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In RuCl$_3$, inelastic neutron scattering and Raman spectroscopy reveal a continuum of non-spin-wave excitations that persists to high temperature, suggesting the presence of a spin liquid state on a honeycomb lattice. In the context of the Kitaev model, magnetic fields introduce finite interactions between the elementary excitations, and thus the effects of high magnetic fields - comparable to the spin exchange energy scale - must be explored. Here we report measurements of the magnetotropic coefficient - the second derivative of the free energy with respect to magnetic field orientation - over a wide range of magnetic fields and temperatures. We find that magnetic field and temperature compete to determine the magnetic response in a way that is independent of the large intrinsic exchange interaction energy. This emergent scale-invariant magnetic anisotropy provides evidence for a high degree of exchange frustration that favors the formation of a spin liquid state in RuCl$_3$., Comment: arXiv admin note: substantial text overlap with arXiv:1901.09245. Nature Physics

Magnetoelastic dilatometry of the piezomagnetic antiferromagnet UO(2) was performed via the fiber Bragg grating method in magnetic fields up to 150 T generated by a single-turn coil setup. We show that in microsecond timescales, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from 10 to 300 K, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100-µs observation window, are attributed to the strong magnetoelastic coupling in UO(2) combined with the high crystalline quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known nonmonotonic magnetostriction background. A clear phase shift of π in the lattice oscillations is observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch field H [Formula: see text] T. We present a theoretical argument that explains this unexpected behavior as a result of the reversal of the antiferromagnetic order parameter at H [Formula: see text].

Spin-valley locking in monolayer transition metal dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a Dirac semimetal BaMnSb2. This is revealed by comprehensive studies using first principles calculations, tight-binding and effective model analyses, angle-resolved photoemission spectroscopy measurements. Moreover, this material also exhibits a stacked quantum Hall effect (QHE). The spin-valley degeneracy extracted from the QHE is close to 2. This result, together with the Landau level spin splitting, further confirms the spin-valley locking picture. In the extreme quantum limit, we also observed a plateau in the z-axis resistance, suggestive of a two-dimensional chiral surface state present in the quantum Hall state. These findings establish BaMnSb2 as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states., Valley dependent spin polarization called spin-valley locking appears in absence of magnetism but it is limited to rare examples of transition metal dichalcogenides. Here, the authors report evidence of spin-valley locking and stacked quantum Hall effect in a bulk Dirac semimetal BaMnSb2.

MnBi${}_{2}$Te${}_{4}$ and MnSb${}_{2}$Te${}_{4}$ are the first examples of antiferromagnetic topological insulators. However, the presence of magnetic defects in the form of antisite mixing between Mn and Bi or Sb introduces defect-driven ferrimagnetism. The application of a large magnetic field of ~50 Tesla aligns the magnetic defects with the main Mn layer, allowing for estimates of the defect concentration and the strength of the antiferromagnetic coupling (${J}^{\ensuremath{'}}$) between Mn defects and the main layer, which is by far the largest magnetic coupling in the system.

Using time-domain terahertz spectroscopy in pulsed magnetic fields up to 31 T, we measure the terahertz optical conductivity in an optimally doped thin film of the high-temperature superconducting cuprate ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_{4}$. We observe systematic changes in the circularly polarized complex optical conductivity that are consistent with cyclotron absorption of $p$-type charge carriers characterized by a cyclotron mass of $4.9{m}_{\mathrm{e}}\ifmmode\pm\else\textpm\fi{}0.8{m}_{\mathrm{e}}$ and a scattering rate that increases with magnetic field. These results open the door to studies aimed at characterizing the degree to which electron-electron interactions influence carrier masses in cuprate superconductors.

