Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB6 may be a realization of a topological Kondo insulator. We have performed an angleresolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like 5d bulk band centred at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo bandgap (∆B ∼ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the centerΓ point and boundaryX point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridisation gap and/or the presence of a topological Kondo insulating state. A three-dimensional (3D) topological insulator (TI) is an unusual topological quantum state associated with unique metallic surface states that appear within the bulk bandgap [1,2]. Owing to the peculiar spin texture protected by time-reversal symmetry, the Dirac fermions in TIs are forbidden from scattering due to nonmagnetic impurities and disorder [3,4]. Hence they carry dissipationless spin current [5], making it possible to explore fundamental physics, spintronics, and quantum computing [1,2]. However, even after extensive materials synthesis efforts [6][7][8][9][10], impurities in the bulk of these materials make them metallic, prompting us to search for new types of TIs with truly insulating bulks.The 3D Kondo insulator SmB 6 may open a new route to realizing topological surface states. SmB 6 is a typical heavy fermion material with strong electron correlation. Localized f electrons hybridize with conduction electrons, leading to a narrow bandgap on the order of 10 meV opening at low temperatures, with the chemical potential lying in the gap [11][12][13][14]. Due to the opening of the bandgap, the conductivity changes from metallic to insulating behavior with decreasing temperature. It saturates to a constant value below about 1 K, which is thought to be caused by in-gap states [15]. Theoretical studies have proposed that SmB 6 may host threedimensional topological insulating phases [16,17]. Recently, transport experiments employing a novel geometry [18] showed convincing evidence of a distinct surface contribution to the conductivity that is unmixed with the bulk contribution, suggesting SmB 6 is an ideal topological insulator with a perfectly insulating bulk. Pointcontact spectroscopy revealed that the low-temperature Kondo insulating state harbors conduction states on the surface, in support of predict...

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba 3 ZnIr 2 O 9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba 3 ZnIr 2 O 9 with Ir 5þ (5d 4 ) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J ¼ 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir 2 O 9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK. DOI: 10.1103/PhysRevLett.116.097205 5d transition metal compounds often exhibit unusual electronic and magnetic properties due to the presence of strong spin-orbit coupling (SOC), comparable to their onsite Coulomb (U) and crystal field (Δ CFE ) interactions [1,2]. In the strong spin-orbit coupling regime, M J ( P m j ) becomes the only valid quantum number instead of m l (orbital) and m s (spin), and the multiplets and their degeneracies are solely determined by the total angular momentum J. The electronic and magnetic responses of a system in such limits are not yet well understood and have generated significant curiosity in recent times. For example, the curious insulating state of the layered tetravalent iridates (Ir 4þ ; 5d 5 ) has recently been explained within single particle theories assuming splitting of t 2g bands into a set of fully filled quartet bands separated from another set of half-filled narrow doublet bands due to finite SOC. The half-filled doublet bands further split into fully occupied lower and empty upper Hubbard bands in the presence of relatively small Hubbard U [3-5].The pentavalent Iridates (Ir 5þ ; 5d 4 ) are more intriguing, where in the strong SOC limit all the spin-orbit entangled electrons will be confined to singlet J ¼ 0 (M J ¼ 0) ground state, with no net moment. The evolution of ground and excited states of a low spin 5d t 4 2g Ir 5þ ion as a function of SOC parameter λ 0 is illustrated in Fig. 1(a) and a J ¼ 0 ground state is realized in the strong SOC limit [6]. A possibility of excitonic magnetism has been predicted for these systems where the energy scale of the singlet-triplet splitting determined by SOC is comparable to superexchange interaction promoted by hopping [10]. The breakdown of the J ¼ 0 nonmagnetic state in d 4 systems can also be realized within a single electron picture primarily driven by band-structure effect that allows the hybridization between the quartet and the doublet redistributed orbitals (eigenstates of the spin-orbit coupled Hamiltonian). Overall, d 4 Ir compounds in the strong SOC limit may host weak magnetic moment unless the λ 0 becomes so large that any excitonic or hopping-assisted magnetism become...

We report superconductivity at T(c) ≈ 2.6 K in a new layered bismuth oxyselenide LaO(0.5)F(0.5)BiSe2 with the ZrCuSiAs-type structure composed of alternating superconducting BiSe2 and blocking LaO layers. The superconducting properties of LaO(0.5)F(0.5)BiSe2 were investigated by means of dc magnetization, resistivity and muon-spin rotation experiments, revealing the appearance of bulk superconductivity with a rather large superconducting volume fraction of ≈ 70% at 1.8 K.

Temperature dependence of the electronic structure of SmB6 is studied by high-resolution ARPES down to 1 K. We demonstrate that there is no essential difference for the dispersions of the surface states below and above the resistivity saturating anomaly (∼ 3.5 K). Quantitative analyses of the surface states indicate that the quasi-particle scattering rate increases linearly as a function of temperature and binding energy, which differs from Fermi-Liquid behavior. Most intriguingly, we observe that the hybridization between the d and f states builds gradually over a wide temperature region (30 K < T < 110 K). The surface states appear when the hybridization starts to develop. Our detailed temperature-dependence results give a complete interpretation of the exotic resistivity result of SmB6, as well as the discrepancies among experimental results concerning the temperature regions in which the topological surface states emerge and the Kondo gap opens, and give new insights into the exotic Kondo crossover and its relationship with the topological surface states in the topological Kondo insulator SmB6.

