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The upper critical field of multi-band superconductors can be an essential quantity to unravel the nature of superconducting pairing and its interplay with the electronic structure. Here we experimentally map out the complete upper critical field phase diagram of FeSe for different magnetic field orientations at temperatures down to 0.3 K using both resistivity and torque measurements. The temperature dependence of the upper critical field reflects that of a multi-band superconductor and requires a two-band description in the clean limit with band coupling parameters favouring interband over intraband interactions. Despite the relatively small Maki parameter in FeSe of $\alpha \sim 1.6$, the multi-band description of the upper critical field is consistent with the stabilization of a FFLO state below $T/T_{\rm c}\sim 0.3$. We find that the anomalous behaviour of the upper critical field is linked to a departure from the single-band picture, and FeSe provides a clear example where multi-band effects and the strong anisotropy of the superconducting gap need to be taken into account., Comment: 15 pages, 10 figures (manuscript and the supplemental materials

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long-range order is suppressed to zero temperature. So far, this has been mostly studied in magnetically ordered materials. By contrast, the interplay between charge order and superconductivity at a QCP is not fully understood. Here, we present resistance measurements proving that a dome of superconductivity surrounds the charge-density-wave (CDW) QCP in pristine samples of 1$T$-TiSe$_2$ tuned with hydrostatic pressure. Furthermore, we use quantum oscillation measurements to show that the superconductivity sets in at a Lifshitz transition in the electronic band structure. We use density functional theory to identify the Fermi pockets enabling superconductivity: large electron and hole pockets connected by the CDW wave vector $\vec{Q}$ which emerge upon partial suppression of the zero-pressure CDW gap. Hence, we conclude that superconductivity is of interband type enabled by the presence of hole and electron bands connected by the CDW $\vec{Q}$ vector. Earlier calculations show that interband interactions are repulsive, which suggests that unconventional s$_{\pm}$ superconductivity is realised in TiSe$_2$ - similar to the iron pnictides. These results highlight the importance of Lifshitz transitions in realising unconventional superconductivity and help understand its interaction with CDW order in numerous materials., Comment: 21 pages, 5 figures

The motivation to develop materials for quantum technologies has put exploration of novel quantum states of matter at the focus of several research fields, with particular efforts towards understanding and controlling the behaviour of quantum entangled and other strongly interacting electronic states. Experimental investigation is of primary importance, but requires measurements at ultra-low temperatures where the quantum states of interest have long lifetimes. Under these conditions, low energy interactions, such as hyperfine or nuclear exchange interactions, become relevant, and can modify electronic ground states and their associated excitations in multiple ways that are not well understood or characterised. In this work, we use a recently developed magnetic susceptibility technique, compatible with ultra-low temperatures and high magnetic fields, to probe the influence of nuclear interactions on superconducting and multipole ordered ground states in the strongly correlated electron system PrOs4Sb12. We find that the multipole order develops a novel, entangled nuclear-electronic character at the lowest temperatures, which significantly modifies the phase boundary and leads to a nuclear quantum critical point. In the superconducting phase, we find that hyperfine interactions suppress superconductivity in a manner that provides evidence for superconducting pairing mediated by crystal field excitations. Our results on PrOs4Sb12 experimentally establish a new type of non-magnetic, nuclear quantum critical point, and give revealing insight into a highly unusual superconducting state. They also demonstrate more generally the feasibility of exploiting hyperfine interactions as a tuning parameter for experimental creation and investigation of a variety of quantum states and phenomena in correlated electron materials.

We performed de Haas-van Alphen (dHvA) experiments in the spin-triplet superconductor UTe2 for magnetic field along the c-axis above 15T. Three fundamental dHvA frequencies, named alpha1, alpha2 and beta corresponding to the cross sections of cylindrical Fermi surfaces (FSs) with large cyclotron effective masses (33-43 m0) were detected. No other fundamental dHvA frequencies were detected at high frequency range, suggesting a cylindrical-shaped electron FS without connecting at the Z point of the Brillouin zone. However, the existence of small pocket FSs associated with extremely heavy masses cannot be fully excluded., Comment: accepted for publication in J. Phys. Soc. Jpn. as "short notes", 3 pages, 2 figures

