Not your usual superconductorMost superconductors have only one superconducting phase. Khim et al. measured the magnetic susceptibility of the heavy fermion material CeRh2As2 to reveal the presence ...of two distinct superconducting phases, one of which emerges from the other when an external magnetic field is applied (see the Perspective by Pourret and Knebel). The researchers ascribe the unusual properties of CeRh2As2 to its crystal structure, which is globally centrosymmetric but consists of noncentrosymmetric layers.Science, abe7518, this issue p. 1012; see also abj8193, p. 962Materials with multiple superconducting phases are rare. Here, we report the discovery of two-phase unconventional superconductivity in CeRh2As2. Using thermodynamic probes, we establish that the superconducting critical field of its high-field phase is as high as 14 tesla, even though the transition temperature is only 0.26 kelvin. Furthermore, a transition between two different superconducting phases is observed in a c axis magnetic field. Local inversion-symmetry breaking at the cerium sites enables Rashba spin-orbit coupling alternating between the cerium sublayers. The staggered Rashba coupling introduces a layer degree of freedom to which the field-induced transition and high critical field seen in experiment are likely related.
Abstract
The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the ...Dirac semimetal ZrTe
5
. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe
5
samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe
5
electronic structure and its Dirac-type semi-metallic character.
We report on negative thermal expansion (NTE) in the high-field, half-magnetization plateau phase of the frustrated magnetic insulator CdCr_{2}O_{4}. Using dilatometry, we precisely map the phase ...diagram at fields of up to 30 T and identify a strong NTE associated with the collinear half-magnetization plateau for B>27 T. The resulting phase diagram is compared with a microscopic theory for spin-lattice coupling, and the origin of the NTE is identified as a large negative change in magnetization with temperature, coming from a nearly localized band of spin excitations in the plateau phase. These results provide useful guidelines for the discovery of new NTE materials.
When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron-electron interactions are expected to play a significant role in determining the electronic ground ...state. Graphite is known to host a sequence of magnetic field-induced states driven by such interactions. Three decades after their discovery, thermodynamic signatures of these instabilities are still elusive. Here we report the detection of these transitions with sound velocity measurements. The evolution of elastic constant anomalies with temperature and magnetic field allows to draw a detailed phase diagram which shows that the ground state evolves in a sequence of thermodynamic phase transitions. Our analysis indicates that the electron-electron interaction is not the sole driving force of these transitions and that lattice degrees of freedom play an important role.
Electron-like carriers in bismuth are described by the Dirac Hamiltonian, with a band mass becoming a thousandth of the bare electron mass along one crystalline axis. The existence of three ...anisotropic valleys offers electrons an additional degree of freedom, a subject of recent attention. Here, we map the Landau spectrum by angle-resolved magnetostriction, and quantify the carrier number in each valley: while the electron valleys keep identical spectra, they substantially differ in their density of states at the Fermi level. Thus, the electron fluid does not keep the rotational symmetry of the lattice at low temperature and high magnetic field, even in the absence of internal strain. This effect, reminiscent of the Coulomb pseudogap in localized electronic states, affects only electrons in the immediate vicinity of the Fermi level. It presents the most striking departure from the non-interacting picture of electrons in bulk bismuth.
CeRh2As2has recently been reported to be a rare case of a multiphase unconventional superconductor close to a quantum critical point (QCP). Here, we present a comprehensive study of its normal-state ...properties and of the phase (I) belowT0≈0.4Kwhich preempts superconductivity atTc=0.26K. The second-order phase transition atT0presents signatures in specific heat and thermal expansion but none in magnetization and ac susceptibility, indicating a nonmagnetic origin of phase I. In addition, an upturn of the in-plane resistivity atT0points to a gap opening at the Fermi level in the basal plane. Thermal expansion indicates a strong-positive-pressure dependence ofT0,dT0/dp=1.5K/GPa, in contrast to the strong-negative-pressure coefficient observed for magnetic order in Ce-based Kondo lattices close to a QCP. Similarly, an in-plane magnetic field shiftsT0to higher temperatures and transforms phase I into another nonmagnetic phase (II) through a first-order phase transition at about 9 T. Using renormalized band-structure calculations, we find that the Kondo effect (TK≈30K) leads to substantial mixing of the excited crystalline-electric-field states into the ground state. This allows quadrupolar degrees of freedom in the resulting heavy bands at the Fermi level which are prone to nesting. The huge sensitivity of the quadrupole moment on hybridization together with nesting causes an unprecedented case of phase transition into a quadrupole-density-wave state at a temperatureT0≪TK, which explains the nature of phases I and II.
We report on a comprehensive study of CePd(1-x)Rh(x) (0.6 <or= x <or= 0.95) poly- and single crystals close to the ferromagnetic instability by means of low-temperature ac susceptibility, ...magnetization, and volume thermal expansion. The signature of ferromagnetism in this heavy-fermion system can be traced from 6.6 K in CePd down to 25 mK for x = 0.87. Despite pronounced non-Fermi-liquid effects in both specific heat and thermal expansion, the Grüneisen ratio does not diverge as T --> 0, providing evidence for the absence of a quantum critical point. Instead, a peculiar "Kondo-cluster-glass" state is found for x >or= 0.65, and the non-Fermi-liquid effects in the specific heat, ac susceptibility, and magnetization are compatible with the quantum Griffiths phase scenario.
PDF
