements of electrical resistivity (rho), magnetic susceptibility, and X-ray scattering; the upper transition takes place at T1 approx
. 210 K, and it is accompanied by a large anomaly in rho(T), whereas the lower transition is marked by a much more subtle anomaly at T2 approx. 185 K. We studied the effect of hydrostatic pressure (P) on the formation of the upper CDW state in pure and doped La1-xRxAgSb2 (R = Ce, Nd) compounds, by means of measurements of rho(T) for P < 23 kbar. We found that the hydrostatic pressure, as well as the chemical pressure introduced by the partial substitution of the smaller Ce and Nd ions for La, result in the suppression of the CDW ground state, e.g. the reduction of the ordering temperature T1. The values of dT1/dP are approx. 2-4 times higher for the Ce-doped samples as compared to pure LaAgSb2, or even La0.75Nd0.25AgSb2 Nd-doped with a comparable T1 (P=0). This increased sensitivity to pressure may be due to increasing Ce- hybridization under pressure. The magnetic ordering temperature of the cerium-doped compounds is also reduced by pressure, and the high pressure behavior of the Ce-doped samples is dominated by Kondo impurity scattering.
We studied single-crystals of the antiferromagnetic compound UNi0.5Sb2 (TN ~ 161 K) by means of measurements of magnetic susceptibility (chi), specific heat (Cp), and electrical resistivity (rho) at ambient pressure, and resistivity under hydrostatic
pressures up to 20 kbar, in the temperature range from 1.9 to 300 K. The thermal coefficient of the electrical resistivity (drho/dT) changes drastically from positive below TN to negative above, reflecting the loss of spin-disorder scattering in the ordered phase. Two small features in the rho vs T data centered near 40 and 85 K correlate well in temperature with features in the magnetic susceptibility and are consistent with other data in the literature. These features are quite hysteretic in temperature, i.e., the difference between the warming and cooling cycles are about 10 and 6 K, respectively. The effect of pressure is to raise TN at the approximate rate of 0.76 K/kbar, while progressively suppressing the amplitude of the small features in rho vs T at lower temperatures and increasing the thermal hysteresis.
