The aim of this study is to compare the magnetic behavior of URu2Si2 under uniaxial stress along the a-axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress ...presents a critical stress {\sigma}xa smaller (0.33(5)GPa) than the hydrostatic critical pressure px =0.5 GPa where the ground state switches from HO (hidden order) to AF (antiferromagnetic) ground state. From these critical values and from Larmor neutron diffraction (LND), we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a-axis before the appearance of the large AF moment which indicates no-mixture of order parameter (OP) between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q0=(1,0,0) and Q1=(0.6,0,0) were measured in the HO state: the excitation at Q0 decreases in energy while the excitation at Q1 increases in energy with the uniaxial stress along the a-axis. The decrease of the energy of the excitation at Q0 seems to indicate a critical energy gap value of 1.2(1) meV at {\sigma}xa. A similar value was derived from studies under hydrostatic pressure at px.
Systematic measurements of temperature dependent magnetization, resistivity and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 ...GPa were conducted on single crystals of CrAuTe\(_4\). Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic \(c\)-axis below \(T_\textrm{N}\) = 255 K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The Neel temperature decreases monotonically under pressure, decreasing to \(T_\textrm{N}\) = 236 K at 5.22 GPa. The pressure dependencies of (i) \(T_\textrm{N}\), (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states.
We report the temperature-pressure phase diagram of CaKFe\(_4\)As\(_4\) established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 ...GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe\(_4\)As\(_4\) is suppressed and then disappears at \(p\) \(\gtrsim\) 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe\(_4\)As\(_4\) under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for \(p\) \(\geq\) 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe\(_4\)As\(_4\): a half-collapsed-tetragonal phase.
Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field ...H_c2. H_c2 for H // b-axis (H_c2^b) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H_c2^a shows upward curvature with a low temperature value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H // b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H_c2^b curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.
Resistivity and specific heat measurements were performed in the low carrier unconventional superconductor URu2Si2 on various samples with very different qualities. The superconducting transition ...temperature (TSC) and the hidden order transition temperature (THO) of these crystals were evaluated as a function of the residual resistivity ratio (RRR). In high quality single crystals the resistivity does not seem to follow a T2 dependence above TSC, indicating that the Fermi liquid regime is restricted to low temperatures. However, an analysis of the isothermal longitudinal magnetoresistivity points out that the T2 dependence may be "spoiled" by residual inhomogeneous superconducting contribution. We discuss a possible scenario concerning the distribution of TSC related with the fact that the hidden order phase is very sensitive to the pressure inhomogeneity.
Combination of neutron scattering and thermal expansion measurements under pressure shows that the so-called hidden order phase of URu2Si2 reenters in magnetic field when antiferromagnetism (AF) ...collapses at H_AF (T). Macroscopic pressure studies of the HO-AF boundaries were realized at different pressures via thermal expansion measurements under magnetic field using a strain gauge. Microscopic proof at a given pressure is the reappearance of the resonance at Q_0=(1,0,0) under field which is correlated with the collapse of the AF Bragg reflections at Q_0.
Low-temperature measurements of the magnetocrystalline anisotropy energy \(K\) in (Fe\(_{1-x}\)Co\(_{x}\))\(_{2}\)B alloys are reported, and the origin of this anisotropy is elucidated using a ...first-principles electronic structure analysis. The calculated concentration dependence \(K(x)\) with a maximum near \(x=0.3\) and a minimum near \(x=0.8\) is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, \(K\) depends strongly on the tetragonality and doubles under a modest 3% increase of the \(c/a\) ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.
We review our recent results on ferromagnetic superconductors, URhGe and UCoGe. High quality single crystals of both compounds were successfully grown. The specific heat shows a clear jump related to ...the superconducting transition in UCoGe. The finite values of C/T at 0K are discussed in terms of the self-induced vortex state and the value of the ordered moment. With increasing fields for H // b-axis in URhGe, the jump of thermal expansion increases and shifts to lower temperature. The re-entrant and S-shaped superconducting phases for URhGe and UCoGe respectively are explained by the unusual field dependence of the effective mass, which is induced by the ferromagnetic instability when the field is applied along the hard magnetization b-axis. The magnetic fluctuations are very sensitive to the field orientation. This is reflected in the Hc2 and the anisotropy of the effective mass.
We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results ...of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field Hc2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge Hc2 along the hard-magnetization axis, Hc2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of Hc2, namely dHc2/dT (H -> 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.
The temperature-pressure phase diagram of the ferromagnet LaCrGe\(_3\) is determined for the first time from a combination of magnetization, muon-spin-rotation and electrical resistivity ...measurements. The ferromagnetic phase is suppressed near \(2.1\)~GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM\(_Q\). Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors \(Q\) allowing for the potential of an ordering wave vector evolving from \(Q=0\) to finite \(Q\), as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe\(_3\) is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.