Magnetoelastic measurements in the piezomagnetic antiferromagnet UO$_{2}$ were performed via the fiber Bragg grating method in magnetic fields up to $150\,\mathrm{T}$ generated by a single-turn coil setup. We show that in short timescales, order of a few micro seconds, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from $10\,\mathrm{K}$ to $300\,\mathrm{K}$, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100 ms observations, are attributed to the strong magnetoelastic coupling in UO$_{2}$ combined with the high crystallographic quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known non-monotonic magnetostriction background. A clear phase-shift of $\pi$ in the lattice oscillations is, unexpectedly, observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch-field $H_c \simeq -18\,\mathrm{T}$. We further present simulations and a theoretical argument to explain the observed phenomena., Comment: 9 pages, 5 figures

High magnetic fields have revealed a surprisingly small Fermi surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue concerns the doping extent of such a state and its relationship to the principal pseudogap and superconducting phases. We employ pulsed magnetic-field measurements on the cuprate [Formula: see text] Cu [Formula: see text] to identify signatures of Fermi-surface reconstruction from a sign change of the Hall effect and a peak in the temperature-dependent planar resistivity. We trace the termination of Fermi-surface reconstruction to two hole concentrations where the superconducting upper critical fields are found to be enhanced. One of these points is associated with the pseudogap endpoint near optimal doping. These results connect the Fermi-surface reconstruction to both superconductivity and the pseudogap phenomena.

The rare-earth tritellurides ($R$Te$_3$, where $R$ = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y) form a charge density wave state consisting of a single unidirectional charge density wave for lighter $R$, with a second unidirectional charge density wave, perpendicular and in addition to the first, also present at low temperatures for heavier $R$. We present a quantum oscillation study in magnetic fields up to 65T that compares the single charge density wave state with the double charge density wave state both above and below the magnetic breakdown field of the second charge density wave. In the double charge density wave state it is observed that there remain several small, light pockets with the largest occupying around 0.5% of the Brillouin zone. By applying magnetic fields above the independently determined magnetic breakown field, the quantum oscillation frequencies of the single charge density wave state are recovered, as expected in a magnetic breakdown scenario. Measurements of the electronic effective mass do not show any divergence or significant increase on the pockets of Fermi surface observed here as the putative quantum phase transition between the single and double charge density wave states is approached., Comment: 11 pages, 7 figures

Author(s): Breznay, NP; Hayes, IM; Nair, NL; Helm, T; Analytis, JG; McDonald, RD; Zhu, Z; Krockenberger, Y; Irie, H; Yamamoto, H; Modic, KA; Frano, A; Shafer, P; Arenholz, E | Abstract: The discovery of quantum oscillations in hole- A nd electron-doped cuprate families has underscored the importance of the Fermi surface in cuprate superconductivity. While the observed quantum oscillations in both families have revealed the presence of reconstructed Fermi surfaces, there remains an important distinction between the two. In hole-doped cuprates the oscillations are thought to arise from the effects of a charge density wave, while in the electron-doped cuprates it is thought that these oscillations occur from an antiferromagnetically reconstructed Fermi surface, despite the fact that the oscillations are observed in overdoped compounds, far from the putative antiferromagnetic critical point. In this work we study thin films of Pr2CuO4±δ, whose apparent doping can be finely tuned by annealing, allowing studies of quantum oscillations in samples straddling the critical point. We show that even though there is a mass enhancement of the quasiparticles, there are only small changes to the Fermi surface itself, suggesting that charge order is a more likely origin, with electronic correlations that are strongly dependent on the structural parameters.

