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Stripe quadrupole order in the nematic phase of FeSe$_{1-x}$S$_x$Oct 26 2017May 18 2018We use polarization-resolved electronic Raman spectroscopy to study charge dynamics in non-magnetic FeSe$_{1-x}$S$_x$ superconductor. We observe two features of the $XY$ quadrupole symmetry: a low-energy quasi-elastic peak (QEP) and an electronic continuum. ... More

Superconducting gap structure of BaFe_2(As_{1-x}P_{x})_2Sep 26 2014We present a study of the superconducting gap structure in the iron-pnictide series BaFe2(As1-xPx)2. By measuring the variation of the specific heat as a function of temperature and magnetic field we are able to determine the number and Fermi surface ... More

Thermodynamic evidence for broken fourfold rotational symmetry in the hidden-order phase of URu2Si2Jul 17 2012Jul 19 2012Despite more than a quarter century of research, the nature of the second-order phase transition in the heavy-fermion metal URu$_2$Si$_2$ remains enigmatic. The key question is which symmetry is being broken below this "hidden order" transition. We review ... More

Rotational Symmetry Breaking in the Hidden-Order Phase of URu2Si2Jul 27 2011A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called "hidden order" phase transition in the heavy fermion compound URu2Si2, at transition temperature T_h=17.5 K, has posed ... More

Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquidMay 14 2018Aug 01 2018The quantum Hall effect (QHE) in two-dimensional (2D) electron gases, which is one of the most striking phenomena in condensed matter physics, involves the topologically protected dissipationless charge current flow along the edges of the sample. Integer ... More

Broken symmetries in URu2Si2Jun 19 2014To resolve the nature of the hidden order below 17.5\,K in the heavy fermion compound URu$_2$Si$_2$, identifying which symmetries are broken below the hidden order transition is one of the most important steps. Several recent experiments on the electronic ... More

Cyclotron Resonance in the Hidden-Order Phase of URu2Si2Jul 17 2012We 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 ... More

Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2May 17 2017Jul 28 2017A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion ... More

Zeeman and Orbital Limiting Fields: Separated Spin and Charge Degrees of Freedom in Cuprate SuperconductorsMay 18 2004Recent in-plane thermal (Nernst) and interlayer (tunnelling) transport experiments in Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ high temperature superconductors report hugely different limiting magnetic fields. Based on pairing (and the uncertainty principle) combined ... More

FFLO state in thin superconducting filmsOct 18 2007We present the analysis of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in thin superconducting films in the parallel magnetic field. For the tetragonal crystal symmetry (relevant to CeCoIn$_{5}$ - the most probable ... More

Quantum critical point lying beneath the superconducting dome in iron-pnictidesApr 23 2013Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermion compounds and most recently iron-pnictides. ... More

Coexistence of orbital and quantum critical magnetoresistance in FeSe$_{1-x}$S$_{x}$Mar 13 2019The recent discovery of a non-magnetic nematic quantum critical point (QCP) in the iron chalcogenide family FeSe$_{1-x}$S$_{x}$ has raised the prospect of investigating, in isolation, the role of nematicity on the electronic properties of correlated metals. ... More

Fully gapped superconductivity with no sign change in the prototypical heavy-fermion CeCu2Si2Mar 08 2017In exotic superconductors including high-$T_c$ copper-oxides, the interactions mediating electron Cooper-pairing are widely considered to have a magnetic rather than the conventional electron-phonon origin. Interest in such exotic pairing was initiated ... More

Superconducting gap with sign reversal between hole pockets in heavily hole-doped Ba1-xKxFe2As2Jul 12 2013To gain insight into the unconventional superconductivity of Fe-pnictides with no electron pockets, we measure the thermal conductivity $\kappa$ and penetration depth $\lambda$ in the heavily hole-doped regime of Ba$_{1-x}$K$_x$Fe$_2$As$_2$. The residual ... More

