Georg Knebel

Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe

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.

Pressure-Temperature Phase Diagram of Polycrystalline UCoGe Studied by Resistivity Measurement

Recently, coexistence of ferromagnetism ( T Curie = 2.8 K) and superconductivity ( T sc =0.8 K) has been reported in UCoGe, a compound close to a ferromagnetic instability at ambient pressure P . Here we present resistivity measurements under pressure on a UCoGe polycrystal. The phase diagram obtained from resistivity measurements on a polycrystalline sample is found to be qualitatively different to those of all other ferromagnetic superconductors. By applying high pressure, ferromagnetism is suppressed at a rate of 1.4 K/GPa. No indication of ferromagnetic order has been observed above P ≈1 GPa. The resistive superconducting transition is, however, quite stable in temperature and persists up to the highest measured pressure of about 2.4 GPa. Superconductivity would therefore appear also in the paramagnetic phase. However, the appearance of superconductivity seems to change at a characteristic pressure \(P^{\star}\sim 0.8\) GPa. Close to a ferromagnetic instability, the homogeneity of the sample can influ...

Evolution toward Quantum Critical End Point in UGe2

We report on Hall resistivity and electrical resistivity measurements under pressure and magnetic field in UGe 2 with ferromagnetic (FM) tricriticality. The Hall resistivity sensitively detects the first order metamagnetic transition from a paramagnetic (PM) phase to a FM phase in a large pressure range almost up to the quantum critical end point (QCEP). The drastic change in the Fermi surface at the PM–FM transition is detected up to the vicinity of the QCEP, while a strong modification in the field variation of the inelastic scattering between electrons is observed toward the QCEP. The comparison with the theoretical predictions is made.

First Observation of Quantum Oscillations in the Ferromagnetic Superconductor UCoGe

We succeeded in growing high quality single crystals of the ferromagnetic superconductor UCoGe and measured the magnetoresistance at fields up to 34 T. The Shubnikov–de Haas signal was observed for the first time in a U-111 system (UTGe, UTSi, T: transition metal). A small pocket Fermi surface ( F ∼1 kT) with large cyclotron effective mass 25 m 0 was detected at high fields above 22 T, implying that UCoGe is a low carrier system accompanyed with heavy quasi-particles. The observed frequency decreases with increasing fields, indicating that the volume of detected Fermi surface changes nonlinearly with field. The cyclotron mass also decreases, which is consistent with the decrease of the A coefficient of resistivity.

Field-Induced Antiferromagnetic State in Non-centrosymmetric Superconductor CeIrSi3

We have carried out the electrical resistivity and ac specific heat measurements under pressure P and magnetic field H in a pressure-induced superconductor CeIrSi 3 with the noncentrosymmetric tetragonal crystal structure. Superconductivity is observed in a wide pressure region from 1.9 GPa to about 3.5 GPa. The antiferromagnetic state disappears at P c =2.25 GPa under H =0, but in magnetic field we have found an antiferromagnetic state. The corresponding electrical resistivity in constant magnetic field decreases rather steeply below the Neel temperature T N under pressures up to about 2.4 GPa, while it shows an upturn anomaly at T N under pressures larger than 2.4 GPa. The ac specific heat also indicates a clear jump at T N in magnetic field. When the antiferromagnetic phase exists in magnetic field, the superconducting transition becomes broad, possessing a transition width Δ T sc =0.2–0.5 K. On the other hand, it becomes extremely sharp, with Δ T sc =0.03 K even at 17 T under P =2.55 GPa, for example,...

Antiferromagnetism and superconductivity in cerium based heavy-fermion compounds

The study of competing ground states is a central issue in condensed matter physics. In this article we will discuss the interplay of antiferromagnetic order and unconventional superconductivity in Ce based heavy-fermion compounds. In all examples discussed superconductivity appears at the border of magnetic order. Special focus is given on the pressure–temperature–magnetic field phase diagram of CeRhIn5 and CeCoIn5 which allows one to discuss microscopic coexistence of magnetic order and superconductivity in detail. A striking point is the similarity of the phase diagram of different classes of strongly correlated systems which is discussed briefly. The recently discovered non-centrosymmetric superconductors will open a new access with the possible mixing of odd and even parity pairing.

Field Reentrance of the Hidden Order State of URu2Si2 under Pressure

Combination of neutron scattering and thermal expansion measurements under pressure shows that the so-called hidden order phase of URu 2 Si 2 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 .

Thermoelectricity of the ferromagnetic superconductor UCoGe

UCoGe exhibits superconductivity in the presence of ferromagnetism. When a field is applied along the b axis (perpendicular to the easy axis), ferromagnetism is weakened and superconductivity is enhanced. This enhancement has been attributed to an increase in coupling as observed in the enhanced effective mass produced by the critical fluctuations as the ferromagnetic transition is strongly suppressed. However it is also important to know if and how the Fermi surface changes near the critical point. Here we report measurements of the thermoelectricity of UCoGe which reveal a low carrier density metal. Under magnetic field applied along the b axis, a sharp peak is observed in the thermopower of UCoGe at H*=11.1T and low temperature which becomes broader at higher temperatures. At higher field, the thermopower changes sign which suggests a modification of the Fermi Surface. We analyze these results using a topological change in Fermi surface and show that this can explain both the thermopower and the enhanced superconductivity.

High Pressure Phase Diagram of the Non-centrosymmetric Antiferromagnet CeCoGe3

We present a detailed study of the high pressure phase diagram of the non-centrosymmetric antiferromagnet CeCoGe 3 by measurements of the ac calorimetry in a diamond anvil pressure cell. Under high pressure the magnetic ordering temperature shows a step like decrease due to various successive magnetic phases under high pressure. The magnetic order disappears at a critical pressure of p c ≈5.5 GPa. Around the critical pressure bulk superconductivity has been detected in the ac calorimetric measurement.

Verification of the Wiedemann-Franz law in YbRh2Si2 at a quantum critical point.

The thermal conductivity measurements are performed on the heavy-fermion compound YbRh(2)Si(2) down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at B(c)=0.66  T∥c axis. In the limit as T→0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of the unconventional quantum criticality of YbRh(2)Si(2).