The nature of quantum spin liquids, a novel state of matter where strong quantum fluctuations destroy the long-range magnetic order even at zero temperature, is a long-standing issue in physics. We measured the low-temperature thermal conductivity of the recently discovered quantum spin liquid candidate, the organic insulator EtMe3Sb[Pd(dmit)2]2. A sizable linear temperature dependence term is clearly resolved in the zero-temperature limit, indicating the presence of gapless excitations with an extremely long mean free path, analogous to excitations near the Fermi surface in pure metals. Its magnetic field dependence suggests a concomitant appearance of spin-gap-like excitations at low temperatures. These findings expose a highly unusual dichotomy that characterizes the low-energy physics of this quantum system.

The notion of quantum spin-liquids (QSLs), antiferromagnets with quantum fluctuation-driven disordered ground states, is now firmly established in one-dimensional (1D) spin systems as well as in their ladder cousins. The spin-1/2 organic insulator κ-(bis(ethylenedithio)-tetrathiafulvalene) 2 Cu 2 (CN) 3 (κ-(BEDT-TTF) 2 Cu 2 (CN) 3 ; ref. 1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in D ≥ 2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations 2,3 , particularly the presence/absence of a 'spin gap', which will provide vital information on the universality class of this putative QSL. Here, we report on our thermal-transport measurements carried out down to 80 mK. We find, rather unexpectedly, unambiguous evidence for the absence of a gapless excitation, which sharply contradicts recent reports of heat capacity measurements 4 . The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase.In antiferromagnetically coupled spin systems, geometrical frustrations enhance quantum fluctuations. Largely triggered by the proposal of the resonating-valence-bond theory for S = 1/2 degrees of freedom residing on a frustrated twodimensional (2D) triangular lattice 5-7 and its possible application to high-T c cuprates with a doped 2D square lattice 8,9 , realizing/detecting QSLs in 2D systems has been a longsought goal. Recently, discoveries of QSL states on S = 1/2 triangular lattices have been reported in organic compounds, κ-(BEDT-TTF) 2 Cu 2 (CN) 3 (Fig. 1, inset) 1,10,11 , C 2 H 5 (CH 3 ) 3 Sb

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (Tc) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ∼15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ∼6 GPa the sudden enhancement of superconductivity (Tc≤38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-Tc cuprates.

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 K͒ 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, K͒Fe 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 .

When interacting electrons are confined to low-dimensions, the electron-electron correlation effect is enhanced dramatically, which often drives the system into exhibiting behaviours that are otherwise highly improbable. Superconductivity with the strongest electron correlations is achieved in heavy-fermion compounds, which contain a dense lattice of localized magnetic moments interacting with a sea of conduction electrons to form a three-dimensional (3D) Kondo lattice 1 . It had remained an unanswered question whether superconductivity would persist upon effectively reducing the dimensionality of these materials from three to two. Here we report on the observation of superconductivity in such an ultimately strongly-correlated system of heavy electrons confined within a 2D square-lattice of Ce-atoms (2D Kondo lattice), which was realized by fabricating epitaxial superlattices 2,3 built of alternating layers of heavy-fermion CeCoIn 5 4 and conventional metal YbCoIn 5 . The field-temperature phase diagram of the superlattices exhibits highly unusual behaviours, including a striking enhancement of the upper critical field relative to the transition temperature. This implies that the force holding together the superconducting electron-pairs takes on an extremely strong coupled nature as a result of two-dimensionalisation. The layered heavy-fermion compound CeCoIn 5 has the highest superconducting transition temperature (T c =2.3 K) among rare-earth-based heavy-fermion materials 4 . Its electronic properties are characterized by anomalously large value of the linear contribution to the specific heat (Sommerfeld coefficient ~1 J/mol K 2 ) indicating heavy effective masses of the 4f electrons which contribute greatly to the Fermi surface. The tetragonal CeCoIn 5 crystal structure is built from alternating layers of CeIn 3 and CoIn 2 stacked along the [001] direction. This compound possesses several key features for understanding the unconventional superconductivity in strongly correlated systems [5][6][7] . The superconductivity with d x 2 -y 2 pairing symmetry 8-11 which occurs in the proximity of a magnetic instability is a manifestation of magnetic fluctuations mediated superconductivity [5][6][7]12 .A very strong coupling superconductivity, where electron-pairs are bound together by strong forces, is revealed by a large specific heat jump 4 at T c representing a steep drop of the entropy below T c , and a large superconducting energy gap  needed to break the electron-pair , all indicate that the electronic, magnetic and superconducting properties are essentially 3D rather than 2D. Therefore it is still unclear to which extent the 3D nature is essential for the superconductivity of CeCoIn 5 .Recently the state-of-the-art technique has been developed to reduce the dimensionality of the heavy electrons in a controllable fashion by the layer-by-layer epitaxial growth of Ce-based materials. Previously a series of antiferromagnetic superlattices CeIn 3 /LaIn 3 have been successfully grown 2 , but it remains open whethe...