Reply to Comment by Borisenko et al. on article `A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs'Aug 19 2011Recently, Borisenko et al have posted a Comment (arXiv:1108.1159) where they suggest an alternative interpretation of our de Haas-van Alphen (dHvA) measurements on the superconductor LiFeAs. In our original paper (arXiv:1107.4375) we concluded that our ... More

Evolution of the Fermi surface of the nematic superconductors FeSe1-xSxNov 22 2016We 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. ... More

Vortex Matter Transition in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ under Tilted FieldsNov 06 2000Dec 13 2000Vortex phase diagram under tilted fields from the $c$ axis in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ is studied by local magnetization hysteresis measurements using Hall probes. When the field is applied at large angles from the $c$ axis, an anomaly ($H_p^\ast$) ... More

A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAsJul 21 2011Nov 22 2011We 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 ... More

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSeDec 22 2015The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (${T_{\rm c}}$) superconductors. Unlike other iron-based superconductors, FeSe ... More

Asymmetric Field Profile in Bose Glass Phase of Irradiated YBa2Cu3O7-d: Loss of Interlayer Coherence around 1/3 of Matching FieldApr 07 2001Apr 12 2001Magneto-optical imaging in YBa2Cu3O7-d with tilted columnar defects (CD's) shows an asymmetric critical-state field profile. The observed hysteretic shift of the profile ridge (trough) from the center of the sample is explained by in-plane magnetization ... More

Quasiparticle mass enhancement close to the quantum critical point in BaFe$_2$(As$_{1-x}$P$_x$)$_2$Mar 14 2013Jun 27 2013We report a combined study of the specific heat and de Haas-van Alphen effect in the iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Our data when combined with results for the magnetic penetration depth give compelling evidence for the existence ... More

Anomalous critical fields in quantum critical superconductorsFeb 06 2014Dec 15 2014Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. ... More

Evolution of Paramagnetic Quasiparticle Excitations Emerged in the High-Field Superconducting Phase of CeCoIn5Mar 08 2011We present In NMR measurements in a novel thermodynamic phase of CeCoIn5 in high magnetic field, where exotic superconductivity coexists with the incommensurate spin-density wave order. We show that the NMR spectra in this phase provide direct evidence ... More

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductorsApr 24 2009In this article, we review our recent studies of microwave penetration depth, lower critical fields, and quasiparticle conductivity in the superconducting state of Fe-arsenide superconductors. High-sensitivity microwave surface impedance measurements ... More

Diagonal Nematicity in the Pseudogap Phase of HgBa$_2$CuO$_{4+δ}$May 01 2018The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. ... More

High-Tc superconductivity in FeSe at high pressure: Dominant hole carriers and enhanced spin fluctuationsNov 23 2016The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (Tc). However, high-Tc superconductivity without hole carriers has been suggested in FeSe single-layer films ... More

Emergent Rank-5 'Nematic' Order in URu2Si2Apr 18 2012Jul 18 2012Novel electronic states resulting from entangled spin and orbital degrees of freedom are hallmarks of strongly correlated f-electron systems. A spectacular example is the so-called 'hidden-order' phase transition in the heavy-electron metal URu2Si2, which ... More

Abrupt recovery of Fermi-liquid transport by the c-axis collapse in CaFe2(As1-xPx)2 single crystalsDec 05 2010Single crystals of CaFe$_2$(As$_{1-x}$P$_x$)$_2$ are found to exhibit the tetragonal (T) to collapsed-tetragonal (cT) transition at $T_{\rm cT} \lesssim100$\,K for $x>0.05$. The c-axis shrinks by $\sim9$% below $T_{\rm cT}$, which substantially diminishes ... More

Nesting of electron and hole Fermi surfaces in non-superconducting BaFe_2P_2Apr 01 2011We report detailed measurements of the de Haas-van Alphen (dHvA) effect in BaFe_2P_2 which is the end member of the superconducting series, BaFe_2(As_{1-x}P_x)_2. Using high purity samples we are able to observe dHvA oscillations from all the sheets of ... More