The field of topological electronic materials has seen rapid growth in recent years, in particular with the increasing number of weakly interacting systems predicted and observed to host topologically non-trivial bands. Given the broad appearance of topology in such systems, it is expected that correlated electronic systems should also be capable of hosting topologically non-trivial states. Interest in correlated platforms is heightened by the prospect that collective behavior therein may give rise to new types of topological states and phenomena not possible in non-interacting systems. However, to date only a limited number of correlated topological materials have been definitively reported due to both the challenge in calculation of their electronic properties and the experimental complexity of correlation effects imposed on the topological aspects of their electronic structure. Here, we report a de Haas-van Alphen (dHvA) study of the recently discovered kagome metal Fe$_3$Sn$_2$ mapping the massive Dirac states strongly coupled to the intrinsic ferromagnetic order. We observe a pair of quasi-two-dimensional Fermi surfaces arising from the massive Dirac states previously detected by spectroscopic probes and show that these band areas and effective masses are systematically modulated by the rotation of the ferromagnetic moment. Combined with measurements of Berry curvature induced Hall conductivity, we find that along with the Dirac fermion mass, velocity, and energy are suppressed with rotation of the moment towards the kagome plane. These observations demonstrate that strong coupling of magnetic order to electronic structure similar to that observed in elemental ferromagnets can be extended to topologically non-trivial electronic systems, suggesting pathways for connecting topological states to robust spintronic technologies., Comment: 16 pages, 4 figures; corrected typos and labeling

Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{3}$$\end{document}3Sn\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2S\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect. We show that the coupling of this material’s topological properties to its magnetic texture leads to a strongly exchange biased anomalous Hall effect. We argue that this is likely caused by the coexistence of ferromagnetism and geometric frustration intrinsic to the kagome network of magnetic ions, giving rise to spin-glass behavior and an exchange bias., The Weyl semimetal Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{3}$$\end{document}3Sn\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2S\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2 exhibits a combination of magnetic ordering with a large anomalous Hall effect. Lachman et al. find an intrinsic exchange bias of this anomalous Hall effect and attribute it to the coexistence of ferromagnetism and spin glass behaviour.

Time reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in URu2Si2 at cryogenic temperatures and magnetic fields more than enough to suppress the hidden order state at H_HO(T = 0.66 K) approximately 35 T. We report a significant crystal lattice volume expansion at and above H_HO(T), and even above T_HO, possibly a consequence of field-induced f-electron localization, and hysteresis at some high field phase boundaries that confirm volume involvement. We investigate in detail the magnetostriction and magnetization as the temperature is reduced over two decades from 50 K where the system is paramagnetic, to 0.5 K in the realms of the hidden order state. We find a dominant quadratic-in-field dependence delta L/L proportional to H^2, a result consistent with a state that is symmetric under time reversal. The data shows, however, an incipient yet unmistakable asymptotic approach to linear (delta L/L proportional to 1-H/H_0) for 15 T < H < H_HO(0.66 K) approximately 35 T at the lowest temperatures. We discuss these results in the framework of a Ginzburg-Landau formalism that proposes a complex order parameter for the HO to model the (H,T,p) phase diagram.

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The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice - Dirac fermions and topological flat bands - have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization- and termination-dependent angle-resolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

In the iron-based superconductors, both nematic and magnetic fluctuations are expected to enhance superconductivity and may originate from a quantum critical point hidden beneath the superconducting dome. The behavior of the non-superconducting state can be an important piece of the puzzle, motivating in this paper the use of high magnetic fields to suppress superconductivity and measure the nematic susceptibility of the normal state at low temperatures. We describe experimental advances which make it possible to measure a resistive gauge factor (which is a proxy for the nematic susceptibility) in the field-induced normal state in a 65 T pulsed magnet, and report measurements of the gauge factor of a micromachined single crystal of Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$ at temperatures down to 1.2 K. The nematic susceptibility increases monotonically in the field-induced normal state as the temperature decreases, consistent with the presence of a quantum critical point nearby in composition., Comment: 11 pages, 11 figures

We present a combined experimental and theoretical study of the evolution of the Fermi surface of the anomalous superconductor ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Tl}}_{x}\mathrm{Te}$ as a function of thallium concentration, drawing on a combination of magnetotransport measurements (Shubnikov--de Haas oscillations and the Hall coefficient), angle resolved photoemission spectroscopy, and density functional theory calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value, the Fermi energy coincides with resonant impurity states in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Tl}}_{x}\mathrm{Te}$, and we rule out the presence of an additional valence band maximum at the Fermi energy. A comparison to nonsuperconducting ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Te}$ implies that the presence of these impurity states at the Fermi energy provides the enhanced pairing interaction and thus also the anomalously high temperature superconductivity in this material.