Lifshitz transitions are being increasingly recognised as significant in a wide variety of strongly correlated and topological materials, and understanding the origin and influence of Lifshitz transitions is leading to deeper understanding of key aspects of magnetic, transport or quantum critical behavior. In the ferromagnetic superconductor UCoGe, a magnetic field applied along the c-axis has been shown to induce a series of anomalies in both transport and thermopower that may be caused by Lifshitz transitions. The need to understand the subtleties of the relationship between magnetism, superconductivity and a heavy electron Fermi surface in the ferromagnetic superconductors makes it important to explore if and why a series of magnetic-field-induced Lifshitz transitions occurs in UCoGe. Here we report magnetic susceptibility measurements of UCoGe, performed at temperatures down to 45 mK and magnetic fields (B ||c) up to 30 T. We observe a series of clearly-defined features in the susceptibility, and multiple sets of strongly field-dependent de Haas-van Alphen oscillations, from which we extract detailed field-dependence of the quasiparticle properties. We complement our experimental results with density functional theory bandstructure calculations, and include a simple model of the influence of magnetic field on the calculated Fermi surface. By comparing experimental and calculated results, we determine the likely shape of the Fermi surface and identify candidate Lifshitz transitions that could correspond to two of the features in susceptibility. We connect these results to the development of magnetization in the system., Comment: 10 pages, 4 figures, plus Supplementary Information

We investigate the high-pressure phase of the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using transport and tunnel diode oscillator studies. We construct detailed pressure-temperature phase diagrams that indicate that outside of the nematic phase, the superconducting critical temperature reaches a minimum before it is quickly enhanced towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. Further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state despite a continuous finite superconducting transition temperature, and any remaining resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-$T_c$ phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness which can trigger a first-order transition. These high-pressure regions could be understood assuming a real-space phase separation caused by concomitant electronic and structural instabilities., Comment: 11 pages, 6 figures

The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally-dependent correlation effects, enhanced interband spin-fluctuations or a Lifshitz-like transition which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface., Comment: 22 pages, 14 figures

Non-magnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can influence significantly the transport and the superconducting behaviour. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states, and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small amount of Cu impurity, we detect a significant reduction in the mobility of the charge carriers by a factor of ~3. While the electronic conduction is strongly disrupted by Cu substitution, we identify additional signatures of anisotropic scattering which manifest in linear resistivity at low temperatures and $H^{1.6}$ dependence of magnetoresistance. The suppression of superconductivity by Cu substitution is consistent with a sign-changing $s_{\pm}$ order parameter. Additionally, in the presence of compressive strain, the superconductivity is enhanced, similar to FeSe., Comment: 16 pages, 9 figures, to appear in Physcial Review B (2022)

Metal-insulator transitions in clean, crystalline solids can be driven by two distinct mechanisms. In a conventional insulator, the charge carrier concentration vanishes, when an energy gap separates filled and unfilled electronic states. In the established picture of a Mott insulator, by contrast, electronic interactions cause coherent charge carriers to slow down and eventually stop in electronic grid-lock without materially affecting the carrier concentration itself. This description has so far escaped experimental verification by quantum oscillation measurements, which directly probe the velocity distribution of the coherent charge carriers. By extending this technique to high pressure we were able to examine the evolution of carrier concentration and velocity in the strongly correlated metallic state of the clean, crystalline material NiS$_2$, while tuning the system towards the Mott insulating phase. Our results confirm that pronounced electronic slowing down indeed governs the approach to the insulating state. However, the critical point itself, at which the carrier velocity would reach zero and the effective carrier mass diverge, is concealed by the insulating sector of the phase diagram. In the resulting, more nuanced view of Mott localisation, the inaccessibility of the low temperature Mott critical point resembles that of the threshold of magnetic order in clean metallic systems, where criticality is almost universally interrupted by first order transitions, tricritical behaviour or novel emergent phases such as unconventional superconductivity., Comment: 8 pages, 3 figures