Microscopic Evidence of Direct Coupling between Magnetic and Superconducting Order Parameters in BaFe$_2$(As$_{1-x}$P$_x$)$_2$Nov 30 2011Feb 11 2012The coexistence of magnetism and superconductivity in the isovalent-P-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ has been investigated microscopically by $^{31}$P-NMR measurements. We found that superconducting (SC) transition occurs in a magnetic region ... More

Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSeJan 26 2019FeSe is argued as a superconductor in the Bardeen-Cooper-Schrieffer Bose-Einstein-condensation crossover regime where the superconducting-gap size and the superconducting transition temperature Tc are comparable to the Fermi energy. In this regime, vortex ... More

Contrasts in electron correlations and inelastic scattering between LiFeAs and LiFeP revealed by charge transportDec 23 2011Dec 28 2011By using high-quality single crystals, we quantitatively compare the transport properties between LiFeAs and LiFeP superconductors with compensated electron and hole carriers. The low-temperature resistivity follows the Fermi-liquid $AT^2$ dependence ... More

Thermal conductivity evidence for d_{x^2-y^2} pairing symmetry in the heavy-fermion CeIrIn5 superconductorDec 17 2007Quasi-two dimensional CeIrIn5 contains two distinct domes with different heavy fermion superconducting states in its phase diagram. Here we pinned down the superconducting gap structure of CeIrIn5 in the second dome, located away from the antiferromagnetic ... More

Entanglement of Solid Vortex Matter: A Boomerang Shaped Reduction Forced by Disorder in Interlayer Phase Coherence in Bi2Sr2CaCu2O8+yJun 15 2008We present evidence for entangled solid vortex matter in a glassy state in a layered superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ containing randomly splayed linear defects. The interlayer phase coherence--probed by the Josephson plasma resonance--is ... More

Specific Heat Discontinuity, deltaC, at Tc in BaFe2(As0.7P0.3)2 - Consistent with Unconventional SuperconductivityMay 21 2011We report the specific heat discontinuity, deltaC/Tc, at Tc = 28.2 K of a collage of single crystals of BaFe2(As0.7P0.3)2 and compare the measured value of 38.5 mJ/molK**2 with other iron pnictide and iron chalcogenide (FePn/Ch) superconductors. This ... More

Stable ultrahigh-density magneto-optical recordings using introduced linear defectsMay 01 2001The stability of data bits in magnetic recording media at ultrahigh densities is compromised by thermal `flips' -- magnetic spin reversals -- of nano-sized spin domains, which erase the stored information. Media that are magnetized perpendicular to the ... More

Inter-plane resistivity of isovalent doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$Feb 26 2013Temperature-dependent inter-plane resistivity, $\rho_c(T)$, was measured for the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a broad isoelectron phosphorus substitution range from $x$=0 to $x$=0.60, from non-superconducting parent compound ... More

Field-induced quantum critical route to a Fermi liquid in high-temperature superconductorsMay 15 2008In high transition temperature (T_c) superconductivity, charge doping is a natural tuning parameter that takes copper oxides from the antiferromagnet to the superconducting region. In the metallic state above T_c the standard Landau's Fermi-liquid theory ... More

Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped SuperconductivityMar 02 2006Jun 07 2006To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly ... More

Closing the Pseudogap by Zeeman Splitting in Bi_2Sr_2CaCu_2O_8+y at High Magnetic FieldsApr 13 2001Interlayer tunneling resistivity is used to probe the low-energy density-of-states (DOS) depletion due to the pseudogap in the normal state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$. Measurements up to 60 T reveal that a field that restores DOS to its ungapped ... More

Unconventional superconductivity and antiferromagnetic quantum critical behavior in the isovalent-doped BaFe2(As1-xPx)2May 17 2010Sep 03 2010Spin dynamics evolution of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ was probed as a function of P concentration via $^{31}$P NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility ... More