The phenomenon of T-linear resistivity commonly observed in a number of strange metals has been widely seen as evidence for the breakdown of the quasiparticle picture of metals. This study shows that a recently discovered H/T scaling relationship in the magnetoresistance of the strange metal BaFe_{2}(As_{1-x}P_{x})_{2} is independent of the relative orientations of current and magnetic field. Rather, its magnitude and form depend only on the orientation of the magnetic field with respect to a single crystallographic axis: the direction perpendicular to the magnetic iron layers. This finding suggests that the magnetotransport scaling does not originate from the conventional averaging or orbital velocity of quasiparticles as they traverse a Fermi surface, but rather from dissipation arising from two-dimensional correlations.

The observation of a reconstructed Fermi surface via quantum oscillations in hole-doped cuprates opened a path towards identifying broken symmetry states in the pseudogap regime. However, such an identification has remained inconclusive due to the multi-frequency quantum oscillation spectra and complications accounting for bilayer effects in most studies. We overcome these impediments with high-resolution measurements on the structurally simpler cuprate HgBa2CuO4+δ (Hg1201), which features one CuO2 plane per primitive unit cell. We find only a single oscillatory component with no signatures of magnetic breakdown tunnelling to additional orbits. Therefore, the Fermi surface comprises a single quasi-two-dimensional pocket. Quantitative modelling of these results indicates that a biaxial charge density wave within each CuO2 plane is responsible for the reconstruction and rules out criss-crossed charge stripes between layers as a viable alternative in Hg1201. Lastly, we determine that the characteristic gap between reconstructed pockets is a significant fraction of the pseudogap energy., The identification of broken symmetry states in underdoped cuprate superconductors via quantum oscillation measurements remains inconclusive. Here, Chan et al. report the reconstructed Fermi surface of HgBa2CuO4+δ comprises only a single pocket indicating a biaxial charge-density-wave order within each CuO2 plane.

The title compounds are bimetallic MOFs containing [Cu(pyz)2]^2+ square lattices linked by MF6^n- octahedra. In each, only the Cu^2+ spins exhibit long-range magnetic order below 3.5 K (M = V^4+) and 2.6 K (M = Ga^3+). The V^4+ spins remain disordered down to 0.5 K.

High magnetic fields have revealed a surprisingly small Fermi-surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue concerns the doping extent of this state and its relationship to the principal pseudogap and superconducting phases. We employ pulsed magnetic field measurements on the cuprate HgBa$_2$CuO$_{4+\delta}$ to identify signatures of Fermi surface reconstruction from a sign change of the Hall effect and a peak in the temperature-dependent planar resistivity. We trace the termination of Fermi-surface reconstruction to two hole concentrations where the superconducting upper critical fields are found to be enhanced. One of these points is associated with the pseudogap end-point near optimal doping. These results connect the Fermi-surface reconstruction to both superconductivity and the pseudogap phenomena., Comment: 5 pages. 3 Figures. PNAS (2020)

PbTe is a leading mid-range thermoelectric material with a $zT$ that has been enhanced by, amongst other methods, band engineering. Here we present an experimental study of the Hall effect, quantum oscillations, specific heat, and electron microprobe analysis that explores the evolution of the electronic structure of PbTe heavily doped with the `ideal' acceptor Na up to the solubility limit. We identify two phenomenological changes that onset as the electronic structure deviates from a Kane-type dispersion at around 180meV; a qualitative change in the field dependence of the Hall effect indicative of an increase in the high-field limit and a change in the Fermiology, and a sharp increase in the density of states as a function of energy. Following consideration of three possible origins for the observed phenomenology we conclude that the most likely source is non-ellipsoidicity of the $L$-pocket upon approach to a saddle point in the band structure, which is evidenced directly by our quantum oscillation measurements. Comparison to density functional theory calculations imply that this evolution of the electronic structure may be a key contributor to the large thermopower in PbTe., 11 pages, 10 figures

