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Since the 1985 discovery of the phase transition at $T_{\rm HO}=17.5$ K in the heavy-fermion metal URu$_2$Si$_2$, neither symmetry change in the crystal structure nor magnetic ordering have been observed, which makes this "hidden order" enigmatic. Some high-field experiments have suggested electronic nematicity which breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state at zero magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at $T_{\rm HO}$ with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure sample, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice., Comment: 17 pages, 3 figures. Submitted version. Revisions have been made through the review process. See the published version in Nature Communications

The observation of cyclotron resonance in ultra-clean crystals of URu2Si2 [S. Tonegawa et al., PRL 109, 036401 (2012)] provides another route besides quantum oscillations to the determination of the bulk electronic structure in the hidden order phase. We report detailed analyses of the resonance lines, which fully resolve the cyclotron mass structure of the main Fermi surface sheets. A particular focus is given to the anomalous splitting of the sharpest resonance line near the [110] direction under in-plane magnetic-field rotation, which implies peculiar electronic structure in the hidden order phase. The results under the field rotation from [110] toward [001] direction reveal that the splitting is a robust feature against field tilting from the basal plane. This is in sharp contrast to the reported frequency branch alpha in the quantum oscillation experiments showing a three-fold splitting that disappears by a small field tilt, which can be explained by the magnetic breakdown between the large hole sphere and small electron pockets. Our analysis of the cyclotron resonance profiles reveals that the heavier branch of the split line has a larger scattering rate, providing evidence for the existence of hot-spot regions along the [110] direction. These results are consistent with the broken fourfold rotational symmetry in the hidden-order phase, which can modify the interband scattering in an asymmetric manner. We also extend our measurements down to 0.7 K, which results in the observation of cyclotron resonance in the superconducting state, where novel effects of vortex dynamics may enter. We find that the cyclotron mass undergoes no change in the superconducting state. In contrast, the quasiparticle scattering rate shows a rapid decrease below the vortex-lattice melting transition temperature, which supports the formation of quasiparticle Bloch state in the vortex lattice phase., Comment: 15 pages, 12 figures

In Mott insulators, the strong electron-electron Coulomb repulsion prevents metallicity and charge excitations are gapped. 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 (QSL) states. However, their magnetic ground states have been a long-standing mystery. Here we show that a QSL state in the organic Mott insulator EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ 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) reveal distinct residual paramagnetic susceptibility comparable to that in a half-filled two-dimensional metal. This demonstrates that the system is in a magnetically gapless ground state, a critical state with infinite magnetic correlation length. 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., Comment: 7 pages, 3 figures. Published in Nature communication

We report the first observation of cyclotron resonance in the hidden-order phase of ultra-clean URu$_2$Si$_2$ crystals, which allows the full determination of angle-dependent electron-mass structure of the main Fermi-surface sheets. We find an anomalous splitting of the sharpest resonance line under in-plane magnetic-field rotation. This is most naturally explained by the domain formation, which breaks the fourfold rotational symmetry of the underlying tetragonal lattice. The results reveal the emergence of an in-plane mass anisotropy with hot spots along the [110] direction, which can account for the anisotropic in-plane magnetic susceptibility reported recently. This is consistent with the `nematic' Fermi liquid state, in which itinerant electrons have unidirectional correlations., Comment: 5 pages, 3 figures

Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice. The electron nematicity has been recently observed in the iron-pnictide and cuprate high-temperature superconductors. Whether such a tendency of electrons to self-organise unidirectionally has a common feature in these superconductors is, however, a highly controversial issue. In the cuprates, the nematicity has been suggested as a possible source of the pseudogap phase, whilst in the iron-pnictides, it has been commonly associated with the tetragonal-to-orthorhombic structural phase transition at $T_s$. Here, we provide the first thermodynamic evidence in BaFe2(As1-xPx)2 that the nematicity develops well above the structural transition and persists to the nonmagnetic superconducting regime, resulting in a new phase diagram strikingly similar to the pseudogap phase diagram in the cuprates. Our highly sensitive magnetic anisotropy measurements using microcantilever torque-magnetometry under in-plane field rotation reveal pronounced two-fold oscillations, which break the tetragonal symmetry. Combined with complementary high-resolution synchrotron X-ray and resistivity measurements, our results consistently identify two distinct temperatures - one at $T^{\ast}$, signifying a true nematic transition, and the other at $T_s (< T^{\ast})$, which we show to be not a true phase transition, but rather what we refer to as a "meta-nematic transition", in analogy to the well-known metamagnetic transition in the theory of magnetism. Our observation of the extended nematic phase above the superconducting dome establishes that the nematicity has primarily an electronic origin, inherent in the normal state of high-temperature superconductors., Comment: See the Nature website for the published version with more detailed doping dependence and discussions

Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe$_2$As$_2$ is an unconventional superconductor with strong electron correlations., Comment: 6 pages, 6 figures, 1 table. Extended version to be published in Phys. Rev. B

Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe_2(As_{1-x}P_x)_2 as function of isoelectric substitution (As/P) for 0.41

We report magnetic penetration depth and thermal conductivity data for high-quality single crystals of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ ($T_c=30$\,K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba,K)Fe$_2$As$_2$ which has similar $T_c$ and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-$T_c$ superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable $T_c$., Comment: 4 pages, 3 figures, revised version to be published in Phys. Rev. B Rapid Communication

The superconducting gap structure of LaFePO ($T_c=7.4 $K) is studied by thermal conductivity ($\kappa$) at low temperatures in fields $H$ parallel and perpendicular to the c axis. A clear two-step field dependence of $\kappa(H)$ with a characteristic field $H_s(\sim 350$ Oe) much lower than the upper critical field $H_{c2}$ is observed. In spite of large anisotropy of $H_{c2}$, $\kappa(H)$ in both $H$-directions is nearly identical below $H_s$. Above $H_s$, $\kappa(H)$ grows gradually with $H$ with a convex curvature, followed by a steep increase with strong upward curvature near $H_{c2}$. These results indicate the multigap superconductivity with active two-dimensional (2D) and passive 3D bands having contrasting gap values. Together with the recent penetration depth results, we suggest that the 2D bands consist of nodal and nodeless ones, consistent with the extended s-wave symmetry.

The normal-state charge transport is studied systematically in high-quality single crystals of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ ($0 \leq x \leq 0.71$). By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase ($T_c \lesssim 31$ K) appears. Near the SDW end point ($x\approx0.3$), we observe striking linear temperature ($T$) dependence of resistivity in a wide $T$-range, and remarkable low-$T$ enhancement of Hall coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler's rule and is well scaled by the Hall angle $\Theta_H$ as $\Delta\rho_{xx}/\rho_{xx} \propto \tan^2\Theta_H$. These non-Fermi liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferromagnetic fluctuations., Comment: 4 pages, 4 figures

We report high-sensitivity microwave measurements of the in-plane penetration depth $\lambda_{ab}$ and quasiparticle scattering rate $1/\tau$ in several single crystals of hole-doped Fe-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x\approx 0.55$). While power-law temperature dependence of $\lambda_{ab}$ with the power $\sim 2$ is found in crystals with large $1/\tau$, we observe exponential temperature dependence of superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different level of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe-arsenides., Comment: 4 pages, 4 figures, 1 table. Accepted for publication in Phys. Rev. Lett. Changed title as suggested by the PRL editors