Robust Fermi-Surface Morphology of CeRhIn_{5} across the Putative Field-Induced Quantum Critical Point.

We report a comprehensive de Haas-van Alphen (dHvA) study of the heavy-fermion material CeRhIn_{5} in magnetic fields up to 70 T. Several dHvA frequencies gradually emerge at high fields as a result of magnetic breakdown. Among them is the thermodynamically important ß_{1} branch, which has not been observed so far. Comparison of our angle-dependent dHvA spectra with those of the non-4f compound LaRhIn_{5} and with band-structure calculations evidences that the Ce 4f electrons in CeRhIn_{5} remain localized over the whole field range. This rules out any significant Fermi-surface reconstruction, either at the suggested nematic phase transition at B^{*}≈30 T or at the putative quantum critical point at B_{c}≃50 T. Our results rather demonstrate the robustness of the Fermi surface and the localized nature of the 4f electrons inside and outside of the antiferromagnetic phase.

Fermi surface of LaFe2P2-a detailed density functional study.

Angular-dependent de Haas-van Alphen measurements allow the mapping of Fermi surfaces in great detail with high accuracy. Density functional electronic-structure calculations can be carried out with high precision, but depend crucially on the used structural information and the applied calculational approximations. We report in a detailed study the sensitivity of the calculated electronic band structure of the 122 compound LaFe2P2 on (i) the exact P position in the unit cell, parametrized by a so-called z parameter, and on (ii) the treatment of the La 4f  states. Depending on the chosen exchange and correlation-potential approximation, the calculated z parameter varies slightly and corresponding small but distinctive differences in the calculated band structure and Fermi-surface topology appear. Similarly, topology changes appear when the energy of the mostly unoccupied La 4f  states is corrected regarding their experimentally observed position. The calculated results are compared to experimental de Haas-van Alphen data. Our findings show a high sensitivity of the calculated band structure on the pnictide z position and the need for an accurate experimental determination of this parameter at low temperatures, and a particular need for a sophisticated treatment of the La 4f  states. Thus, this is not only crucial for the special case of LaFe2P2 studied here, but of importance for the precise determination of the band structure of related 122 materials and La containing compounds in general.

Field-Induced Lifshitz Transition without Metamagnetism in CeIrIn(5).

We report high magnetic field measurements of magnetic torque, thermoelectric power, magnetization, and the de Haas-van Alphen effect in CeIrIn_{5} across 28 T, where a metamagnetic transition was suggested in previous studies. The thermoelectric power displays two maxima at 28 and 32 T. Above 28 T, a new, low de Haas-van Alphen frequency with a strongly enhanced effective mass emerges, while the highest frequency observed at low field disappears entirely. This suggests a field-induced Lifshitz transition. However, longitudinal magnetization does not show any anomaly up to 33 T, thus ruling out a metamagnetic transition at 28 T.

Incomplete devil's staircase in the magnetization curve of SrCu2(BO3)2.

We report on NMR and torque measurements on the frustrated quasi-two-dimensional spin-dimer system SrCu(2)(BO(3))(2) in magnetic fields up to 34 T that reveal a sequence of magnetization plateaus at 1/8, 2/15, 1/6, and 1/4 of the saturation and two incommensurate phases below and above the 1/6 plateau. The magnetic structures determined by NMR involve a stripe order of triplets in all plateaus, suggesting that the incommensurate phases originate from proliferation of domain walls. We propose that the magnetization process of SrCu(2)(BO(3))(2) is best described as an incomplete devil's staircase.

Novel Pauli-paramagnetic quantum phase in a Mott insulator.

In Mott insulators, the strong electron-electron Coulomb repulsion localizes electrons. In dimensions greater than one, their spins are usually ordered antiferromagnetically at low temperatures. Geometrical frustrations can destroy this long-range order, leading to exotic quantum spin liquid states. However, their magnetic ground states have been a long-standing mystery. Here we show that a quantum spin liquid state in the organic Mott insulator EtMe(3)Sb[Pd(dmit)(2)](2) (where Et is C(2)H(5)-, Me is CH(3)-, and dmit is 1,3-dithiole-2-thione-4,5-dithiolate) with two-dimensional triangular lattice has Pauli-paramagnetic-like low-energy excitations, which are a hallmark of itinerant fermions. Our torque magnetometry down to low temperatures (30 mK) up to high fields (32 T) reveals distinct residual paramagnetic susceptibility comparable to that in a half-filled two-dimensional metal, demonstrating the magnetically gapless nature of the ground state. Moreover, our results are robust against deuteration, pointing toward the emergence of an extended 'quantum critical phase', in which low-energy spin excitations behave as in paramagnetic metals with Fermi surface, despite the frozen charge degree of freedom.

Using the de Haas-van Alphen effect to map out the closed three-dimensional Fermi surface of natural graphite.

The Fermi surface of graphite has been mapped out using de Haas-van Alphen (dHvA) measurements at low temperature with in-situ rotation. For tilt angles θ>60° between the magnetic field and the c axis, the majority electron and hole dHvA periods no longer follow a cos(θ) behavior demonstrating that graphite has a three-dimensional closed Fermi surface. The Fermi surface of graphite is accurately described by highly elongated ellipsoids. A comparison with the calculated Fermi surface suggests that the Slonczewski-Weiss-McClure trigonal warping parameter γ(3) is significantly larger than previously thought.

Fermi-surface reconstruction by stripe order in cuprate superconductors.

The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa(2)Cu(3)O(y) (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La(1.8-x)Eu(0.2)Sr(x)CuO(4) (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates.