Quantum oscillation measurements in the new unconventional superconductor YFe$_2$Ge$_2$ resolve all four Fermi surface pockets expected from band structure calculations, which predict an electron pocket in the Brillouin zone corner and three hole pockets enveloping the centers of the top and bottom of the Brillouin zone. The carrier masses are uniformly renormalized by about a factor of five and broadly account for the enhanced heat capacity Sommerfeld coefficient $\simeq 100$ mJ/molK$^2$. Our data highlight the key role of the electron pocket, which despite its small volume accounts for about half the total density of states, and point towards a predominantly local mechanism underlying the mass renormalization in YFe$_2$Ge$_2$., Comment: 5 pages, 5 figures

We present a comprehensive ultrasound study of the prototypical heavy-fermion material CeRhIn$_5$, examining the origin of the enigmatic 30 T transition. For a field applied at 2$^\circ$ from the $c$ axis, we observed two sharp anomalies in the sound velocity, at $B_m \approx$ 20 T and $B^* \approx$ 30 T, in all the symmetry-breaking ultrasound modes at low temperatures. The lower-field anomaly corresponds to the well-known first-order metamagnetic incommensurate-to-commensurate transition. The higher-field anomaly takes place at 30 T, where an electronic-nematic transition was previously suggested to occur. Both anomalies, observed only within the antiferromagnetic state, are of similar shape, but the corresponding changes of the ultrasound velocity have opposite signs. Based on our experimental results, we suggest that a field-induced magnetic transition from a commensurate to another incommensurate antiferromagnetic state occurs at $B^*$. With further increasing the field angle from the $c$ axis, the anomaly at $B^*$ slowly shifts to higher fields, broadens, and becomes smaller in magnitude. Traced up to 30$^\circ$ from the $c$ axis, it is no longer observed at 40$^\circ$ below 36 T., Comment: 9 pages, 10 figures. To be published in Phys. Bev. B

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 $\beta_1$ branch, which has not been observed so far. Comparison of our angule-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 $B^{*}\approx$ 30~T or at the putative quantum critical point at $B_c \simeq$ 50~T. Our results rather demonstrate the robustness of the Fermi surface and the localized nature of the 4$f$ electrons inside and outside of the antiferromagnetic phase., Comment: 7 pages, 4 figures + Supplemental Material (5 pages, 10 figures)

We report the magnetic structure of nominally 10% Cd-doped CeIrIn$_5$, CeIr(In$_{0.9}$Cd$_{0.1}$)$_5$, determined by elastic neutron scattering. Magnetic intensity was observed only at the ordering wave vector $Q_{AF} = (1/2,1/2,1/2)$, commensurate with the crystal lattice. A staggered moment of 0.47(3)$\mu_B$ at 1.8 K resides on the Ce ion. The magnetic moments are found to be aligned along the crystallographic $c$ axis. This is further confirmed by magnetic susceptibility data, which suggest the $c$ axis to be the easy magnetic axis. The determined magnetic structure is strikingly different from the incommensurate antiferromagnetic ordering of the closely related compound CeRhIn$_5$, in which the magnetic moments are antiferromagnetically aligned within the tetragonal basal plane., Comment: 6 pages, 5 figures. Accepted for publication in Phys. Rev. B

Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haasvan Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial Berry phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semi-classical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror (ab) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present., Comment: accepted Physical Review Research; 12 pages, 10 figures including supplemental materials

FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes while its monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16T and temperatures down to 2K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. In the thin limit we detect signatures of a crossover towards two-dimensional behaviour from the observation of the vortex-antivortex unbinding transition and strongly enhanced anisotropy. Our study provides detailed insights into the evolution of the superconducting properties from three-dimensional bulk behaviour towards the two-dimensional limit of FeSe in the absence of a dopant substrate., Comment: 18 pages, 13 figures