Specific Heat vs Field in the 30 K Superconductor BaFe2(As0.7P0.3)2Feb 17 2010May 20 2010We report specific heat measurements on the Fe-based superconductor BaFe2(As0.7P0.3)2, a material on which previous penetration depth, NMR, and thermal conductivity measurements have observed a high density of low-energy excitations, which have been interpreted ... More

Highly Anisotropic Superconducting Gap in Nematically Ordered and Tetragonal Phases of FeSe$_{1-x}$S$_x$May 25 2017FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering at ambient pressure. Here, to study how the pairing interaction evolves with nematicity, we measured the thermal conductivity ... More

Normal state spin dynamics in the iron-pnictide superconductors BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 probed with NMR measurementsMay 13 2013The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are analyzed based on the self-consistent renormalization (SCR) spin fluctuation theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as well as the electrical ... More

Diamagnetic Vortex Barrier Stripes in Underdoped $BaFe_2(As_{1-x}P_x)_2$Oct 29 2015May 31 2016We report magnetic force microscopy (MFM) measurements on underdoped $BaFe_2(As_{1-x}P_x)_2$ ($x=0.26$) that show enhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel vortices when ... More

Nematic quantum critical point without magnetism in FeSe1-xSx superconductorsApr 01 2016Apr 04 2016The importance of antiferromagnetic fluctuations are widely acknowledged in most unconventional superconductors. In addition, cuprates and iron pnictides often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, ... More

Optical Conductivity Evidence of Clean-Limit Superconductivity in LiFeAsApr 26 2015We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. ... More

Vortex Redistribution below the First-Order Transition Temperature in the β-Pyrochlore Superconductor KOs_2O_6Oct 31 2007A miniature Hall sensor array was used to detect magnetic induction locally in the vortex states of the $\beta$-pyrochlore superconductor KOs$_2$O$_6$. Below the first-order transition at $T_{\rm p}\sim 8$ K, which is associated with a change in the rattling ... More

High-Field Quasiparticle Tunneling in Bi_2Sr_2CaCu_2O_8+delta: Negative Magnetoresistance in the Superconducting StateDec 10 1999We report on the c-axis resistivity rho_c(H) in Bi_2Sr_2CaCu_2O_{8+\delta} that peaks in quasi-static magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity \sigma_c (H), we find that ... More

Nodeless vs nodal order parameters in LiFeAs and LiFeP superconductorsJul 22 2011High-precision measurements of magnetic penetration depth $\lambda$ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting low-energy quasiparticle excitations. In LiFeAs the low-temperature $\lambda(T)$ shows a flat dependence ... More

Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting $β$-Pyrochlore KOs$_2$O$_6$May 17 2007Microwave penetration depth $\lambda$ and surface resistance at 27 GHz are measured in high quality crystals of KOs$_2$O$_6$. Firm evidence for fully-gapped superconductivity is provided from $\lambda(T)$. Below the second transition at $T_{\rm p}\sim ... More

Anisotropic Superconducting Properties of Optimally Doped BaFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ under PressureJun 30 2010Sep 02 2010Magnetic measurements on optimally doped single crystals of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ ($x\approx0.35$) with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence ... More

Signature of Unconventional Superconductivity in a Copper-based Metal-Organic Framework with Perfect Kagome StructureOct 01 2018Recently, the superconductivity in a metal-organic framework (MOF) has been discovered for the first time in copper(II) benzenehexathiolate ([Cu$_3$(C$_6$S$_6$)]$_n$, Cu-BHT). The Cu atoms form a two-dimensional perfect Kagome lattice, which has the potential ... More

Competition between unconventional superconductivity and incommensurate antiferromagnetic order in CeRh1-xCoxIn5Apr 18 2007Apr 23 2007Elastic neutron diffraction measurements were performed on the quasi-two dimensional heavy fermion system CeRh1-xCoxIn5, ranging from an incommensurate antiferromagnet for low x to an unconventional superconductor on the Co-rich end of the phase diagram. ... More

Possible Phase Transition Deep Inside the Hidden Order Phase of Ultraclean URu2Si2Mar 23 2009To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu2Si2, we measure electrical transport properties of ultraclean crystals in a high field/low temperature regime. Unlike previous studies, the present system ... More

