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The superconducting diode effect (SDE), where zero-resistance states appear nonreciprocally during current injection, is receiving tremendous interest in both fundamental and applied physics because the SDE is a novel manifestation of symmetry breaking and enables the creation of a novel diode without energy loss. In particular, magnetic-field-free SDEs have been extensively investigated because of their potential to serve as building blocks for superconducting circuit technology. In this letter, we report the field-free SDE in a layered superconductor, FeSe. Its underlying physics is clarified by systematic controlled experiments to be an interplay of a large thermoelectric response and geometrical asymmetry in FeSe. Our findings can pave a new avenue for the construction of novel material and device platforms utilizing SDEs., Comment: 16 pages, 5 figures

We provide bulk transport evidence for twin-boundary pinning of vortices in FeSe. We measure interlayer resistance in FeSe in magnetic fields and find that, as the field is rotated in the $ab$ plane, the flux-flow resistivity is suppressed when the field direction is parallel to twinning planes. The width of the associated dip in the resistance vs in-plane field direction curve varies as $T^{1/2}B^{-3/4}$, consistent with the creation of kinked vortices near the parallel field geometry., Comment: 5 pages, 6 figures

The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, Bardeen-Cooper-Schrieffer Bose-Einstein-condensation (BCS-BEC) crossover, and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking. Here we report the observation of spin fluctuations originating from BFSs in the superconducting (SC) state via $^{77}$Se-nuclear magnetic resonance measurements to 100 mK. In a heavily S-substituted FeSe, we found an anomalous enhancement of low-energy spin fluctuations deep in the SC state, which cannot be explained by an impurity effect. Such unusual behavior implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs.

Continued advances in quantum technologies rely on producing nanometer-scale wires. Although several state-of-the-art nanolithographic technologies and bottom-up synthesis processes have been used to engineer such wires, critical challenges remain in growing uniform atomic-scale crystalline wires and constructing their network structures. Here we discover a simple method to fabricate atomic-scale wires with various arrangements, including stripes, X-, Y-junctions, and nanorings. Single-crystalline atomic-scale wires of a Mott insulator, whose band gap is comparable to those of wide-gap semiconductors, are spontaneously grown on graphite substrates \DEL{and epitaxial monolayer graphene on SiC }by pulsed-laser deposition. These wires are one-unit-cell-thick and have an exact width of two- and four-unit-cells (1.4 and 2.8\,nm) and lengths up to a few $\mu m$. We show that the non-equilibrium reaction-diffusion processes may play an essential role in atomic pattern formation. Our findings offer a new perspective on the non-equilibrium self-organization phenomena on an atomic scale, paving a unique way for the quantum architecture of nano-network.

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator beta'-EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in beta'-EtMe3Sb[Pd(dmit)2]2., Comment: 14 pages, 4 figures, to appear in Scientific Reports

In correlated electronic systems, strong interactions and the interplay between different degrees of freedom may give rise to anomalous charge transport properties, which can be tuned by external parameters like temperature and magnetic field. Recently, magnetic quantum oscillations and metallic low-temperature thermal conductivity have been observed in the Kondo insulator YbB$_{12}$, whose resistivity is a few orders of magnitude higher than those of conventional metals. As yet, these unusual observations are not fully understood. Here we present a detailed investigation of the behavior of YbB$_{12}$ under intense magnetic fields using both transport and torque magnetometry measurements. A low-field Hall anomaly, reminiscent of the Hall response associated with "strange-metal" physics, develops at $T < 1.5$ K. At two characteristic magnetic fields ($\mu_0H_1= 19.6$ T and $\mu_0H_2 \sim 31$ T), signatures appear in the Hall coefficient, magnetic torque, and magnetoresistance. We suggest that they are likely to be field-induced Lifshitz transitions. Moreover, above 35 T, the background resistivity displays an unusual, nonmetallic $T^{\alpha}$-behavior, with $\alpha$ being field-dependent and varying between -1.5 and -2. By normalizing the Shubnikov-de Haas oscillation amplitude to this $T^{\alpha}$-dependence, the calculated cyclotron mass becomes more consistent with that deduced from de Haas-van Alphen oscillations. Our results support a novel two-fluid scenario in YbB$_{12}$: a Fermi-liquid-like fluid of charge-neutral quasiparticles coexists with charge carriers that remain in a nonmetallic state. The former experience successive Lifshitz transitions and develop Landau quantization in applied magnetic fields, whilst scattering between both fluids allows the Shubnikov-de Haas effect to be observed in the electrical transport., Comment: 25 pages, 14 figures