The phase diagram of the filled skutterudite CeOs$_4$Sb$_{12}$ has been mapped in fields $H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and MHz conductivity. The valence transition separating the semimetallic, low-$H$, low-$T$, $\cal{L}$ phase from the metallic high-$H$, high-$T$ $\cal{H}$ phase exhibits a very unusual, wedge-shaped phase boundary, with a non-monotonic gradient alternating between positive and negative. This is quite different from the text-book "elliptical" phase boundary usually followed by valence transitions. Analysis of Shubnikov-de Haas oscillations within the $\cal{H}$ phase reveals an effective mass that increases as $H$ drops toward the $\cal{H-L}$ phase boundary, suggesting proximity to a quantum-critical point. The associated magnetic fluctuations may be responsible for the anomalous $H,T$ dependence of the valence transition at high $H$, whereas the low$-H$, high$-T$ portion of the phase boundary may rather be associated with the proximity of CeOs$_4$Sb$_{12}$ to a topological semimetal phase induced by uniaxial stress., Comment: 7 pages, 4 figures and Supplementary Information of 4 pages, 2 figures

Combining strong electron correlations [1-4] and nontrivial electronic topology [5] holds great promise for discovery. So far, this regime has been rarely accessed and systematic studies are much needed to advance the field. Here we demonstrate the control of topology in a heavy fermion system. We use magnetic field to manipulate Weyl nodes in a Weyl-Kondo semimetal [6-8], up to the point where they annihilate in a topological quantum phase transition. The suppression of the topological characteristics occurs in an intact and only weakly varying strongly correlated "background". Thus, topology is changing per se and not as a consequence of a change of the correlation state, for instance across a magnetic, electronic or structural phase transition. Our demonstration of genuine topology tuning in a strongly correlated electron system sets the stage for establishing global phase diagrams of topology, an approach that has proven highly valuable to explore and understand topologically trivial strongly correlated electron systems [1-4]. Our work also lays the ground for technological exploitations of controlled electronic topology., Comment: 4 figures, 17 pages

Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe$_{1-x}$S$_x$ across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition $x$ and the impurity level. Near the nematic end point, we find an extended temperature regime with $T^{1.5}$ resistivity. The transverse magnetoresistance inside the nematic phase has as a $H^{1.55}$ dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder, Comment: 18 pages, 14 figures

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.

We report a combined study of transport and thermodynamic measurements on the layered pnictide material SrAg4As2. Upon cooling, a drop in electrical and Hall resistivity, a jump in heat capacity and an increase in susceptibility and magnetoresistance (MR) are observed around 110 K. These observations suggest that non-magnetic phase transitions emerge at around 110 K, that are likely associated with structural distortions. In sharp contrast with the first-principles calculations based on the crystal structure at room temperature, quantum oscillations reveal small Fermi pockets with light effective masses, suggesting a significant change in the Fermi surface topology caused by the low temperature structural distortion. No superconductivity emerges in SrAg$_4$As$_2$ down to 2 K and under pressures up to 2.13 GPa; instead, the low temperature structural distortion increases linearly with temperature at a rate of ~13 K/GPa above 0.89 GPa.

Relating the band structure of correlated semimetals to their transport properties is a complex and often open issue. The partial occupation of numerous electron and hole bands can result in properties that are seemingly in contrast with one another, complicating the extraction of the transport coefficients of different bands. The 5d oxide SrIrO3 hosts parabolic bands of heavy holes and light electrons in gapped Dirac cones due to the interplay between electron-electron interactions and spin-orbit coupling. We present a multifold approach relying on different experimental techniques and theoretical calculations to disentangle its complex electronic properties. By combining magnetotransport and thermoelectric measurements in a field-effect geometry with first-principles calculations, we quantitatively determine the transport coefficients of different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients, yielding a holelike response under field-effect and thermoelectric measurements and a linear, electronlike Hall effect up to 33 T., Comment: 5 pages, 4 figures

We report measurements of the de Haas-van Alphen effect in the layered heavy-fermion compound CePt$_2$In$_7$ in high magnetic fields up to 35 T. Above an angle-dependent threshold field, we observed several de Haas-van Alphen frequencies originating from almost ideally two-dimensional Fermi surfaces. The frequencies are similar to those previously observed to develop only above a much higher field of 45 T, where a clear anomaly was detected and proposed to originate from a change in the electronic structure [M. M. Altarawneh et al., Phys. Rev. B 83, 081103 (2011)]. Our experimental results are compared with band structure calculations performed for both CePt$_2$In$_7$ and LaPt$_2$In$_7$, and the comparison suggests localized $f$ electrons in CePt$_2$In$_7$. This conclusion is further supported by comparing experimentally observed Fermi surfaces in CePt$_2$In$_7$ and PrPt$_2$In$_7$, which are found to be almost identical. The measured effective masses in CePt$_2$In$_7$ are only moderately enhanced above the bare electron mass $m_0$, from 2$m_0$ to 6$m_0$., Comment: 10 pages, 10 figures, published in Phys. Rev. B