Nernst and Seebeck coefficients of the cuprate superconductor YBa2Cu3O6.67: a study of Fermi surface reconstruction.

The Seebeck and Nernst coefficients S and nu of the cuprate superconductor YBa{2}Cu{3}O{y} (YBCO) were measured in a single crystal with doping p=0.12 in magnetic fields up to H=28 T. Down to T=9 K, nu becomes independent of field by H approximately 30 T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, S/T and nu/T are both large and negative in the T-->0 limit, with the magnitude and sign of S/T consistent with the small electronlike Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in S(T) at T approximately 50 K is remarkably similar to that observed in La2-xBaxCuO4, La{2-x-y}Nd{y}Sr_{x}CuO{4}, and La{2-x-y}Eu{y}Sr{x}CuO{4}, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.

Magnetic breakdown in the electron-doped cuprate superconductor Nd(2-x)Ce(x)CuO4: the reconstructed Fermi surface survives in the strongly overdoped regime.

We report on semiclassical angle-dependent magnetoresistance oscillations and the Shubnikov-de Haas effect in the electron-overdoped cuprate superconductor Nd(2-x)CexCuO4. Our data provide convincing evidence for magnetic breakdown in the system. This shows that a reconstructed multiply connected Fermi surface persists, at least at strong magnetic fields, up to the highest doping level of the superconducting regime.

Magnetic phase diagram of the S = 1/2 triangular layered compound NaNiO(2): a single crystal study.

Using magnetic torque measurements on a NaNiO(2) single crystal, we have established the magnetic phase diagram of this triangular compound. It presents five different phases depending on the temperature (4-300 K) and magnetic field (0-22 T) revealing several spin reorientations coupled to different magnetic anisotropies.

Quantum phase interference and Néel-vector tunneling in antiferromagnetic molecular wheels.

The antiferromagnetic molecular wheel Fe18 of 18 exchange-coupled Fe;{III} ions has been studied by magnetic torque, magnetization, and inelastic neutron scattering. The combined data show that the low-temperature magnetism of Fe18 is very accurately described by the Néel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory. In addition, the magnetic torque as a function of applied field exhibits oscillations that reflect the oscillations in the NVT splitting with field due to quantum phase interference.

Coexistence and interplay of superconductivity and ferromagnetism in URhGe.

As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe(2), URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was first observed in 2001. Recently, we discovered a second pocket of superconductivity. This new pocket of superconductivity appears at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at H(R) = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. The possibility that magnetic fluctuations emerging from a quantum critical point provide the pairing mechanism for superconductivity is discussed.

Study of the Fermi surface of ZrB12 using the de Haas-van Alphen effect.

The de Haas-van Alphen (dHvA) effect in the cluster superconductor ZrB12 was studied by magnetic torque measurements in magnetic fields up to 28 T at temperatures down to 0.07 K. The dHvA oscillations due to orbits from the neck sections and "cubic box" of the Fermi surface were detected. The dHvA frequencies as well as the cyclotron effective masses were calculated using the full-potential linear muffin-tin orbitals method within the generalized gradient approximation. A comparison of the experimental and calculated cyclotron mass shows unusually large electron-phonon interaction on the neck (lambdaep=0.95) and box (lambdaep=1.07) sections of the Fermi surface on the Brillouin zone boundaries.

Anisotropic multiband many-body interactions in LuNi2B2C.

We present a comprehensive de Haas-van Alphen study on the nonmagnetic borocarbide superconductor LuNi2B2C. The analysis of the angular-dependent effective masses for different bands in combination with full-potential density functional calculations allowed us to determine the mass-enhancement factors, lambda, for the different electronic bands and their wave-vector dependences. Our data clearly show the anisotropic multiband character of the superconductivity in LuNi2B2C.

Angle-dependent magnetoresistance in the weakly incoherent interlayer transport regime in a layered organic conductor.

We present comparative studies of the orientation effect of a strong magnetic field on the interlayer resistance of alpha-(BEDT-TTF)2KHg(SCN)4 samples characterized by different crystal quality. We find striking differences in their behavior, which is attributed to the breakdown of the coherent charge transport across the layers in the lower quality sample. In the latter case, the nonoscillating magnetoresistance background is essentially a function of only the out-of-plane field component, in contradiction to the existing Fermi-liquid theories.

Evidence for a new magnetic field scale in CeCoIn5.

The Nernst coefficient of displays two distinct anomalies in magnetic field. The feature detected at Hk approximately 23 T is similar to what is observed in CeRu2Si2 at Hm = 7.8 T where a metamagnetic transition occurs. In CeCoIn5, new frequencies are observed in de Haas-van Alphen oscillations when the field exceeds 23 T where the Dingle temperature decreases by about 30%. Based on the Nernst coefficient anomalies, the magnetic phase diagram of CeCoIn5 is revised.

Field-induced magnetoelastic instabilities in antiferromagnetic molecular wheels.

The magnetic torque of the antiferromagnetic molecular wheel CsFe8 was studied down to 50 mK and up to 28 T. Below about 0.5 K phase transitions were observed at the field-induced level crossings (LCs). Intermolecular magnetic interactions are very weak excluding field-induced magnetic ordering. A magnetoelastic coupling was considered. A generic model shows that the wheel structure is unconditionally unstable at the LCs, and the predicted torque curves explain the essential features of the data well.

Magnetic field-induced superconductivity in the ferromagnet URhGe.

In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.

Specific heat of single crystal MgB2: a two-band superconductor with two different anisotropies.

Heat-capacity measurements of a 39 microg MgB2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence, and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the smaller gap being isotropic, that carrying the larger gap having an anisotropy of approximately 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.