Electronic nematicity in correlated metals often occurs alongside another instability such as magnetism. As a result, the question remains whether nematicity alone can drive unconventional superconductivity or anomalous (quantum critical) transport in such systems. In FeSe, nematicity emerges in isolation, providing a unique opportunity to address this question. Studies to date, however, have proved inconclusive; while signatures of nematic criticality are observed upon sulfur substitution, they appear to be quenched under the application of pressure due to the emergent magnetism. Here, we study the temperature and pressure dependence of the low-temperature resistivity of FeSe$_{1-x}$S$_{x}$ crystals at $x$ values just beyond the nematic quantum critical point. Two distinct components to the resistivity are revealed; one whose magnitude falls with increasing pressure and one which grows upon approaching the magnetic state at higher pressures. These findings indicate that nematic and magnetic critical fluctuations in FeSe$_{1-x}$S$_{x}$ are completely decoupled, in marked contrast to other Fe-based superconductors, and that nematic fluctuations alone may be responsible for the transport signatures of quantum criticality found in FeSe$_{1-x}$S$_{x}$ at ambient pressure.

Within condensed-matter systems, strong electronic interactions often lead to exotic quantum phases. A recent manifestation of this is the unexpected observation of magnetic quantum oscillations and metallic thermal transport, both properties of systems with Fermi surfaces of itinerant quasiparticles, in the Kondo insulators SmB6 and YbB$_{12}$. To understand these phenomena, it is informative to study their evolution as the energy gap of the Kondo-Insulator state is closed by a large magnetic field. We show here that both the quantum-oscillation frequency and the cyclotron mass display a strong field dependence in the resulting high-field metallic state in $_{12}$. By tracking the Fermi-surface area, we conclude that the same quasiparticle band gives rise to the quantum oscillations in both insulating and metallic states. These data are understood most simply using a two-fluid picture where unusual quasiparticles, contributing little or nothing to charge transport, coexist with conventional fermions. In the metallic state this leads to a heavy-fermion bad metal with negligible magnetoresistance, relatively high resistivity and a very large Kadowaki-Woods ratio, underlining the exotic nature of the fermion ensemble inhabiting $_{12}$., Comment: 11 pages, 3 figures. This is the original submitted version. Final version is accepted for publication in Nature Physics

Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials, including high-$T_c$ cuprates, iron pnictides, and heavy-fermion compounds. A new approach to this most fundamental and hotly debated subject focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. A recent state-of-the-art molecular-beam-epitaxy technique has enabled us to fabricate superlattices consisting of different heavy-fermion compounds with atomic thickness. These Kondo superlattices provide a unique opportunity to study the mutual interaction between unconventional superconductivity and magnetic order through the atomic interface. Here, we design and fabricate hybrid Kondo superlattices consisting of alternating layers of superconducting CeCoIn$_5$ with $d$-wave pairing symmetry and nonmagnetic metal YbCoIn$_5$ or antiferromagnetic heavy fermion metals, such as CeRhIn$_5$ and CeIn$_3$. In these Kondo superlattices, superconducting heavy electrons are confined within the two-dimensional CeCoIn$_5$ block layers and interact with the neighboring nonmagnetic or magnetic layers through the interface. In CeCoIn$_5$/YbCoIn$_5$ superlattices, the superconductivity is strongly influenced by the local inversion symmetry breaking at the interface. In CeCoIn$_5$/CeRhIn$_5$ and CeCoIn$_5$/CeIn$_3$ superlattices, the superconducting and antiferromagnetic states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting and magnetic properties. The fabrication of a wide variety of hybrid superlattices paves a new way to study the relationship between unconventional superconductivity and magnetism in strongly correlated materials., Comment: 18 pages, 17 figures. arXiv admin note: text overlap with arXiv:1904.06937