The magnetic field response of the Mott-insulating honeycomb iridate Na$_{2}$IrO$_{3}$ is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15~K$) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material $\alpha-$RuCl$_{3}$., Comment: 12 pages, 20 figures; text modified, one figure shifted to main text, table added to main text

We present a de Haas-van Alphen study of the Fermi surface of the low temperature antiferromagnet CeZn11 and its non-magnetic analogue LaZn11, measured by torque magnetometry up to fields of 33T and at temperatures down to 320 mK. Both systems possess similar de Haas-van Alphen frequencies, with three clear sets of features - ranging from 50T to 4kT - corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in CeZn11. The effective masses of the charge carriers are very light (<1 me) in LaZn11 but a factor of 2 - 4 larger in CeZn11, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for CeZn11 and find that only DFT+U calculations with U=1.5 eV, which localize the 4f states, provide a good match to the measured de Haas-van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Our study suggests that the Fermi surface of CeZn11 is very close to that of LaZn11 being dominated by Zn 3d, as the Ce 4f states are localised and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations., Comment: 13 pages, 12 figures

We investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. The unusually small and strongly elongated Fermi surface of FeSe increases monotonically with chemical pressure, x, due to the suppression of the in-plane anisotropy except for the smallest orbit which suffers a Lifshitz-like transition once nematicity disappears. Even outside the nematic phase the Fermi surface continues to increase, in stark contrast to the reconstructed Fermi surface detected in FeSe under applied external pressure. We detect signatures of orbital-dependent quasiparticle mass renomalization suppressed for those orbits with dominant dxz=yz character, but unusually enhanced for those orbits with dominant dxy character. The lack of enhanced superconductivity outside the nematic phase in FeSe1-xSx suggest that nematicity may not play the essential role in enhancing Tc in these systems., Comment: 5 pages, 3 figures

We report high magnetic field measurements of magnetic torque, thermoelectric power (TEP), magnetization, and de Haas-van Alphen (dHvA) effect in CeIrIn$_5$ across 28 T, where a metamagnetic transition was suggested in previous studies. The TEP displays two maxima at 28 T and 32 T. Above 28 T, a new, low dHvA frequency with 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., Comment: accepted to Phys. Rev. Lett

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state and elastoresistance measurements. Our high resolution ARPES allows us to track the Fermi surface deformation from four-fold to two-fold symmetry across the structural transition at ~87 K which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character. The low temperature Fermi surface is that a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility as approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice effects and, in the absence of magnetic fluctuations above the structural transition, points clearly towards an electronically driven transition in FeSe stabilized by orbital-charge ordering., Comment: Latex, 8 pages, 4 figures

Magnetoresistivity \r{ho}xx and Hall resistivity \r{ho}xy in ultra high magnetic fields up to 88T are measured down to 0.15K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel and independent approach to identify the sign of corresponding cyclotron orbit in a compensated metal from magnetotransport measurements. The observed significant differences in the relative amplitudes of the quantum oscillations between the \r{ho}xx and \r{ho}xy components, together with the positive sign of the high-field \r{ho}xy , reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron bands, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility., Comment: Latex, 4 pages (2 figures, 1 table), and supplemental material

We report the observation of the de Haas-van Alphen effect in IrTe2 measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33T. IrTe2 undergoes a major structural transition around 283 K due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than 1 me), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas-van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling IrTe2., Comment: Latex, 4 pages, 3 figures

Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to construct the field-temperature phase diagram. The results are discussed in the context of a frustrated easy-axis Heisenberg model for the localized moments where intermediate applied magnetic fields are predicted to stabilize a magnetic supersolid phase. Deviations in the measured phase diagram from this model predictions are attributed to the role played by the itinerant electrons., Comment: possible experimental signature of a magnetic supersolid phase