The phenomenon of T-linear resistivity commonly observed in a number of strange metals has been widely seen as evidence for the breakdown of the quasiparticle picture of metals. This study shows that a recently discovered H/T scaling relationship in the magnetoresistance of the strange metal BaFe_{2}(As_{1-x}P_{x})_{2} is independent of the relative orientations of current and magnetic field. Rather, its magnitude and form depend only on the orientation of the magnetic field with respect to a single crystallographic axis: the direction perpendicular to the magnetic iron layers. This finding suggests that the magnetotransport scaling does not originate from the conventional averaging or orbital velocity of quasiparticles as they traverse a Fermi surface, but rather from dissipation arising from two-dimensional correlations.

Strong magnetic field induces at least two phase transitions in graphite beyond the quantum limit where many-body effects are expected. We report on a study using a state-of-the-art non-destructive magnet allowing to attain 90.5 T at 1.4 K, which reveals a new field-induced phase and evidence that the insulating state destroyed at 75 T is an excitonic condensate of electron-hole pairs. By monitoring the angle dependence of in-plane and out-of-plane magnetoresistance, we distinguish between the role of cyclotron and Zeeman energies in driving various phase transitions. We find that, with the notable exception of the transition field separating the two insulating states, the threshold magnetic field for all other transitions display an exact cosine angular dependence. Remarkably, the threshold field for the destruction of the second insulator (phase B) is temperature-independent with no detectable Landau-level crossing nearby. We conclude that the field-induced insulator starts as a weak-coupling spin-density-wave, but ends as a strong-coupling excitonic insulator of spin-polarized electron-hole pairs., Main text: 6 pages, 4 figures. Supplemental Materials included

We synthesize single crystals of ${\mathrm{PuB}}_{4}$ using an Al-flux technique. Single-crystal diffraction data provide structural parameters for first-principles density functional theory (DFT) calculations. By computing the density of states, the ${Z}_{2}$ topological invariant using the Wilson loop method, and the surface electronic structure from slab calculations, we find that ${\mathrm{PuB}}_{4}$ is a nonmagnetic strong topological insulator with a band gap of 254 meV. Our magnetic susceptibility, heat capacity, and resistivity measurements are consistent with this analysis, albeit with a smaller gap of 35 meV. DFT plus dynamical mean-field theory calculations show that electronic correlations reduce the size of the band gap, and provide better agreement with the value determined by resistivity. These results demonstrate that ${\mathrm{PuB}}_{4}$ is a promising actinide material to investigate the interplay of electronic correlations and nontrivial topology.

We have studied the electronic structure of electron-doped cuprate superconductors via measurements of high-field Shubnikov-de Haas oscillations in thin films. In optimally doped Pr$_{2-x}$Ce$_{x}$CuO$_{4\pm\delta}$ and La$_{2-x}$Ce$_{x}$CuO$_{4\pm\delta}$, quantum oscillations indicate the presence of a small Fermi surface, demonstrating that electronic reconstruction is a general feature of the electron-doped cuprates, despite the location of the superconducting dome at very different doping levels. Negative high-field magnetoresistance is correlated with an anomalous low-temperature change in scattering that modifies the amplitude of quantum oscillations. This behavior is consistent with effects attributed to spin fluctuations.

Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac–fermion excitations.