To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first-principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe L$_3$ absorption resonance of the example material BaFe$_2$As$_2$, we show that the RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel contribution, including the Raman-like response below threshold, which we explain as a consequence of the finite core-hole lifetime broadening. Calculations are initially performed within the first-principles Bethe-Salpeter framework, which we then significantly improve by convolution with an effective spectral function for the intermediate-state excitation. We construct this spectral function, also from first-principles, by employing the cumulant expansion of the Green's function and performing a real-time time dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this allows us to evaluate the indirect RIXS response from first-principles, accounting for the full periodicity of the crystal structure and with dependence on the momentum transfer., Comment: 18 pages, submitted

High-temperature superconductivity and a wide variety of exotic superconducting states discovered in FeSe-based materials have been at the frontier of research on condensed matter physics over the past decade. Unique properties originating from the multiband electronic structure, strongly orbital-dependent phenomena, extremely small Fermi energy, electronic nematicity, and topological aspects give rise to many distinct and fascinating superconducting states. Here, we provide an overview of our current understanding of the superconductivity of {\it bulk} FeSe-based materials, focusing on FeSe and the isovalent substituted FeSe$_{1-x}$S$_{x}$ and FeSe$_{1-x}$Te$_{x}$. We discuss the highly nontrivial superconducting properties in FeSe, including extremely anisotropic pairing states, crossover phenomena from Bardeen--Cooper--Schrieffer (BCS) to Bose--Einstein condensation (BEC) states, a novel field-induced superconducting phase, and broken time-reversal symmetry. We also discuss the evolution of the superconducting gap function with sulfur and tellurium doping, paying particular attention to the impact of quantum critical nematic fluctuations and the topological superconductivity. FeSe-based materials provide an excellent playground to study various exotic superconducting states., Comment: 33 pages, 32 figures, invited review article

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe-K$_\beta$ emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$ (0.00$\leq$x$\leq0.$52). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome., Comment: Accepted in Communications Physics

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the $^{31}$P-NMR relaxation rate on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x = 0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x = 0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4~GPa, whereas the superconducting transition temperature $T_{\rm c}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and $T_{\rm c}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the $T_{\rm c}$ dome as a function of Weiss temperature $\theta$, which measures the distance to the QCP. Moreover, the $T_{\rm c}$-$\theta$ curve under pressure precisely coincides with that with chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$., Comment: 5 pages, 6 figures

Photo-excitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron-, lattice-, and spin-degrees of freedom change. One of the most spectacular phenomena is photo-induced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from original Tc with significant lattice modulations. Here we show another photo-induced high-Tc superconducting state in the iron-based superconductor FeSe with semi-metallic hole and electron bands. The transient electronic state in the entire Brillouin zone is directly observed by the time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors on timescales from 50 fs to 800 ps consistently support the favorable superconducting state after photo-excitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photo-induced superconductors.

The 12%-S doped FeSe system has a high Tc of 30 K at a pressure of 3.0 GPa. We have successfully investigated its microscopic properties for the first time via $^{77}$Se-NMR measurements under pressure. The antiferromagnetic (AFM) fluctuations at the optimal pressure (~3 GPa) exhibited unexpected suppression compared with the AFM fluctuations at ambient pressure, even though the optimal pressure is close to the phase boundary of the AFM phase induced at the high-pressure region. In addition, we revealed that the SC phase at an applied field of 6.02 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zero field., Comment: arXiv admin note: substantial text overlap with arXiv:1807.07118

It 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 thin films of CeRhIn$_5$, a prototypical heavy fermion compound with antiferromagnetic ground state. The conductance spectra above the N\'eel temperature $T_N$ clearly resolve the energy gap due to the hybridization between local 4$f$ electrons and conduction bands as well as the crystal electric field excitations. These structures persist even below $T_N$. Moreover, an additional dip in the conductance spectra develops due to the antiferromagnetic order. These results provide direct evidence for the presence of itinerant heavy $f$-electrons participating in the Fermi surface even in the magnetically ordered state of CeRhIn$_5$.

Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of 59Co nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN ~ 3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties., Comment: 5 pages, 4 figures

Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the {$in$} {$situ$} scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In$_{1-x}$Zn$_x$)$_5$ films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the $c$-$f$ hybridization gap, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn$_5$. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn$_5$, in both of which $d$-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

Since the progress in the fabrication techniques of thin-films of exotic materials such as strongly correlated heavy-fermion compounds, microscopic studies of the magnetic and electronic properties inside the films have been needed. Herein, we report the first observation of 115In nuclear quadrupole resonance (NQR) in an epitaxial film of the heavy-fermion superconductor CeCoIn5, for which the microscopic field gradient within the unit cell as well as magnetic and superconducting properties at zero field are evaluated. We find that the nuclear spin-lattice relaxation rate in the film is in excellent agreement with that of bulk crystals, whereas the NQR spectra show noticeable shifts and significant broadening indicating a change in the electric-field distribution inside the film. The analysis implies a displacement of In layers in the film, which however does not affect the magnetic fluctuations and superconducting pairing. This implies that inhomogeneity of the electronic field gradient in the film sample causes no pair breaking effect., Comment: 12 pages, 6 figures

In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature $T_c$ is depressed at high concentrations ($x\gtrsim$0.28), it shows an initial increase at lower $x$. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-$T_c$ family., Comment: 5 pages, 4 figures

When quantum fluctuations destroy underlying long-range ordered states, novel quantum states emerge. Spin-liquid (SL) states of frustrated quantum antiferromagnets, in which highly-correlated spins keep to fluctuate down to very low temperatures, are prominent examples of such quantum states. SL states often exhibit exotic physical properties, but the precise nature of the elementary excitations behind such phenomena remains entirely elusive. Here we utilize thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report on the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity (k_xy) which the charge-neutral spin excitations in a gapless SL state of the two-dimensional kagome insulator volborthite Cu_3V_2O_7(OH)_2 \cdot 2H_2O exhibit, in much the same way in which charged electrons give rise to the conventional electric Hall effect. We find that k_xy is absent in the high-temperature paramagnetic state and develops upon entering the SL state in accordance with the growth of the short-range spin correlations, demonstrating that k_xy is a key signature of the elementary excitation formed in the SL state. These results suggest the emergence of nontrivial elementary excitations in the gapless SL state which feel the presence of fictitious magnetic flux, whose effective Lorentz force is found to be less than 1/100 of that experienced by free electrons., Comment: 9 pages, 4 figures, Proc. Natl. Acad. Sci. USA (2016)

There is growing evidence that the superconducting semimetal FeSe ($T_c\sim8$ K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, $\kappa_{xx}$ and $\kappa_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $\kappa_{xx}$ and $\kappa_{xy}$ imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below $\sim1.0$ K, both $\kappa_{xx}$ and $\kappa_{xy}$ exhibit distinct anomalies (kinks) at the upper critical field $H_{c2}$ and at a field $H^*$ slightly below $H_{c2}$. The analysis of the thermal Hall angle ($\kappa_{xy}/\kappa_{xx}$) indicates a change of the quasiparticle scattering rate at $H^*$. These results provide strong support to the previous suggestion that above $H^*$ a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance., Comment: 7 pages, 3 figures, to be published in JPSJ

The resistivity $\rho$ and Hall resistivity $\rho_H$ are measured on FeSe at pressures up to $P$ = 28.3 kbar in magnetic fields up to $B$ = 14.5 T. The $\rho(B)$ and $\rho_H(B)$ curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of $P$ and temperature ($ T\leqslant$ 110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order., Comment: 6 pages, 5 figures + SM ( 2 pages, 2 figures)

Realizing new classes of ground states in strongly correlated electron systems continues to be at the forefront of condensed matter physics. Heavy-fermion materials, whose electronic structure is essentially three-dimensional, are one of the most suitable systems for obtaining novel electronic states because they demonstrate many fascinating properties. Recently, a state-of-the-art MBE technique has been developed to reduce the dimensionality of the heavy electrons by fabricating heavy fermion superlattices, which can produce new electronic states present in two-dimensional (2D) heavy fermion system. In superlattices of antiferromagnetic heavy fermion CeIn3 and conventional metal LaIn3, the magnetic order is suppressed by reducing the thickness of the CeIn3 layers. The 2D confinement of heavy fermion also leads to the enhancement of the effective mass and the deviation from the Fermi liquid properties, which are associated with the dimensional tuning of quantum criticality. In superconducting superlattices of heavy fermion superconductor CeCoIn5 and nonmagnetic metal YbCoIn5, superconductivity is realized even at one-unit-cell-thick layer of CeCoIn5. The thickness reduction of the CeCoIn5 layers drastically changes the temperature and angular dependencies of the upper critical field. This result would be attributed to a suppression of the Pauli pair-breaking effect through the local inversion symmetry breaking (ISB) at the interfaces of CeCoIn5 block layers. The importance of the ISB in this system has also been supported by site-selective nuclear magnetic resonance spectroscopy. In addition, recent experiment of CeCoIn5/YbCoIn5 superlattices have shown that the degree of the ISB are controllable, which offers the prospect of achieving even more fascinating superconducting states. These Kondo superlattices, thus, pave the way for exploring unusual metallic and superconducting states., Comment: An invited review article for Reports on Progress in Physics, 23 pages, 13 figures, and 1 table

We discuss the charge carrier dynamics of the heavy-fermion compound CeCoIn$_5$ in the metallic regime measured by means of quasi-optical THz spectroscopy. The transmittance of electromagnetic radiation through a CeCoIn$_5$ thin film on a dielectric substrate is analyzed in the single-particle Drude framework. We discuss the temperature dependence of the electronic properties, such as the scattering time and dc-conductivity and compare with transport measurements of the sheet resistance. Towards low temperatures, we find an increasing mismatch between the results from transport and Drude-analyzed optical measurements and a growing incapability of the simple single-particle picture describing the charge dynamics, likely caused by the evolving heavy-fermion nature of the correlated electron system.

In this report, we comprehensively study the effect of H$^+$ irradiation on the critical current density, $J_c$, and vortex pinning in FeSe single crystal. It is found that the value of $J_c$ for FeSe is enhanced more than twice after 3-MeV H$^+$ irradiation. The scaling analyses of the vortex pinning force based on the Dew-Hughes model reveal that the H$^+$ irradiation successfully introduce point pinning centers into the crystal. We also find that the vortex creep rates are strongly suppressed after irradiation. Detailed analyses of the critical current dependent pinning energy based on the collective creep theory and extend Maley's method show that the H$^+$ irradiation enhances the value of $J_c$ before the flux creep, and also reduces the size of flux bundle, which will further reduce the field dependence of $J_c$ due to vortex motion., Comment: 4 pages, 4figures

We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor CeCoIn5 and conventional-metal YbCoIn5 by site-selective nuclear magnetic resonance(NMR) spectroscopy, which uniquely offers spatially-resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce-BLs, indicating that magnetic degrees of freedom of f-electrons are quenched inside the Yb-BLs. Contrary to simple expectations that the two-dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce-BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion-symmetry breaking at the interfaces., Comment: 5 pages, 4 figures, accepted for publication as a Rapid Communication in Physical Review B

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal, which is free from doping introduced inhomogeneities and charged quasi-particle-scattering because of its innate superconductivity. Critical current density, $J_c$, is found to be almost isotropic, and reaches a value $\sim$ 3 $\times$ 10$^4$ A/cm$^2$ at 2 K (self-field) for both $H$ $\|$ $c$ and $ab$. The normalized magnetic relaxation rate $S$ (= $\mid$dln$M$/dln$t$$\mid$) shows a temperature insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate ($S$ $\sim$ 0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fish-tail effect is absent for both $H$ $\|$ $c$ and $ab$. Based on this observation, the origin of the fish-tail effect is also discussed. Combining the results of $J_c$ and $S$, vortex motion in FeSe single crystal is found to be dominated by sparse strong point-like pinning from nm-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed., Comment: 7 pages, 5 figures

To elucidate the origin of nematic order in Fe-based superconductors, we report a Raman scattering study of lattice dynamics, which quantify the extent of $C_4$-symmetry breaking, in BaFe$_2$As$_2$ and FeSe. FeSe possesses a nematic ordering temperature $T_\mathrm{s}$ and orbital-related band-energy split below $T_\mathrm{s}$ that are similar to those in BaFe$_2$As$_2$, but unlike BaFe$_2$As$_2$ it has no long-range magnetic order. We find that the $E_g$ phonon-energy split in FeSe sets in only well below $T_\mathrm{s}$, and its saturated value is substantially smaller than that in BaFe$_2$As$_2$. Together with reported results for the Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ family, the data suggest that magnetism exerts a major influence on the lattice., Comment: 4 pages, 4 figures

We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to $P$ = 16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at $P$ $\sim$ 8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative d$T_c$/d$P$ observed in a range between $P$ $\sim$ 8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: $k_BT_c/E_F$ $\sim$ 0.1 even at $P$ = 16.1 kbar., Comment: 12 pages , 10 figures

We report on $^{75}$As-NMR measurements in single crystalline BaFe$_{2}$(As$_{0.96}$P$_{0.04}$)$_{2}$ for magnetic fields parallel to the orthorhombic $[110]_{\rm o}$ and $[100]_{\rm o}$ directions above the structural transition temperature $T_{\rm S}\simeq121$ K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-$4p_{x}$ and $4p_{y}$ orbitals, which reaches $|n_x-n_y|/|n_x+n_y| \sim 15$\% at $T\rightarrow T_{\rm S}$ and is a natural consequence of the orbital ordering of Fe-3$d_{xz}$ and $d_{yz}$ electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below $\sim 160$ K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state., Comment: 6 pages, 6 figures. Supplemental Materials are included. To appear in J. Phys. Soc. Jpn. (Vol.84 No. 4)

We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above $\sim$18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at $\sim12$ kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in $\mu$SR measurements [M. Bendele \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to $\sim25$ kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists., Comment: 3 figures

We use magnetic force microscopy (MFM) to characterize superconductivity across the superconducting dome in ${\rm {BaFe}_2 \left(As_{1-x}P_x\right)_2}$, a pnictide with a peak in the penetration depth ($\lambda_{ab}$) at optimal doping ($\rm x_{opt}$), as shown in sample-wide measurements. Our local measurements show a peak at $\rm x_{opt}$ and a $T_C$ vs. $\lambda_{ab}^{-2}$ dependence similar on both sides of $\rm x_{opt}$. Near the underdoped edge of the dome $\lambda_{ab}$ increases sharply suggesting that superconductivity competes with another phase. Indeed MFM vortex imaging shows correlated defects parallel to twin boundaries only in underdoped samples and not for $\rm x\ge x_{opt}$., Comment: 23 pages (11 main text, 12 support material)

We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering., Comment: 6 pages, 6 figures, 1 table, published in Phys. Rev. B

The optical properties of thin films of the heavy-fermion compound CeCoIn5, which were deposited by molecular beam epitaxy onto MgF2 substrates, have been studied at frequencies 7 to 45 cm^{-1} (corresponding to 0.2 to 1.3 THz) and temperatures 2 to 300 K. We observe an electrodynamic behavior which is typical for heavy fermions, namely Drude-like conductivity with a relaxation rate at rather low frequencies. This relaxation rate increases almost linearly with temperature up to at least 30 K. The coherent heavy-fermion state, characterized by an increase of the effective mass, continuously evolves upon cooling and is not fully developed for temperatures as low as 5 K., Comment: 5 pages, 4 figures

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66}, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals. Analysis reveals that the pinning force and -energy distributions depend on the P-content $x$. However, they are always much narrower than in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_{c}$ in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\bar{\mathcal L} \sim 90$ nm, increasing with increasing P-content $x$. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution., Comment: 11 pages

High energy electron irradiation is used to controllably introduce atomic-scale point defects into single crystalline Ba(Fe_1-xCo_x)_2As_2, Ba(Fe_1-xNi_x)_2As_2, and BaFe_2(As_1-xP_x)_2. The appearance of the collective pinning contribution to the critical current density in BaFe_2(As_1-xP_x)_2, and the magnitude of its enhancement in Ba(Fe_1-xCo_x)_2As_2, conform with the hypothesis of quasi-particle scattering by Fe vacancies created by the irradiation. Whereas the insignificant modification of the temperature dependence of the superfluid density in Ba(Fe_1-xCo_x)_2As_2 and Ba(Fe_1-xNi_x)_2As_2 points to important native disorder present before the irradiation, the critical temperatures of these materials undergo a suppression equivalent to that observed in the much cleaner BaFe_2(As_1-xP_x)_2. This lends credence to the hypothesis of line nodes of the order parameter (at finite k_{z}) in the former two materials., Comment: 17 pages, soumis pour publications dans les actes de M2S-2012; Materials and Mechanisms of Superconductivity 2012, Washington DC : United States (2012)

Quantum spin liquids (QSLs) are fluid-like states of quantum spins where its long-range ordered state is destroyed by quantum fluctuations. The ground state of QSL and its exotic phenomena, which have been extensively discussed for decades, have yet to be identified. We employ thermal transport measurements on newly discovered QSL candidates, $\kappa$-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, and report that the two organic insulators possess different QSLs characterized by different elementary excitations. In $\kappa$-(BEDT-TTF)2Cu2(CN)3, heat transport is thermally activated at low temperatures, suggesting presence of a spin gap in this QSL. In stark contrast, in EtMe3Sb[Pd(dmit)2]2, a sizable linear temperature dependence of thermal conductivity is clearly resolved in the zero-temperature limit, showing gapless excitation with a long mean free path (~1,000 lattice distances). Such a long mean free path demonstrates a novel feature of QSL as a quantum-condensed state with long-distance coherence., Comment: 5 pages, 3 figures; invited mini-review article for ChemPhysChem

Measurements of the superconducting transition temperature, T_c, under hydrostatic pressure via bulk AC susceptibility were carried out on several concentrations of phosphorous substitution in BaFe_2(As_{1-x}P_x)_2. The pressure dependence of unsubstituted BaFe_2As_2, phosphorous concentration dependence of BaFe_2(As_{1-x}P_x)_2, as well as the pressure dependence of BaFe_2(As_{1-x}P_x)_2 all point towards an identical maximum T_c of 31 K. This demonstrates that phosphorous substitution and physical pressure result in similar superconducting phase diagrams, and that phosphorous substitution does not induce substantial impurity scattering., Comment: 5 pages, 4 figures, to be published in Journal of the Physical Society of Japan

Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material., Comment: 4 pages

^{31}P and ^{75}As NMR measurements were performed in superconducting BaFe_2(As_{0.67}P_{0.33})_2 with T_c = 30 K. The nuclear-spin-lattice relaxation rate T_1^{-1} and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T_1^{-1} in the superconducting (SC) state decreases without a coherence peak just below T_c, as observed in (Ba_{1-x}K_{x})Fe_2As_2. In contrast to other iron arsenide superconductors, the T_1^{-1} \propto T behavior is observed below 4K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe_2(As_{1-x}P_{x})_2 despite a comparable T_c value, an analogous phase diagram, and similar Fermi surfaces to (Ba_{1-x}K_{x})Fe_2As_2., Comment: 4 pages, 5 figures