We present the results of a systematic investigation of the magnetic properties of the 3D Kondo topological insulator SmB6 using magnetization and muon spin relaxation/rotation (µSR) measurements. The µSR measurements exhibit magnetic field fluctuations in SmB6 below ∼ 15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between µSR, magnetization and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature.PACS numbers: 71.27.+a, 74.25.Jb, 75.70.Tj, 76.75.+i Kondo insulators are mostly realized in strongly correlated rare-earth material systems. At high temperature, these materials behave as highly correlated metals, while at low temperature they are simply band insulators due to the formation of an energy gap at the Fermi level [1][2][3]. The opening of a gap at low temperature is attributed to hybridization between the localized f electrons (mostly from unfilled 4f -shells of the rare earth atoms) and the conduction electrons. SmB 6 , a mixed valence heavy fermion compound, more frequently referred to as Kondo insulator (even though Sm has non-integer chemical valence close to 2.5), has been very well known for many years due to its exotic low temperature transport properties. In this material, as the temperature is reduced, its resistivity increases exponentially as expected for a normal insulator. However, as the temperature is reduced further below 4 K, the resistivity saturates at a finite value (a few Ω.cm) [4]. This behavior was attributed to certain "in-gap" states [5], whose true nature was revealed only recently, when SmB 6 was predicted theoretically to be a 3D topological insulator. As such, it features topologically protected metallic surface states at low temperature [6][7][8][9], which lie in the bulk gap. Several ARPES measurements conducted on SmB 6 reveal a Kondo gap of a few meV in the bulk and also identify the low-lying in-gap states close to the Fermi level [10][11][12][13][14]. These in-gap states are found to disappear as the temperature is raised above ∼ 15 K [11]. Although, other ARPES results suggest that the transition is very broad and that the in-gap states disappear completely at a much higher temperature [10]. A very recent ARPES study has further suggested that the in-gap states gradually transform from 2D to 3D nature with increasing temperature [15,16]. These insights are comple-mented by surface-related transport measurements which also suggest that the surface conductivity can be ascribed to topologically protected surface states [17][18][19][20].Besides this intriguing charge response, SmB 6 also shows peculiar magnetic properties. NMR measurements have shown an enhanced spin lattice relaxation in high applied magnetic f...

Muon-spin-spectroscopy measurements have been used to study the superconducting state of FeTe 0.5 Se 0.5 . The temperature dependence of the in-plane magnetic penetration depth, ab ͑T͒, is found to be compatible with either a two-gap s + s-wave or an anisotropic s-wave model. The value for ab ͑T͒ at T = 0 K is estimated to be ab ͑0͒ = 534͑2͒ nm.

We report microscopic studies by muon spin rotation/relaxation as a function of pressure of the Ca3Ir4Sn13 and Sr3Ir4Sn13 cubic compounds, which are members of the (Ca1−xSrx)3Ir4Sn13 system displaying superconductivity and a structural phase transition associated with the formation of a charge density wave (CDW). We find a strong enhancement of the superfluid density and a dramatic increase of the pairing strength above a pressure of ≈ 1.6 GPa giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases has the same s-wave symmetry. In spite of the conventional phonon-mediated BCS character of the weakly correlated (Ca1−xSrx)3Ir4Sn13 system, the dependence of the effective superfluid density on the critical temperature puts this compound in the "Uemura" plot close to unconventional superconductors. This system exemplifies that conventional BCS superconductors in the presence of competing orders or multi-band structure can also display characteristics of unconventional superconductors. INTRODUCTIONThe interplay between different electronic ground states is one of the fundamental topics in condensed matter physics and is well apparent in phase diagrams as a function of doping, pressure or magnetic fields, resulting in various forms of coexistence, cooperation or competition of the order parameters [1][2][3][4]. Particularly interesting are the regions at phase boundaries or at quantum critical points (QCPs) where different quantum states meet [5]. Very often magnetism and superconductivity are involved and, in spite of diverse structural and physical properties, many compounds show characteristic phase diagrams where superconductivity is found in the vicinity of electronic instabilities of magnetic (mainly antiferromagnetic) origin. In this case spin fluctuations are predominantly considered at the heart of the mechanisms leading to pairing and superconductivity is unconventional. Less common is the case where the electronic instability is linked to the formation of a charge density wave (CDW), which is based on the same electron-phonon interaction found in conventional superconductors.Ternary intermetallic stannide compounds such as R 3 T 4 Sn 13 , where R =La, Ca, Sr and T =Ir, Rh [6,7] are of particular interest because they exhibit many physical properties such as superconductivity, magnetic or charge order, and structural instabilities. The quasiskutteridite cubic superconductor (Ca,Sr) 3 Ir 4 Sn 13 and the related (Ca,Sr) 3 Rh 4 Sn 13 have recently attracted attention because of the presence of a pressure induced structural phase transition at a temperature T * , the possible coexistence of superconducting and charge density wave states, and a putative quantum critical point [8][9][10][11][12][13][14][15][16]. The role and interplay of these degrees of freedom remain a central issue also in many unconventional superconductors, as demonstrated by the recent observa...

By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized Cu 0.75 Mo 0.25 Sr 2 YCu 2 O 7.54 , in which overdoping is achieved up to p ∼ 0.46 hole/Cu, well beyond the T c -p superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with T c = 84 K and superfluid density similar to those of optimally doped YBa 2 Cu 3 O 7−δ . On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La 2−x Sr x CuO 4 . These results point at an unusual high-T c phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high T c of cuprates originates from a non-Fermi-liquid ground state.