We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85 T. There is a clear signal, periodic in inverse field, with frequency 660+/-15 T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m(*)=3.0+/-0.3m(e). In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.

We report quantum oscillation measurements in SrFe 2 As 2 -which is an antiferromagnetic parent of the iron-arsenide family of superconductors -known to become superconducting under doping and the application of pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55 T in the liquid helium temperature range suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high T c superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state.

Abstract. We present a review of quasi-two-dimensional organic superconductors. These systems exhibit many interesting phenomena, including reduced dimensionality, strong electron-electron and electron-phonon interactions and the proximity of antiferromagnetism, insulator states and superconductivity. Moreover, it has been possible to measure the electronic bands of many of the organics in great detail, in contrast to the situation in other well-known systems in which similar phenomena occur. We describe the crystal structure and normal-state properties of the organics, before presenting the experimental evidence for and against exotic superconductivity mediated by antiferromagnetic fluctuations. Finally, three instances of field-induced unconventional superconductivity will be described.

Results are presented of anisotropic temperature and field-dependent magnetization M (H,T) and resistivity (H,T) measurements on high quality single crystals of the light rare-earth diantimonides RSb 2 , RϭLa-Nd, Sm. All of these, excepting LaSb 2 , magnetically order at low temperatures, and for CeSb 2 and NdSb 2 several magnetically ordered phases were observed in low-field magnetization and zero-field resistivity measurements. For RϭCe-Sm strong anisotropies, associated with crystalline electric field ͑CEF͒ splitting of the R 3ϩ ion, were found in M (T) measurements both below and above magnetic ordering temperatures. Furthermore, for RϭCe-Nd metamagnetic transitions were observed in M (H) and (H) for Hʈ(ab) in the magnetically ordered state. In addition, above 15 kG de Haas-van Alphen oscillations are observed for SmSb 2 and Shubnikov-de Haas quantum oscillations are observed above ϳ120 kG for NdSb 2 and SmSb 2 . The zero-field in-plane resistivity ab of all of the compounds is metallic (d/dTϾ0), with residual resistance ratios ranging from 40 to 750. The c-axis resistivity is also metallic, but appears to be considerably larger than the in-plane value, consistent with the diantimonides being quasi-two-dimensional materials. The magnetoresistance of all members of the series is large, approximately linear in H at moderate fields, and is also dependent on the relative orientation of the applied magnetic fields to the crystallographic axes. The extreme case of SmSb 2 has ͓(55 kG)Ϫ(0)]/(0)Ͼ50 000% at Tϭ2 K and Hʈc. ͓S0163-1829͑98͒03621-2͔

The kagome lattice1, which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders2, 3,4,5,6,7,8 and, most rarely, to unconventional su-perconductivity6,9. Recently, charge sensitive probes have indicated that the kagome superconductors AV3Sb5 (A = K, Rb, Cs)9,10,11 exhibit unconventional chiral charge order12, 13,14,15,16,17,18,19, which is analogous to the long-sought-after quantum order in the Haldane model20 or Varma model21. However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV3Sb5. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV3Sb5 and that the Tc/−2ab ratio (where Tc is the superconducting transition temperature and ab is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high T c cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa 2 Cu 3 O 6þx , revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in T c in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability.fermi surface | high temperature superconductivity | metal-insulator transition | quantum oscillations | quantum critical point A continuous zero temperature instability between different ground states-termed as a quantum critical point-is characterized by a divergence in a relevant susceptibility (1-3). In strongly correlated systems (4), the influence of criticality on the entire body of itinerant electrons results in a global divergence of the effective mass-which is recognized as the key defining experimental signature of quantum criticality (4, 5). The growth of electronic correlations on the zero temperature approach to the critical instability can be experimentally accessed by the tuning of parameters such as pressure and doping. Quantum oscillation measurements are ideally suited to investigate the effects of such tuning due to the direct access they provide to the effective mass of the elementary fermionic excitations that can be traced across the quantum critical point (QCP) (6). Such a direct probe is crucial in superconducting materials, where bulk thermodynamic signatures of quantum critical behavior of the normal quasiparticles (3, 4, 7) are difficult to access due to the overlying superconducting dome.While the emergence of high T c superconductivity in the cuprate family is inextricably linked to the parent Mott insulating compound, remarkably little is known about the physics of the metal-insulator cross-over (8) and its relation to electronic correlations. By using quantum oscillation measurements in strong magnetic fields to access normal state quasiparticles in underdoped YBa 2 Cu 3 O 6þx , we uncover a striking doping-dependent upturn in the effective mass at the location of the metal-insulator cross-over (9-13). Our findings provide bulk thermodynamic evidence for a metal-insulator quantum critical point (QCP) in high T c cuprates (14-19), without requiring extrapolation below the superconducting dome. The effective mass divergence unaccompanied by a change in Fermi surface area away from half-filling signals a unique many-body mechanism (20) that drives insulating behavior in underdoped cuprates.We trace the...

We report quantum oscillations in the underdoped high-temperature superconductor YBa 2 Cu 3 O 6+x over a wide range in magnetic field 28Յ 0 H Յ 85 T corresponding to Ϸ12 oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian et al. ͓Nature ͑London͒ 454, 200 ͑2008͔͒, we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad lowfrequency Fourier peak at Ϸ535 T to be clearly resolved into three separate peaks at Ϸ460, Ϸ532, and Ϸ602 T, in reasonable agreement with the reported frequencies of Audouard et al. ͓Phys. Rev. Lett. 103, 157003 ͑2009͔͒. However, our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket ͑denoted ␣͒ that is almost ideally two-dimensional in form ͑exhibiting negligible interlayer corrugation͒. In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket ͑denoted ␥͒. On comparison with band structure, the d-wave symmetry of the interlayer dispersion locates the minimally corrugated ␣ pocket at the "nodal" point k nodal = ͑ / 2, / 2͒, and the significantly corrugated ␥ pocket at the "antinodal" point k antinodal = ͑ ,0͒ within the Brillouin zone. The differently corrugated pockets at different locations indicate creation by translational symmetry breaking-a spin-density wave has been suggested from the suppression of Zeeman splitting for the spectrally dominant pocket. In a broken-translational symmetry scenario, symmetry points to the nodal ͑␣͒ pocket corresponding to holes, with the weaker antinodal ͑␥͒ pocket corresponding to electrons-likely responsible for the negative Hall coefficient reported by LeBoeuf et al. ͓Nature ͑London͒ 450, 533 ͑2007͔͒. Given the similarity in ␣ and ␥ pocket volumes, their opposite carrier type and the previous report of a diverging effective mass in Sebastian et al. ͓Proc. Nat. Am. Soc. 107, 6175 ͑2010͔͒, we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator quantum critical point. Its potential involvement in the enhancement of superconducting transition temperatures is also discussed.