Superconducting Gap Structure of LaFePO Studied by Thermal ConductivityJun 03 2009The 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 ... More

Tuning of magnetic quantum criticality in artificial Kondo superlattice CeRhIn5/YbRhIn5Apr 30 2016The effects of reduced dimensions and the interfaces on antiferromagnetic quantum criticality are studied in epitaxial Kondo superlattices, with alternating $n$ layers of heavy-fermion antiferromagnet CeRhIn$_5$ and 7 layers of normal metal YbRhIn$_5$. ... More

Evidence for time-reversal symmetry breaking of the superconducting state near twin-boundary interfaces in FeSeApr 09 2015Junctions and interfaces consisting of unconventional superconductors provide an excellent experimental playground to study exotic phenomena related to the phase of the order parameter. Not only the complex structure of unconventional order parameters ... More

Thermal conductivity of quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7Apr 09 2015We report low-temperature thermal conductivity $\kappa$ of pyrochlore Yb$_2$Ti$_2$O$_7$, which contains frustrated spin-ice correlations with significant quantum fluctuations. In the disordered spin-liquid regime, $\kappa(H)$ exhibits a nonmonotonic magnetic ... More

Strong suppression of superconductivity by divalent Ytterbium Kondo-holes in CeCoIn_5Aug 03 2012To study the nature of partially substituted Yb-ions in a Ce-based Kondo lattice, we fabricated high quality Ce_{1-x}Yb_xCoIn_5 epitaxial thin films using molecular beam epitaxy. We find that the Yb-substitution leads to a linear decrease of the unit ... More

Controllable Rashba spin-orbit interaction in artificially engineered superlattices involving the heavy-fermion superconductor CeCoIn5Apr 02 2014By using a molecular beam epitaxy technique, we fabricate a new type of superconducting superlattices with controlled atomic layer thicknesses of alternating blocks between heavy fermion superconductor CeCoIn_5, which exhibits a strong Pauli pair-breaking ... More

Two- and Three-Dimensional Fermi Surfaces and their Nesting Properties in Superconducting BaFe2(As1-xPx)2Aug 12 2010We have studied the three-dimensional shapes of the Fermi surfaces (FSs) of BaFe2(As1-xPx)2 (x=0.38), where superconductivity is induced by isovalent P substitution, by angle-resolved photoemission spectroscopy. Moderately strong electron mass enhancement ... More

Colossal thermomagnetic response in the exotic superconductor URu2Si2Nov 05 2014Nov 06 2014When a superconductor is heated above its critical temperature $T_c$, macroscopic coherence vanishes, leaving behind droplets of thermally fluctuating Cooper pair. This superconducting fluctuation effect above $T_c$ has been investigated for many decades ... More

Evolution from Non-Fermi to Fermi Liquid Transport Properties by Isovalent Doping in BaFe2(As1-xPx)2 SuperconductorsMay 27 2009Feb 09 2010The 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 ... More

Tuning the pairing interaction in a $d$-wave superconductor by paramagnons injected through interfacesDec 01 2017Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and ... More

Line nodes in the energy gap of high-temperature superconducting BaFe_2(As_{1-x}P_x)_2 from penetration depth and thermal conductivity measurementsJul 25 2009May 17 2010We 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 ... More

Anomalous Temperature Dependence of Lower Critical Field in Ultraclean URu$_2$Si$_2$Sep 06 2009Jul 26 2010To investigate a mysterious superconducting state of URu_2Si_2 embedded in the so-called hidden order state, the lower critical field H_{c1} is precisely determined down to 55 mK for H || a and H || c. For this purpose, the positional dependence of the ... More

Emergent XY electronic nematicity in iron-based superconductorsDec 13 2018Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials . In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction ... More

Exotic Superconducting Properties in the Electron-Hole Compensated Heavy Fermion `Semimetal' URu2Si2Aug 08 2007We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including ... More

