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Abstract Superconductivity in quasicrystals poses a new challenge in condensed matter physics. We measured the resistance and ac magnetic susceptibility of a Ta1.6Te dodecagonal quasicrystal, which is superconducting below T c ~ 1 K. We show that the upper critical field increases linearly with a large slope of − 4.4 T/K with decreasing temperature down to 0.04 K, with no tendency to level off. The extrapolated zero-temperature critical field exceeds the Pauli limit by a factor of 2.3. We also observed flux-flow resistance with thermally activated behavior and an irreversibility field that is distinct from the upper critical field. We discuss these peculiarities in terms of the nonuniform superconducting gap and spin-orbit interaction in quasicrystal structures.

In bulk Sr_{2}RuO_{4}, the strong sensitivity of the superconducting transition temperature T_{c} to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr_{2}RuO_{4}, the relationship between T_{c} and the residual resistivity ρ_{0}, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr_{2}RuO_{4}, we show that T_{c} is suppressed in both systems at nearly identical rates. This suggests that part of ρ_{0} in films comes from defects that do not contribute to superconducting pairbreaking and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

Abstract The quest to improve transparent conductors balances two key goals: increasing electrical conductivity and increasing optical transparency. To improve both simultaneously is hindered by the physical limitation that good metals with high electrical conductivity have large carrier densities that push the plasma edge into the ultra-violet range. Technological solutions reflect this trade-off, achieving the desired transparencies only by reducing the conductor thickness or carrier density at the expense of a lower conductance. Here we demonstrate that highly anisotropic crystalline conductors offer an alternative solution, avoiding this compromise by separating the directions of conduction and transmission. We demonstrate that slabs of the layered oxides Sr2RuO4 and Tl2Ba2CuO6+δ are optically transparent even at macroscopic thicknesses >2 μm for c-axis polarized light. Underlying this observation is the fabrication of out-of-plane slabs by focused ion beam milling. This work provides a glimpse into future technologies, such as highly polarized and addressable optical screens.

This review describes the recent progress of floating-zone techniques for bulk single-crystal growth. The most crucial point of the crucible-free technique is to keep the molten zone stable. It has been investigated and reported to yield a steeper temperature gradient at the liquid–solid interface along the growth direction and a homogeneous molten liquid along the rotation axis. This article overviews several recent achievements starting from the conventional setup, particularly for lamps equipped in horizontal or vertical configurations, tilting mirrors, shielding the irradiation, and filament sizes for the optical-lamp floating-zone furnaces. Also, the recently advancing laser-heated floating-zone furnaces are described. Throughout the article, the author emphasizes that the floating-zone technique has been a powerful tool for crystal growth since the 1950s with its roots in the zone-melting method, and it has still been advancing for further materials’ growth such as quantum materials with modern scientific concepts.

The development of robust catalysts that could work under industrial-scale current densities is a challenge for hydrogen production. Here, the authors report an in-situ activation method to produce ferromagnetic Ru clusters that can catalyze the hydrogen evolution reaction at high current densities.

In the superconductor Sr2RuO4, in-plane strain is known to enhance both the superconducting transition temperature Tc and upper critical field Hc2, but the effect of out-of-plane strain has not been studied. Here, the authors find that Hc2 is enhanced under out-of-plane strain, but Tc unexpectedly decreases.

Abstract CaFeAsF is an iron-based superconductor parent compound whose Fermi surface is quasi-two dimensional, composed of Dirac-electron and Schrödinger-hole cylinders elongated along the c axis. We measured the longitudinal and Hall resistivities in CaFeAsF with the electrical current in the a b plane in magnetic fields up to 45 T applied along the c axis and obtained the corresponding conductivities via tensor inversion. We found that both the longitudinal and Hall conductivities approached zero above ~40 T as the temperature was lowered to 0.4 K. Our analysis indicates that the Landau-level filling factor is ν = 2 for both electrons and holes at these high field strengths, resulting in a total filling factor ν = ν hole − ν electron = 0. We therefore argue that the ν = 0 quantum Hall state emerges under these conditions.

Two possible scenarios of the superconducting order parameter in Sr2RuO4 remain difficult to distinguish. Here, the authors observe that the onset temperature of time reversal symmetry breaking tracks the superconducting transition temperature in Sr2RuO4, supporting a d x z ± id y z order parameter.