We report a high magnetic field study up to 55 T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$) ((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ (x=0.078(6)) with a Tc 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22 T, when the magnetic field is applied perpendicular to the ab planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition not of long-range ordered moments, but of nematic-like antiferromagnetic fluctuations., Comment: Latex, 4 figures

We demonstrate that the volume of the Fermi surface, measured very precisely using de Haas-van Alphen oscillations, can be used to probe changes in the nature and occupancy of localized electronic states. In systems with unconventional ordered states, this allows an underlying electronic order parameter to be followed to very low temperatures. We describe this effect in the field-induced antiferroquadrupolar (AFQ) ordered phase of PrOs4Sb12, a heavy fermion intermetallic compound. We find that the phase of de Haas-van Alphen oscillations is sensitively coupled, through the Fermi volume, to the configuration of the Pr f-electron states that are responsible for AFQ order. In particular, the \beta-sheet of the Fermi surface expands or shrinks as the occupancy of two competing localized Pr crystal field states changes. Our results are in good agreement with previous measurements, above 300 mK, of the AFQ order parameter by other methods. In addition, the low temperature sensitivity of our measurement technique reveals a strong and previously unrecognized influence of hyperfine coupling on the order parameter below 300 mK within the AFQ phase. Such hyperfine couplings could provide insight into the nature of hidden order states in other systems., Comment: 11 pages, 7 figures, accepted for publication in Physical Review B

We have performed high field magnetotransport measurements to investigate the interface electron gas in LaAlO3/SrTiO3 heterostructures. Shubnikov-de Haas oscillations reveal several 2D conduction subbands with carrier effective masses between 1 and 3 m_e, quantum mobilities of order 3000 cm^2/V s, and band edges only a few millielectronvolts below the Fermi energy. Measurements in tilted magnetic fields confirm the 2D character of the electron gas, and show evidence of inter-subband scattering.

We report a de Haas-van Alphen (dHvA) oscillation study of the 111 iron pnictide superconductors LiFeAs with T_c ~18K and LiFeP with T_c~5K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and shows quasi-nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs respectively., Comment: This version includes new higher resolution data. 4 pages with figures

We have studied the effect of pressure on the pyrochlore iridate Eu$_2$Ir$_2$O$_7$, which at ambient pressure has a thermally driven insulator to metal transition at $T_{MI}\sim120$\,K. As a function of pressure the insulating gap closes, apparently continuously, near $P \sim 6$\,GPa. However, rather than $T_{MI}$ going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, calling into question the true nature of both ground states. The high temperature state also crosses over near 6 GPa, from an incoherent to a conventional metal, suggesting a connection between the high and the low temperature states., Comment: 7 pages, 7 figures, updated after publication in Physical Review B

Using ac susceptibility, we have determined the pressure dependence of the metamagnetic critical end point temperature T* for field applied in the ab-plane in the itinerant metamagnet Sr3Ru2O7. We find that T* falls monotonically to zero as pressure increases, producing a quantum critical end point (QCEP) at Pc~13.6 kbar. New features are observed near the QCEP -- the slope of T* vs pressure changes at ~12.8 kbar, and weak subsidiary maxima appear on either side of the main susceptibility peak at pressures near Pc -- indicating that some new physics comes into play near the QCEP. Clear signatures of a nematic phase, however, that were seen in field-angle tuning of T*, are not observed. As T* is suppressed by pressure, the metamagnetic peak in the susceptibility remains sharp as a function of applied magnetic field. As a function of temperature, however, the peak becomes broad with only a very weak maximum, suggesting that, near the QCEP, the uniform magnetization density is not the order parameter for the metamagnetic transition.