Unusual behavior of quantum materials commonly arises from their effective low-dimensional physics, which reflects the underlying anisotropy in the spin and charge degrees of freedom. Torque magnetometry is a highly sensitive technique to directly quantify the anisotropy in quantum materials, such as the layered high-T$_c$ superconductors, anisotropic quantum spin-liquids, and the surface states of topological insulators. Here we introduce the magnetotropic coefficient $k=\partial^2 F/\partial \theta^2$, the second derivative of the free energy F with respect to the angle $\theta$ between the sample and the applied magnetic field, and report a simple and effective method to experimentally detect it. A sub-$\mu$g crystallite is placed at the tip of a commercially available atomic force microscopy cantilever, and we show that $k$ can be quantitatively inferred from a shift in the resonant frequency under magnetic field. While related to the magnetic torque $\tau=\partial F/\partial \theta$, $k$ takes the role of torque susceptibility, and thus provides distinct insights into anisotropic materials akin to the difference between magnetization and magnetic susceptibility. The thermodynamic coefficient $k$ is discontinuous at second-order phase transitions and subject to Ehrenfest relations with the specific heat and magnetic susceptibility. We apply this simple yet quantitative method on the exemplary cases of the Weyl-semimetal NbP and the spin-liquid candidate RuCl$_3$, yet it is broadly applicable in quantum materials research., Comment: 7 pages including 6 figures and methods section

In the quest for superconductors with high transition temperatures (T$_\mathrm{c}$s), one emerging motif is that unconventional superconductivity is enhanced by fluctuations of a broken-symmetry phase near a quantum-critical point. While recent experiments have suggested the existence of the requisite broken symmetry phase in the high-T$_\mathrm{c}$ cuprates, the signature of quantum-critical fluctuations in the electronic structure has thus far remained elusive, leaving their importance for high-T$_\mathrm{c}$ superconductivity in question. We use magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+$_\delta$ over an unprecedented range of doping, and magnetic quantum oscillations reveal a strong enhancement in the quasiparticle effective mass toward optimal doping. This mass enhancement is a characteristic signature of quantum criticality, and identifies a quantum-critical point at p$_{crit}$ $\approx$ 0.18. This point also represents the juncture of the vanishing pseudogap energy scale and the disappearance of Kerr rotation, the negative Hall coefficient, and the recently observed charge order, suggesting a mechanism of high-T$_\mathrm{c}$ that is strongest when these definitive experimental signatures of the underdoped cuprates converge at a quantum critical point., Comment: 4 figures. Supplementary info (15 figures). Science. Published online 26 March 2015

We present a detailed study of the low-temperature and high-magnetic-field phases in the chemical substitution series ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2\ensuremath{-}x}{\mathrm{P}}_{x}$ using electrical transport and magnetization in pulsed magnetic fields up to 65 T. Within the hidden-order $x$ regime ($0lx\ensuremath{\lesssim}0.035$) the field-induced ordering that was earlier seen for $x$ = 0 is robust, even as the hidden-order temperature is suppressed. Earlier work shows that for $0.035\ensuremath{\lesssim}x\ensuremath{\lesssim}0.26$ there is a Kondo lattice with a no-ordered state that is replaced by antiferromagnetism for $0.26\ensuremath{\lesssim}x\ensuremath{\lesssim}0.5$. We observe a simplified continuation of the field-induced order in the no-order $x$ regime and an enhancement of the field-induced order upon the destruction of the antiferromagnetism with magnetic field. These results closely resemble what is seen for ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{Si}}_{2}$,1 from which we infer that charge tuning dominantly controls the ground state of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$, regardless of whether $s$/$p$ or $d$ electrons are replaced. Contraction of the unit-cell volume may also play a role at large $x$. This provides guidance for determining the specific factors that lead to hidden order versus magnetism in this family of materials and constrains possible models for hidden order.The concentration in this paper is defined as ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{Si}}_{2}$ while the chemical formula in the literature is given as $\mathrm{U}({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}$)${}_{2}{\mathrm{Si}}_{2}$ [28--30].