The successful growth of single crystals of a cubic Laves-phase material HoAl2 with the space group Fd-3m is reported in this study. The crystals were grown by a floating-zone method with five laser diodes as a heat source. Al-rich feed rods were prepared as compensation for heavy evaporation during the growth. The nominal ratio for the feed rods was optimized as Ho:Al = 1:2.5. Single crystals of HoAl2 with a length of 50 mm were first grown in this technique. Obtaining the large-sized crystal by the floating-zone method enabled us to systematically explore the physical properties using the same batch crystal. The crystal possessed a second-ordered ferromagnetic transition at 29 K and a first-ordered spin-reorientation transition at 20 K. The bulk physical properties, such as specific heat, magnetic susceptibility, isothermal magnetization, and thermal expansion measurements, were measured. Additionally, a magnetocaloric effect was evaluated by the magnetic entropy change. We demonstrate that anisotropic physical properties along the principal axes ([100], [110], and [111]) emerged below the magnetically ordered states, in contrast to the isotropic behavior in the paramagnetic state.

In this study, we report the successful growth of single crystals of a magnetic Weyl semimetal candidate NdAlGe with the space group I41md. The crystals were grown using a floating-zone technique, which used five laser diodes, with a total power of 2 kW, as the heat source. To ensure that the molten zone was stably formed during the growth, we employed a bell-shaped distribution profile of the vertical irradiation intensity. After the nominal powder, crushed from an arc-melted ingot, was shaped under hydrostatic pressure, we sintered the feed and seed rods in an Ar atmosphere under ultra-low oxygen partial pressure (−26 atm) generated by an oxygen pump made of yttria-stabilized zirconia heated at 873 K. Single crystals of NdAlGe were successfully grown to a length of 50 mm. The grown crystals showed magnetic order in bulk at 13.5 K. The fundamental physical properties were characterized by magnetic susceptibility, magnetization, specific heat, thermal expansion, and electrical resistivity measurements. This study demonstrates that the magnetic order induces anisotropic magnetoelasticity, magneto-entropy, and charge transport in NdAlGe.

We report the single-crystal growth of the unconventional superconductor Sr2RuO4, on which research has reached a turning point recently. In order to optimize the quality of crystals grown by the floating-zone method using an infrared image furnace, we focus on an improvement of the structure of the filament in the halogen lamps. By reducing the thickness of the total filament, the form of the molten zone was narrowed. More importantly, the molten zone was observed to be more stable during the growth process. Finally, we obtained the crystals with a length of 12 cm. Additionally, the grown crystal has high quality, displaying the 1.5 K transition temperature expected only for the purest crystals. We also discuss the availability of the newly developed halogen lamps.

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr2RuO4, which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures Tc that are consistently as high as 1.4 K, as well as the properties of these crystals.

In many physical situations in which many-body assemblies exist at temperature T, a characteristic quantum-mechanical time scale of approximately h/kBT can be identified in both theory and experiment, leading to speculation that it may be the shortest meaningful time in such circumstances. This behavior can be investigated by probing the scattering rate of electrons in a broad class of materials often referred to as "strongly correlated metals". It is clear that in some cases only electron-electron scattering can be its cause, while in others it arises from high-temperature scattering of electrons from quantized lattice vibrations, i.e., phonons. In metallic oxides, which are among the most studied materials, analysis of electrical transport does not satisfactorily identify the relevant scattering mechanism at "high" temperatures near room temperature. We therefore employ a contactless optical method to measure thermal diffusivity in two Ru-based layered perovskites, Sr3Ru2O7 and Sr2RuO4, and use the measurements to extract the dimensionless Lorenz ratio. By comparing our results to the literature data on both conventional and unconventional metals, we show how the analysis of high-temperature thermal transport can both give important insight into dominant scattering mechanisms and be offered as a stringent test of theories attempting to explain anomalous scattering. [ABSTRACT FROM AUTHOR]

A key question regarding the unconventional superconductivity of Sr2RuO4 remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few percent of that of the primary superconducting transition. The normalized jump in heat capacity, C=C, grows smoothly as a function of uniaxial pressure, favoring order parameters which are allowed to maximize in the same part of the Brillouin zone as the wellstudied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr2RuO4. [ABSTRACT FROM AUTHOR]

To investigate the electronic state of CeRu2Al10, which exhibits an unusual antiferromagnetic order below TN = 27K, we measured the electrical resistivity, the transverse magnetoresistance (TMR), the Hall resistivity H, and the Shubnikov-de Haas (SdH) effect at low temperatures down to ~20mK and high magnetic fields up to 17.8 T. The temperature dependence of P at zero magnetic field exhibits a T 2 dependence below ~0.7 K, indicating a Fermi-liquid ground state. The TMR increases with increasing the magnetic field for a wide range of field directions except for particular directions around H // α, where the TMR shows a broad maximum at ~12 T. The field dependence of PH for H // α shows an evident increase around this field, suggesting a marked change in the mobility of carriers. The SdH effect measurements at temperatures lower than those of our previous work depicted the Fermi surface (FS) topology in more detail and confirmed the small size of the FS (SdH frequency F ~ 210 T). Relatively heavy cyclotron effective masses of up to 3.6m0 (m0: rest mass of electron) with a large anisotropy were observed despite the small carrier density. [ABSTRACT FROM AUTHOR]