We report a previously unobserved quantum oscillation frequency in YbRh2Si2. This is the first quantum oscillation that can definitely be assigned to the larger of the two major sheets of the Fermi surface predicted by band structure calculations. Previously observed frequencies were interpreted in terms of a large Fermi surface, which includes the Yb 4f-hole in the Fermi volume, that has been so strongly spin-split that it closely resembles the small Fermi surface, which does not include the Yb 4f -hole in the Fermi volume. The new frequency can also be incorporated into this picture, however there are some indications that the a need for more advanced energy band calculations., Comment: 5 pages, 3 figures, submitted to Physica Status Solidi on 24 September 2009, fixed lingering issues with figures

We report transport and thermodynamic properties of stoichiometric single crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows an unusual "non-metallic" dependence on temperature T, rising continuously with decreasing T from ~ 800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, rho(T) = rho_0 - AT^x with x<1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, gamma ~ 30 mJ/mol K^2. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T -> 0 properties suggest a new class of non-Fermi liquid., Comment: 6 pages, 4 figures

Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh2Si2. We have carried out de Haas-van Alphen measurements in the magnetic-field range 8 T <= H <= 16 T, and directly observe field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field "large" Fermi surface, including the Yb 4f quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at H0 ~ 10 T, without requiring 4f localization at H0., Comment: 4 pages, 5 figures, 1 table

We present quantum oscillation measurements of YbRh2Si2 at magnetic fields above the Kondo-suppression scale H0 ~ 10 T. Comparison with electronic structure calculations is complicated because the "small" Fermi surface, where the Yb 4f-quasi-hole is not contributing to the Fermi volume, and "large" Fermi surface, where the Yb 4f-quasi-hole is contributing to the Fermi volume, are related by a rigid Fermi energy shift. This means that spin-split branches of the large Fermi surface can look like unsplit branches of the small surface, and vice-versa. Thus, although the high-field angle dependence of the experimentally-measured oscillation frequencies most resembles the electronic structure prediction for the small Fermi surface, this may instead be a branch of the spin-split large Fermi surface., Comment: 4 pages, 2 figures, LT25 conference proceeding, submitted to Journal of Physics: Conference Series

Combining old and new de Haas-van Alphen (dHvA) and magnetoresistance data, we arrive at a detailed picture of the Fermi surface of the heavy fermion superconductor UPt3. Our work was partially motivated by a new proposal that two 5f valence electrons per formula unit in UPt3 are localized by correlation effects -- agreement with previous dHvA measurements of the Fermi surface was invoked in its support. Comprehensive comparison with our new observations shows that this 'partially localized' model fails to predict the existence of a major sheet of the Fermi surface, and is therefore less compatible with experiment than the originally proposed 'fully itinerant' model of the electronic structure of UPt3. In support of this conclusion, we offer a more complete analysis of the fully itinerant band structure calculation, where we find a number of previously unrecognized extremal orbits on the Fermi surface., Comment: 23 pages, 12 figures, latex, iopart class

Quantum phase transitions (QPTs) have been studied extensively in correlated electron systems. Characterization of magnetism at QPTs has, however, been limited by the volume-integrated feature of neutron and magnetization measurements and by pressure uncertainties in NMR studies using powderized specimens. Overcoming these limitations, we performed muon spin relaxation ($\mu$SR) measurements which have a unique sensitivity to volume fractions of magnetically ordered and paramagnetic regions, and studied QPTs from itinerant heli/ferro magnet to paramagnet in MnSi (single-crystal; varying pressure) and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$ (ceramic specimens; varying $x$). Our results provide the first clear evidence that both cases are associated with spontaneous phase separation and suppression of dynamic critical behavior, revealed a slow but dynamic character of the ``partial order'' diffuse spin correlations in MnSi above the critical pressure, and, combined with other known results in heavy-fermion and cuprate systems, suggest a possibility that a majority of QPTs involve first-order transitions and/or phase separation., Comment: 11 pages, 4 figures, 21 authors, to appear in Nature Physics

We report a metamagnetic critical point at (B*,T*)=(5.1T,1.1K) for magnetic fields applied perpendicular to the (tetragonal) c-axis of Sr3Ru2O7. First-order behaviour well below T* indicates that (B*,T*) marks a critical end-point that terminates a line of first-order metamagnetic transitions. The absence of first-order behaviour in the metamagnetic transition with B||c confirms that the non-Fermi liquid behaviour for B||c is underpinned by a metamagnetic quantum critical end-point for which T*->0. Scaling behaviour of the resistivity under hydrostatic pressure yields the surprising result that although B* increases rapidly with pressure, T* has only a weak pressure dependence., Comment: 4 pages incl 4 eps figures