Although solid helium-4 (4He) may be a supersolid, it also exhibits many phenomena unexpected in that context. We studied relaxation dynamics in the resonance frequency f(T) and dissipation D(T) of a torsional oscillator containing solid 4He. With the appearance of the "supersolid" state, the relaxation times within f(T) and D(T) began to increase rapidly together. More importantly, the relaxation processes in both D(T) and a component of f(T) exhibited a complex synchronized ultraslow evolution toward equilibrium. Analysis using a generalized rotational susceptibility revealed that, while exhibiting these apparently glassy dynamics, the phenomena were quantitatively inconsistent with a simple excitation freeze-out transition because the variation in f was far too large. One possibility is that amorphous solid 4He represents a new form of supersolid in which dynamical excitations within the solid control the superfluid phase stiffness.

The Kitaev quantum spin liquid displays the fractionalization of quantum spins into Majorana fermions. The emergent Majorana edge current is predicted to manifest itself in the form of a finite thermal Hall effect, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity κ_{xy} measurements in α-RuCl_{3}, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction J_{K}/k_{B}∼80  K, positive κ_{xy} develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. Although the zero-temperature property is masked by the magnetic ordering at T_{N}=7  K, the sign, magnitude, and T dependence of κ_{xy}/T at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.

We report a highly unusual angular variation of the upper critical field (Hc2) in epitaxial superlattices CeCoIn5(n)/YbCoIn5(5), formed by alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor CeCoIn5 with a strong Pauli effect and normal metal YbCoIn5, respectively. For the n = 3 superlattice, Hc2(θ) changes smoothly as a function of the field angle θ. However, close to the superconducting transition temperature, Hc2(θ) exhibits a cusp near the parallel field (θ = 0 • ). This cusp behavior disappears for n = 4 and 5 superlattices. This sudden disappearance suggests the relative dominance of the orbital depairing effect in the n = 3 superlattice, which may be due to the suppression of the Pauli effect in a system with local inversion symmetry breaking. Taking into account the temperature dependence of Hc2(θ) as well, our results suggest that some exotic superconducting states, including a helical superconducting state, might be realized at high magnetic fields.PACS numbers: 74.25. Op, 81.15.Hi In the absence of time reversal symmetry or space inversion symmetry, the Fermi surface (FS) can often be split into portions with different spin structures. To stabilize superconductivity under such conditions where spin degeneracy is lifted, unconventional pairing of quasiparticles is needed, leading to exotic superconducting states very different from the conventional BCS pairing state of (k ↑, -k ↓). Considering the situation of the broken time reversal symmetry alone, Fulde and Ferrell [1], and Larkin and Ovchinnikov [2] proposed the pairing state of (k ↑, -k+q ↓) on a Zeeman-split FS. This so-called FFLO pairing state leads to the modulation of the superconducting order parameter in real space with the modulation wavelength of the order of 1/|q|. On the other hand, in the lack of space inversion symmetry, a Rashba-type spin-orbit coupling splits the FS into branches with spins of opposite rotation sense [3]. When the magnetic field is applied to such a system, a pairing state with a finite center-of-mass momentum can also be realized, resulting in a helical superconducting state analogous to the FFLO phase.However, such exotic superconducting states have been poorly explored because of the lack of suitable materials. Recent advancement in heavy fermion thin film fabrication technology [4,5] has enabled the preparation of superlattices formed by alternate stacking of c-axis oriented CeCoIn 5 and YbCoIn 5 with atomic layer thicknesses. The large Fermi velocity mismatch across the interface between CeCoIn 5 and YbCoIn 5 significantly reduces the transmission probability of quasiparticles, thereby ensuring quasi-two-dimensional superconductivity confined within CeCoIn 5 layers [6,7]. This provides a unique opportunity to explore the physics discussed above. This is because bulk CeCoIn 5 with strong Pauli effect has been reported to host the FFLO phase at low temperatures and high magnetic field [8][9][10][11]. In the superlattice, the electronic structure becomes two-dimensional, which is expec...