Magnetic properties of an Yb-based semiconductor YbCuS2 with a zigzag chain of Yb3+ ions were studied. The Curie-Weiss behavior of the magnetic susceptibility χ(T) indicates antiferromagnetic interaction and an isolated Kramers doublet ground state with effective spin-1=2. Magnetic specific heat Cm=T exhibits a sharp peak at TO = 0.95 K, where the magnetic entropy is 20% of R ln 2, indicating strong magnetic fluctuations. In magnetic fields, TO is robust for B < 4 T, and it shifts to higher temperatures but reverses downward for B ≥ 7 T. Below TO, a magnetization plateau manifests itself at around 6 T which is followed by transitions at 9.1 and 15.6 T. The phase diagram suggests that nontrivial phases arise from magnetic frustration in the Yb zigzag chain. [ABSTRACT FROM AUTHOR]

Ln2Co12P7 (Ln = Y, Nd, and Sm) shows a ferromagnetic transition of Co magnetic moments approximately at 150 K. To know how the Nd and Sm moments interact with the Co moments, the magnetocaloric effect of Ln2Co12P7 was studied. For Ln = Nd and Sm, the value of the magnetic entropy change of Ln2Co12P7 is positive below 50 K and increases with decreasing temperature. This behavior of the magnetic entropy change indicates that the Ln moments interact antiferromagnetically with the Co moments. For Ln = Sm, this is the first report to verify the antiferromagnetic interaction with the Co moments. The value of the magnetic entropy change steeply drops below 12 K for Ln = Nd and 20 K for Ln = Sm at 5 T, suggesting antiferromagnetic transitions. [ABSTRACT FROM AUTHOR]

Hard X-ray photoemission studies are carried out to evaluate the electronic structure of iron-based superconductors BaFe2(As1-xPx)2 in a wide range of the P content 0 ≤ x ≤ 0.61. As x increases, the Fe 2p3/2 and Ba 3d5/2 core levels linearly shift to the higher and lower binding energy sides, respectively, while the anion core levels show no appreciable shifts. These results can be explained in terms of the Madelung potential and the change of the hybridization between the Fe 3d and the anion orbitals. The increase of the hybridization with x leads to an increase of the band width, which is consistent with the spectrum shift near the Fermi level. [ABSTRACT FROM AUTHOR]

We report single-crystal growth ofa perovskite ruthenate SrRuO3by the floating-zone methodusing an infrared image furnace.By employing a cold trap for the growth, we prevented the evaporatedRuO2from coating the inner surface of a quartz tube andwere thereby able to maintain a stable molten zone during the growth.Powder X-ray and Laue diffraction measurements confirmed the qualityof the grown single crystal. The crystal was cut into a 1.4 mm ×1.4 mm × 3.0 mm rectangle that was characterized by magneticsusceptibility, magnetization, and specific heat measurements. Thecrystal showed a ferromagnetic transition with a Curie temperatureof 163.5 K, and the coercive field was as low as 0.004 T. The residualresistivity was 1.05 μΩ cm, corresponding to a residualresistivity ratio of 192. These results indicate that the grown crystalis high-quality. Electrical resistivity measurements show temperature-squaredbehavior, revealing that a Fermi-liquid-like electron–electronscattering is dominant. Together with a large electronic specificheat coefficient with 30 mJ K–2mol–1, we confirm that the ferromagnetic SrRuO3is a stronglycorrelated material. [ABSTRACT FROM AUTHOR]

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 ~18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at ~12 kbar. We find another anomaly at 21.2K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in μSR measurements [M. Bendele et al., Phys. Rev. Lett. 104, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to ~25 kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The magnetoresistance and magnetic torque of FeS are measured in magnetic fields B of up to 18 T down to a temperature of 0.03 K. The superconducting transition temperature is found to be Tc=4.1K, and the anisotropy ratio of the upper critical field Bc2 at Tc is estimated from the initial slopes to be Γ(Tc)=6.9. Bc2(0) is estimated to be 2.2 and 0.36 T for B∥ab and c, respectively. Quantum oscillations are observed in both the resistance and torque. Two frequencies F=0.15 and 0.20 kT are resolved and assigned to a quasi-two-dimensional Fermi surface cylinder. The carrier density and Sommerfeld coefficient associated with this cylinder are estimated to be 5.8×10-3 carriers/Fe and 0.48mJ/(K²mol), respectively. Other Fermi surface pockets still remain to be found. Band-structure calculations are performed and compared to the experimental results. [ABSTRACT FROM AUTHOR]