Metals containing cerium exhibit a diverse range of fascinating phenomena including heavy fermion behavior, quantum criticality, and novel states of matter such as unconventional superconductivity. The cubic system CeIn3 has attracted significant attention as a structurally isotropic Kondo lattice material possessing the minimum required complexity to still reveal this rich physics. By using magnetic fields with strengths comparable to the crystal field energy scale, we illustrate a strong field-induced anisotropy as a consequence of non-spherically symmetric spin interactions in the prototypical heavy fermion material CeIn3. This work demonstrates the importance of magnetic anisotropy in modeling f-electron materials when the orbital character of the 4f wavefunction changes (e.g., with pressure or composition). In addition, magnetic fields are shown to tune the effective hybridization and exchange interactions potentially leading to new exotic field tuned effects in f-based materials., npj Quantum Materials, 2 (1), ISSN:2397-4648

Although crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches exist for spatially modulating their properties. In this study, we demonstrate disorder-free control, on the micrometer scale, over the superconducting state in samples of the heavy-fermion superconductor CeIrIn

The anomalous metallic state in high-temperature superconducting cuprates is masked by the onset of superconductivity near a quantum critical point. Use of high magnetic fields to suppress superconductivity has enabled a detailed study of the ground state in these systems. Yet, the direct effect of strong magnetic fields on the metallic behavior at low temperatures is poorly understood, especially near critical doping, $x=0.19$. Here we report a high-field magnetoresistance study of thin films of \LSCO cuprates in close vicinity to critical doping, $0.161\leq x\leq0.190$. We find that the metallic state exposed by suppressing superconductivity is characterized by a magnetoresistance that is linear in magnetic field up to the highest measured fields of $80$T. The slope of the linear-in-field resistivity is temperature-independent at very high fields. It mirrors the magnitude and doping evolution of the linear-in-temperature resistivity that has been ascribed to Planckian dissipation near a quantum critical point. This establishes true scale-invariant conductivity as the signature of the strange metal state in the high-temperature superconducting cuprates., 10 pages, 3 figures

The excitonic insulator phase has long been predicted to form in proximity to a band gap opening in the underlying band structure. The character of the pairing is conjectured to crossover from weak (BCS-like) to strong coupling (BEC-like) as the underlying band structure is tuned from the metallic to the insulating side of the gap opening. Here we report the high-magnetic field phase diagram of graphite to exhibit just such a crossover. By way of comprehensive angle-resolved magnetoresistance measurements, we demonstrate that the underlying band gap opening occurs inside the magnetic field-induced phase, paving the way for a systematic study of the BCS-BEC-like crossover by means of conventional condensed matter probes.

Magneto-quantum oscillation experiments in high temperature superconductors show a strong thermally-induced suppression of the oscillation amplitude approaching critical dopings---in support of a quantum critical origin of their phase diagrams. We suggest that, in addition to a thermodynamic mass enhancement, these experiments may directly indicate the increasing role of quantum fluctuations that suppress the oscillation amplitude through inelastic scattering. We show that the traditional theoretical approaches beyond Lifshitz-Kosevich to calculate the oscillation amplitude in correlated metals result in a contradiction with the third law of thermodynamics and suggest a way to rectify this problem., PRB Rapid commun. (2017)

Controlled access to the border of the Mott insulating state by variation of control parameters offers exotic electronic states such as anomalous and possibly high-transition-temperature (Tc) superconductivity. The alkali-doped fullerides show a transition from a Mott insulator to a superconductor for the first time in three-dimensional materials, but the impact of dimensionality and electron correlation on superconducting properties has remained unclear. Here we show that, near the Mott insulating phase, the upper critical field Hc2 of the fulleride superconductors reaches values as high as ∼90 T—the highest among cubic crystals. This is accompanied by a crossover from weak- to strong-coupling superconductivity and appears upon entering the metallic state with the dynamical Jahn–Teller effect as the Mott transition is approached. These results suggest that the cooperative interplay between molecular electronic structure and strong electron correlations plays a key role in realizing robust superconductivity with high-Tc and high-Hc2., Alkali-doped fullerides are superconductors but the impact of dimensionality and electron correlation remains unclear. Here, Kasahara et al. report an upper critical field about 90 T, suggesting cooperative interplay between molecular electronic structure and strong electron correlations.