Thermodynamic evidence for nematic phase transition at the onset of pseudogap in YBa$_2$Cu$_3$O$_y$Jun 16 2017A central issue in the quest to understand the superconductivity in cuprates is the nature and origin of the pseudogap state, which harbours anomalous electronic states such as Fermi arc, charge density wave (CDW), and $d$-wave superconductivity. A fundamentally ... More

Nematic and meta-nematic transitions in the iron pnictidesJul 04 2012Strongly 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 ... More

Direct Evidence for the Existence of Heavy Quasiparticles in the Magnetically Ordered Phase of CeRhIn$_5$Oct 25 2018It is a long-standing important issue in heavy fermion physics whether $f$-electrons are itinerant or localized when the magnetic order occurs. Here we report the {\it in situ} scanning tunneling microscopy observation of the electronic structure in epitaxial ... More

Nodal gap structure of BaFe_2(As_{1-x}P_x)_2 from angle-resolved thermal conductivity in a magnetic fieldMar 04 2011Aug 25 2011The structure of the superconducting order parameter in the iron-pnictide superconductor BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ ($T_c=31$\,K) with line nodes is studied by the angle-resolved thermal conductivity measurements in a magnetic field rotated within ... More

Extremely strong coupling superconductivity in artificial two-dimensional Kondo latticesSep 12 2011When interacting electrons are confined to low-dimensions, the electron-electron correlation effect is enhanced dramatically, which often drives the system into exhibiting behaviors that are otherwise highly improbable. Superconductivity with the strongest ... More

Novel Pauli-paramagnetic quantum phase in a Mott insulatorOct 01 2012In 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 ... More

Flux Line Lattice Melting and the Formation of a Coherent Quasiparticle Bloch State in the Ultraclean URu$_2$Si$_2$ SuperconductorOct 12 2007Dec 16 2007We find that in ultraclean heavy-fermion superconductor URu$_2$Si$_2$ ($T_{c0}=1.45$ K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number ... More

Evidence for s-wave Pairing with Atomic Scale Disorder in the van der Waals Superconductor NaSn2As2May 31 2018Jul 11 2018The recent discovery of superconductivity in NaSn$_2$As$_2$ with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic ... More

Evolution of quasiparticle excitations with critical mass enhancement in superconducting AFe2As2 (A = K, Rb, and Cs)Oct 08 2015In the heavily hole-doped iron-based superconductors $A$Fe$_2$As$_2$ ($A=$ K, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study how the mass enhancement affects the superconducting state, we measure the London ... More

Magneto-transport properties governed by the antiferromagnetic fluctuations in heavy fermion superconductor CeIrIn$_{5}$Apr 16 2008Apr 18 2008In quasi-two dimensional Ce(Ir,Rh)In$_5$ system, it has been suggested that the phase diagram contains two distinct domes with different heavy fermion superconducting states. We here report the systematic pressure dependence of the electron transport ... More

Cyclotron resonance study of quasiparticle mass and scattering rate in the hidden-order and superconducting phases of URu2Si2Oct 12 2013The 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. ... More

Disorder-induced topological change of the superconducting gap structure in iron pnictidesMay 27 2014May 29 2014In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, e.g. $d$-wave cuprate superconductors, the position and topology ... More

Microwave Penetration Depth and Quasiparticle Conductivity in PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap SuperconductorJun 19 2008Dec 11 2008In-plane microwave penetration depth $\lambda_{ab}$ and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO$_{1-y}$ ($T_c\approx 35$ K) by using a sensitive superconducting cavity resonator. ... More

Spin nematic susceptibility studied by inelastic neutron scattering in FeSeNov 13 2015Sep 22 2016Mechanism of unconventional superconductivity in FeSe has been intensely scrutinized recently because of a variety of exotic properties unprecedented for other iron-based superconductors. A central unanswered question concerns the origin of the interaction ... More

Volovik effect in a highly anisotropic multiband superconductor: experiment and theorySep 02 2011We present measurements of the specific heat coefficient \gamma(= C/T) in the low temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. We find both a linear regime at higher fields ... More

Anomalous Upper Critical Field in CeCoIn_5/YbCoIn_5 Superlattices with a Rashba-type Heavy Fermion InterfaceJul 20 2012Oct 18 2012We report a highly unusual angular variation of the upper critical field (H_c2) in epitaxial superlattices CeCoIn_5(n)/YbCoIn_5(5), formed by alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor CeCoIn_5 with a strong Pauli effect ... More

Weakening of the diamagnetic shielding in FeSe$_{1-x}$S$_x$ at high pressuresMay 22 2017Jul 10 2017The superconducting transition of FeSe$_{1-x}$S$_x$ with three distinct sulphur concentrations $x$ was studied under hydrostatic pressure up to $\sim$70 kbar via bulk AC susceptibility. The pressure dependence of the superconducting transition temperature ... More

Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe$_{2}$As$_{2}$ family of materialsAug 18 2014We report a combination of Fe K$\beta$ x-ray emission spectroscopy and $ab$-intio calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe$_{2}$(As$_{1-x}$P$_{x} $)$_{2}$. The puzzling temperature behavior ... More

Anomalous superfluid density in quantum critical superconductorsFeb 12 2013When a second-order magnetic phase transition is tuned to zero temperature by a non-thermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these `quantum critical' superconductors ... More

Quantum sensing of local magnetic field texture in strongly correlated electron systems under extreme conditionsDec 25 2018An important feature of strong correlated electron systems is the tunability between interesting ground states such as unconventional superconductivity and exotic magnetism. Pressure is a clean, continuous and systematic tuning parameter. However, due ... More

Field-induced superconducting phase of FeSe in the BCS-BEC cross-overNov 05 2014Fermi systems in the crossover regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting ... More

Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossoverAug 05 2016The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits gives a unified framework of quantum bound (superfluid) states of interacting fermions. This crossover has been ... More

Momentum-dependent sign-inversion of orbital polarization in superconducting FeSeApr 04 2015We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy (ARPES) on detwinned single crystals. Across the structural transition, ... More

Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2 studied by thermal expansionMar 09 2012High-resolution thermal-expansion and specific-heat measurements were performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The observation of clear anomalies allows to establish the thermodynamic phase diagram which features a small ... More

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1-xPx)2Dec 21 2012In a superconductor, the ratio of the carrier density, $n$, to their effective mass, $m^*$, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, $\lambda_L$. In two dimensional systems, this ... More

Evolution of the Fermi surface of BaFe_2(As_{1-x}P_x)_2 on entering the superconducting domeOct 19 2009Using 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<x<1 (T_c up to 25 K). We find that the volume of electron and hole Fermi surfaces ... More

Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2Jun 19 2014Since 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 ... More

Lower Critical Fields of Superconducting PrFeAsO$_{1-y}$ Single CrystalsNov 22 2008Jan 14 2009We have studied the lower critical fields H_{c1} of superconducting iron oxipnictide PrFeAsO_{1-y} single crystals for H parallel and perpendicular to the ab-planes. Measurements of the local magnetic induction at positions straddling the sample edge ... More

Microwave Surface-Impedance Measurements of the Magnetic Penetration Depth in Single Crystal Ba1-xKxFe2As2 Superconductors: Evidence for a Disorder-Dependent Superfluid DensityOct 20 2008Apr 24 2009We 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$_2$ ($x\approx 0.55$). ... More

Uncommonly High Upper Critical Field in the Superconducting KOs$_2$O$_6$ PyrochloreMar 11 2006Nov 13 2006The entire temperature dependence of the upper critical field $H_{\rm c2}$ in the $\beta$-pyrochlore KOs$_2$O$_6$ is obtained from high-field resistivity and magnetic measurements. Both techniques identically give $H_{\rm c2}(T \simeq 0 {\rm K})$ not ... More

Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillationsDec 12 2014We 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 ... More