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of Sr2 RuO4 down to 40 mK. For high-quality single crystals with the transition temperature (Tc) of 1.48-1.49 K, an abrupt jump of the torque signal is found near 1.5 T in field parallel to the conducting RuO2 planes below ~0.8K. The jump corresponds to the first order transition recently revealed by magnetocaloric and magnetization measurements [Yonezawa et al., Phys. Rev. Lett. 110, 077003 (2013); Kittaka et al., Phys. Rev. B 90, 220502(R) (2014)]. Furthermore, weak diamagnetic and irreversible signals are found to persist above the first order transition up to 1.85 T. The result indicates the presence of a subphase boundary separating low- and high-field phases in the superconducting phase. The high-field subphase disappears when the field is tilted from the conducting planes only by a few degrees. Quantum oscillation measurements are also reported to clarify the strong sample-quality dependence of the high-field subphase. [ABSTRACT FROM AUTHOR]

The resistivity ρ and Hall resistivity ρH are measured on FeSe at pressures up to P = 28.3 kbar in magnetic fields up to B = 14.5 T. The ρ(B) and ρH(B) curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of P and temperature (T ≤ 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. [ABSTRACT FROM AUTHOR]

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∼8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dTc/dP observed in a range between P∼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: kBTc/EF∼0.1 even at P=16.1 kbar. [ABSTRACT FROM AUTHOR]

Iron oxide photoelectrodes were prepared using pulsed laser deposition with hematite pellet as the target. The dependences of photocurrent density on post-annealing O2 pressure, thickness, heating temperature and surface morphology were investigated in detail. The photoelectrode properties were characterized using Raman spectroscopy, XPS, UV-vis absorption spectroscopy, SEM, current density and photoelectrochemical measurements. The a-Fe2O3 thin film of thickness 70 nm prepared using the SiO2 sphere template method with post-annealing under an O2 pressure of 160 Pa at 550 degC showed the highest photocurrent density (275 uA cm[?]2) under AM1.5 solar simulated light irradiation. [ABSTRACT FROM AUTHOR]

The crystal structure, superconducting properties, and electronic structure of the novel superconducting 122-type antimonide BaPt2Sb2 have been studied by measurements of powder x-ray diffraction patterns, electrical resistivity, ac magnetic susceptibility, specific heat, and by ab initio calculations. The material crystallizes in a new monoclinic variant of the CaBe2Ge2-type structure, in which Pt2Sb2 layers that consist of PtSb4 tetrahedra, and Sb2Pt2 layers that consist of SbPt4 tetrahedra, are stacked alternatively, with Ba atoms located between them. Measurements of electrical resistivity, ac magnetic susceptibility, and specific heat revealed that BaPt2Sb2 is a superconducting material with a critical temperature of 1.8 K. The electronic heat capacity coefficient and the Debye temperature were 8.6(2) mJ/mol K2 and 146(4) K, respectively. The upper critical field and the Ginzburg-Landau coherent length were determined to be 0.27 T and 35 nm, respectively. The calculations showed that the material has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs, leading to anisotropic transport properties. The d states of the Pt atoms in the Pt2Sb2 layers are the main contributors to the density of states at the Fermi level. A comparison between experimental and calculated results indicates that BaPt2Sb2 is a superconducting material with moderate coupling. [ABSTRACT FROM AUTHOR]

We report measurements of ac magnetic susceptibility Xac and de Haas-van Alphen (dHvA) oscillations in KFe2As2 under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature Tc changes from negative to positive across Pc ~ 18 kbar, as previously reported. The ratio of the upper critical field to Tc, i.e., Bc2/Tc, is enhanced above Pc, and the shape of the Xac versus field curves qualitatively changes across Pc, dHvA oscillations smoothly evolve across Pc, indicating no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap s wave as a possible explanation. [ABSTRACT FROM AUTHOR]

Nonequilibrium steady state conditions induced by a dc current can alter the physical properties of strongly correlated electron systems. In this regard, it was recently shown that dc current can trigger novel electronic states, such as current-induced diamagnetism, which cannot be realized in equilibrium conditions. However, reversible control of diamagnetism has not been achieved yet. Here, we demonstrate reversible in situ control between a Mott insulating state and a diamagnetic semimetal-like state by a dc current in the Ti-substituted bilayer ruthenate Ca3(Ru1-xTix)2O7 (x=0.5%). By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the nonequilibrium steady state of this material. The present results open up the possibility of creating novel electronic states in a variety of strongly correlated electron systems under dc current. [ABSTRACT FROM AUTHOR]