The Fermi surface of elemental bismuth consists of three small rotationally equivalent electron pockets, offering a valley degree of freedom to charge carriers. A relatively small magnetic field can confine electrons to their lowest Landau level. This is the quantum limit attained in other dilute metals upon application of sufficiently strong magnetic field. Here, we report on the observation of another threshold magnetic field never encountered before in any other solid. Above this field, $B_{\rm{empty}}$, one or two valleys become totally empty. Drying up a Fermi sea by magnetic field in the Brillouin zone leads to a manyfold enhancement in electric conductance. We trace the origin of the large drop in magnetoresistance across $B_{\rm{empty}}$ to transfer of carriers between valleys with highly anisotropic mobilities. The non-interacting picture of electrons with field-dependent mobility explains most results. Coulomb interaction may play a role in shaping the fine details., Comment: 19 pages, 5 figures, Supplemental Material available upon request

The title compounds are bimetallic MOFs containing [Cu(pyz)

We study magnetotransport properties of the electron-doped superconductor ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ with $x=0.14$ in magnetic fields up to 92 T, and observe Shubnikov-de Haas magnetic quantum oscillations. The oscillations display a single frequency $F=255\ifmmode\pm\else\textpm\fi{}10$ T, indicating a small Fermi pocket that is $\ensuremath{\sim}1%$ of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large holelike cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. Our study demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.

We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (Tc ~ 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor ~2. The quasi-two dimensional Fermi surface consist of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability., to appear in Physical Review Letters

[Ni(HF2)(pyz)2]X {pyz = pyrazine; X = PF6- (1), SbF6- (2)} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN4F2 octahedra. At 298 K, 1 is monoclinic (C2/c) with unit cell parameters, a = 9.9481(3), b = 9.9421(3), c = 12.5953(4) angstrom, and beta = 81.610(3) degrees while 2 is tetragonal (P4/nmm) with a = b = 9.9359(3) and c = 6.4471(2) angstrom and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the c-axis which are linked via mu-pyz bridges in the ab-plane to afford three-dimensional polymeric frameworks with PF6- and SbF6- counterions occupying the interior sites. A major difference between 1 and 2 is that the Ni-F-H bonds are bent (similar to 157 degrees) in 1 but are linear in 2. Ligand field calculations (LFT) based on an angular overlap model (AOM), with comparison to the electronic absorption spectra, indicate greater pi-donation of the HF2- ligand in 1 owing to the bent Ni-F-H bonds. Magnetic susceptibility data for 1 and 2 exhibit broad maxima at 7.4 and 15 K, respectively, and lambda-like peaks in d chi T/dT at 6.2 and 12.2 K that are ascribed to transitions to long-range antiferromagnetic order (TN). Muon-spin relaxation and specific heat studies confirm these TN's. A comparative analysis of chi vs T to various 1D Heisenberg/Ising models suggests moderate antiferromagnetic interactions, with the primary interaction strength determined to be 3.05/3.42 K (1) and 5.65/6.37 K (2). However, high critical fields of 19 and 37.4 T obtained from low temperature pulsed-field magnetization data indicate that a single exchange constant (J(1D)) alone is insufficient to explain the data and that residual terms in the spin Hamiltonian, which could include interchain magnetic couplings (J(perpendicular)), as mediated by Ni-pyz-Ni, and single-ion anisotropy (D), must be considered. While it is difficult to draw absolute conclusions regarding the magnitude (and sign) of J(perpendicular) and D based solely on powder data, further support offered by related Ni(II)-pyz compounds and our LFT and density-functional theory (DFT) results lead us to a consistent quasi-1D magnetic description for 1 and 2.

Although the isotope effect in superconducting materials is well-documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the possible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the N\'{e}el temperature and critical magnetic fields ($\Delta T_{\rm N}/T_{\rm N}\approx 4%$; $\Delta B_{\rm c}/B_{\rm c}\approx 4%$) in a Q2D organic molecular antiferromagnets on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and inter-plane exchange energies evolve as the hydrogens on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds.