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We have investigated the structural, electrical, and magnetic properties of oxychalcogenide LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) superconductors. The LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) samples are composed of a single La2O2Bi3AgS6 phase without any impurity phase. The superconducting onset temperature of LaREO2Bi3Ag0.6Sn0.4S6 (RE = Pr and Nd) is $\sim $ 4.1 K, which is slightly higher than 2.8 K observed for La2O2Bi3Ag0.6Sn0.4S6.

For high-Tc superconductors, clarifying the role and origin of the pseudogap is essential for understanding the pairing mechanism. Among the various models describing the pseudogap, the preformed Cooper pair model is a potential candidate. Therefore, we present experimental evidence for the preformed Cooper pair model by studying the pseudogap spectrum observed in the optical conductivity of a Ca10(Pt4As8)(Fe2As2)5 (Tc = 34.6 K) single crystal. We observed a clear pseudogap structure in the optical conductivity and observed its temperature dependence. In the superconducting (SC) state, one SC gap with a gap size of Δ = 26 cm−1, a scattering rate of 1/τ = 360 cm−1 and a low-frequency extra Drude component were observed. Spectral weight analysis revealed that the SC gap and pseudogap are formed from the same Drude band. This means that the pseudogap is a gap structure observed as a result of a continuous temperature evolution of the SC gap observed below Tc. This provides clear experimental evidence for the preformed Cooper pair model.

Alkali metal dosing (AMD) has been widely used as a way to control doping without chemical substitution. This technique, in combination with angle resolved photoemission spectroscopy (ARPES), often provides an opportunity to observe unexpected phenomena. However, the amount of transferred charge and the corresponding change in the electronic structure vary significantly depending on the material. Here, we report study on the correlation between the sample work function and alkali metal induced electronic structure change for three iron-based superconductors: FeSe, Ba(Fe$_{0.94}$Co$_{0.06}$)$_{2}$As$_{2}$ and NaFeAs which share a similar Fermi surface topology. Electronic structure change upon monolayer of alkali metal dosing and the sample work function were measured by ARPES. Our results show that the degree of electronic structure change is proportional to the difference between the work function of the sample and Mulliken's absolute electronegativity of the dosed alkali metal. This finding provides a possible way to estimate the AMD induced electronic structure change., 4 pages

The temperature dependence of the electric resistivity measured in various magnetic fields was analyzed by the vortex glass theory and the thermally activated flux motion (TAFM) theory. The vortex glass-to-vortex liquid (GTL) transition Tg obtained from the analysis shows a temperature dependence of B g T = B 0 1 - T / T c m . The vortex liquid region is divided into the critical region existing in a finite temperature region just above Tg and the TAFM region present in the finite temperature region above it. In the critical region, the activation energy is expressed as U e f f = k B T T c - T / ( T c - T g ) , whereas in the TAFM region, the activity energy is expressed as temperature-nonlinear U T , B = U 0 B 1 - t q . In the GTL transition, (Ca0.85La0.15)10(Pt3As8)(Fe2As2)5 maintains the 3D vortex structure without exhibiting dimension crossover of the vortex, but Ca10(Pt3As8)((Fe0.92Pt0.08)2As2)5 exhibits the dimension crossover from the 3D vortex glass to the 2D vortex liquid.

We investigate changes in the vortex pinning mechanism caused by proton irradiation through the measurement of the in-plane electrical resistivity for H//c in a pristine and two proton-irradiated (total doses of 1 × 1015 and 1 × 1016 cm−2) SmBa2Cu3O7-δ (SmBCO) superconducting tapes. Even though proton irradiation has no effect on the critical temperature (Tc), the resulting artificial point defect causes an increase in normal state electrical resistivity. The electrical resistivity data around Tc shows no evidence of a phase transition to the vortex glass state but only broadens with increasing magnetic field due to the vortex depinning in the vortex liquid state. The vortex depinning is well interpreted by a thermally activated flux flow model in which the activation energy shows a nonlinear temperature change $${\boldsymbol{U}}{\boldsymbol{(}}{\boldsymbol{T}},{\boldsymbol{H}}{\boldsymbol{)}}{\boldsymbol{=}}{{\boldsymbol{U}}}_{{\boldsymbol{0}}}{\boldsymbol{(}}{\boldsymbol{H}}{\boldsymbol{)}}{{\boldsymbol{(}}{\bf{1}}-{\boldsymbol{T}}{\boldsymbol{/}}{{\boldsymbol{T}}}_{{\boldsymbol{c}}}{\boldsymbol{)}}}^{{\boldsymbol{q}}}$$ U ( T , H ) = U 0 ( H ) ( 1 − T / T c ) q (q = 2). The field dependence of activation energy shows a $${{\boldsymbol{U}}}_{{\bf{0}}}{\boldsymbol{ \sim }}{{\boldsymbol{H}}}^{-{\boldsymbol{\alpha }}}$$ U 0 ~ H − α with larger exponents above 4 T. This field dependence is mainly due to correlated disorders in pristine sample and artificially created point defects in irradiated samples. Compared with the vortex pinning due to correlated disorders, the vortex pinning due to the appropriate amount of point defects reduces the magnitude of Uo(H) in the low magnetic field region and slowly reduces Uo(H) in high magnetic fields.

We measured the optical reflectivity R(ω) for an underdoped (Ca0.935La0.065)10(Pt3As8)(Fe2As2)5 single crystal and obtained the optical conductivity $${\sigma }_{1}(\omega )$$ σ 1 ( ω ) using the K-K transformation. The normal state $${\sigma }_{1}(\omega )$$ σ 1 ( ω ) at 30 K is well fitted by a Drude-Lorentz model with two Drude components (ωp1 = 1446 cm−1 and ωp2 = 6322 cm−1) and seven Lorentz components. Relative reflectometry was used to accurately determine the temperature dependence of the superconducting gap at various temperatures below Tc. The results clearly show the opening of a superconducting gap with a weaker second gap structure; the magnitudes for the gaps are estimated from the generalized Mattis-Bardeen model to be Δ1 = 30 and Δ2 = 50 cm−1, respectively, at T = 8 K, which both decrease with increasing temperature. The temperature dependence of the gaps was not consistent with one-band BCS theory but was well described by a two-band (hence, two gap) BCS model with interband interactions. The temperature dependence of the superfluid density is flat at low temperatures, indicating an s-wave full-gap superconducting state.

A single crystal of rare earth intermetallic compound Dy5Si2Ge2 (Orthorhombic, Space group Pnma) has been prepared by Czochralski method. Magnetization of Dy5Si2Ge2 single crystal has been measured with magnetic field applied (i) along b axis and (ii) parallel to ac plane. Although a same Neel temperature (TN) of ∼55 K is obtained when 50 Oe field is applied along b axis and in ac plane, the magnitude of magnetization is substantially different. This TN value is close to the antiferromagnetic ordering temperature of polycrystalline Dy5Si2Ge2 as well. The magnetization vs field data at 1.8 K shows an ultra-sharp metamagnetic transition when field is applied along both the directions, in a critical field of about 28 kOe. It is observed that b axis is easy direction of magnetization. Magnetocaloric effect of this Dy5Si2Ge2 crystal has been estimated using magnetization vs field data obtained at various temperatures. A maximum magnetic entropy change (ΔSmmax) of ∼−15.6 J/kg/K has been observed at 22 K with field along b axis and ΔSmmax is ∼−11.2 J/kg/K at 54 K when field is parallel to ac plane, for a field change of 50 kOe. Occurrence of metamagnetic transition at temperatures below TN gives rise to a second peak at ∼22 K in the temperature dependent isothermal magnetic entropy change that leads to the observed large magnetocaloric effect.

For newly synthesized hole-doped Ca8.5Na1.5(Pt3As8)(Fe2As2)5 single crystals, we measured the infrared reflectivity spectrum and the magnetic field dependence of magnetoresistivity and Hall resistivity. The results of these two experiments in normal states are well described by two band models. In the normal state below 150 K, the optical conductivity spectrum shows a transfer of spectral weights from the mid-infrared region to the near-infrared region. Meanwhile, the magnetoresistance and Hall resistance show a significant decrease in carrier density at 150 K. These two phenomena are due to the conversion of itinerant electrons to heavy electrons by the strong correlation effect, Hund's coupling. In the superconducting state, the spectral weight in the low frequency region by the superconducting condensate is completely suppressed, which is well analyzed by the generalized Mattis-Bardeen (M-B) model with a two superconducting gap.

α-RuCl3 has recently attracted great interest as a possible experimental realization of the Kitaev model. Neutron scattering measurements of a single crystal of this material reveal signatures of Majorana excitations, consistent with Kitaev’s predictions. Geometrical constraints to the electronic degrees of freedom within condensed-matter systems often give rise to topological quantum states of matter such as fractional quantum Hall states, topological insulators, and Weyl semimetals1,2,3. In magnetism, theoretical studies predict an entangled magnetic quantum state with topological ordering and fractionalized spin excitations, the quantum spin liquid4. In particular, the so-called Kitaev spin model5, consisting of a network of spins on a honeycomb lattice, is predicted to host Majorana fermions as its excitations. By means of a combination of specific heat measurements and inelastic neutron scattering experiments, we demonstrate the emergence of Majorana fermions in single crystals of α-RuCl3, an experimental realization of the Kitaev spin lattice. The specific heat data unveils a two-stage release of magnetic entropy that is characteristic of localized and itinerant Majorana fermions. The neutron scattering results corroborate this picture by revealing quasielastic excitations at low energies around the Brillouin zone centre and an hour-glass-like magnetic continuum at high energies. Our results confirm the presence of Majorana fermions in the Kitaev quantum spin liquid and provide an opportunity to build a unified conceptual framework for investigating fractionalized excitations in condensed matter1,6,7,8.

The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($\sigma(\omega)$) spectra. Remarkably, the temperature variation in the $\sigma(\omega)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $\sigma(\omega)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $\sigma(\omega)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.

A single crystal of a superconducting Ca10(Pt4 As8)(Fe2−xPtxAs2)5 (x = 0.01) was grown by the Bridgman method. The transition temperature was 32 K. The penetration field, H p , was measured and the superconducting parameters, κ, λ, and ξ, were obtained from H p . A fitting function was derived for the field dependence of the magnetization (M−H curve) including the fishtail effect and the secondary peak using a statistical method. The temperature dependence of the upper critical field, H c2, was obtained from the fitting function and H c2(0) was estimated to be 134 T. In addition, the SDW transition in the normal state is reported.

MgB2 samples were prepared from a stoichiometry mixture of Mg and B inside stainless steel tubes. The transition temperature of the specimens was 37.5 K with a sharp transition width within 1 K. From the magnetic hysteresis measurements, flux jump was observed up to 15 K. The flux jump is believed to begin at the fluxes pinned at the defects. An over-moving flux situation formed at the places where there were moving fluxes that had been pinned at the defect and steady state ones. The flux jumps depend not on the amount of impurities and second phases, but on their distribution, pinning strength and heat capacity.

We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1−xCoxAs single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x = 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x ≥ 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around Tc(H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above Tc may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level.

We measured the optical reflectivity R(ω) for an underdoped (Ca

We report on the thermally activated flux flow dependency on the doping dependent mixed state in NaFe1−x Co x As (x = 0.01, 0.03, and 0.07) crystals using the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found clearly that irrespective of the doping ratio, magnetoresistivity showed a distinct tail just above the Tc,offset associated with the thermally activated flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature dependence of the activation energy follows the relation $$U(T,B)={U}_{0}(B){(1-T/{T}_{c})}^{q}$$ U ( T , B ) = U 0 ( B ) ( 1 − T / T c ) q with q = 1.5 in all studied crystals. The magnetic field dependence of the activation energy follows a power law of $${U}_{0}(B)\sim {B}^{-\alpha }$$ U 0 ( B ) ∼ B − α where the exponent α is changed from a low value to a high value at a crossover field of B = ∼2 T, indicating the transition from collective to plastic pinning in the crystals. Finally, it is suggested that the 3D vortex phase is the dominant phase in the low temperature region as compared to the TAFF region in our series samples.

We have measured the reflectivity of the optimally doped ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ single crystal $({T}_{c}=32.8\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ over the broad frequency range from $40{\text{cm}}^{\ensuremath{-}1}$ to 12 $000{\text{cm}}^{\ensuremath{-}1}$ and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region $(l1000\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1})$ in the normal state (80 K $lT\ensuremath{\le}300$ K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around $100\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below ${T}_{c}$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above ${T}_{c}$ into the SC gap structure below ${T}_{c}$. The extracted two SC gap sizes are ${\mathrm{\ensuremath{\Delta}}}_{S}=4.9$ $\text{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{L}=14.2$ $\text{meV}$, suggesting ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.

We report on the thermally activated flux flow dependency on the doping dependent mixed state in NaFe1−xCoxAs (x = 0.01, 0.03, and 0.07) crystals using the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found clearly that irrespective of the doping ratio, magnetoresistivity showed a distinct tail just above the T c,offset associated with the thermally activated flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature dependence of the activation energy follows the relation \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$U(T,B)={U}_{0}(B){(1-T/{T}_{c})}^{q}$$\end{document}U(T,B)=U0(B)(1−T/Tc)q with q = 1.5 in all studied crystals. The magnetic field dependence of the activation energy follows a power law of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${U}_{0}(B)\sim {B}^{-\alpha }$$\end{document}U0(B)∼B−α where the exponent α is changed from a low value to a high value at a crossover field of B = ∼2 T, indicating the transition from collective to plastic pinning in the crystals. Finally, it is suggested that the 3D vortex phase is the dominant phase in the low temperature region as compared to the TAFF region in our series samples.

Ru1Sr2Gd1Cu2O8 and Sr2Yb1Ru0.9Cu0.1O6 were synthesized using a solid-state reaction. X-ray diffraction showed that Ru1Sr2Gd1Cu2O8 crystallized into the tetragonal Ru-1212 structure (space group P4/mmm), whereas the Sr2Yb1Ru0.9Cu0.1O6 had a monoclynic structure (space group P21/n). Sr2Yb1Ru0.9Cu0.1O6 showed a possible magnetic transition ( T m a g ) at 44 K with a superconducting transition temperature T c at 30 K. In contrast, Ru1Sr2Gd1Cu2O8 showed a magnetic transition at 140 K with a T c at 25 K. The magnetization data for Sr2Yb1Ru0.9Cu0.1O6 suggests that this sample exhibits antiferromagnetic ordering in zero-field-cooled (ZFC) magnetization, whereas it reveals a weak ferromagnetic component during field-cooled (FC) cycles. In addition, larger diamagnetic signals were observed during FC cycle compared to the ZFC. These results suggest that the valence state of the Yb ion have a significant effect on the magnetic state of the Sr2Yb1Ru0.9Cu0.1O6 superconductor.

The influence of proton irradiation on the fluctuation-induced magnetoconductivity of high quality FeSe1−xTex (x=0.4, 0.55) (FST) thin films has been investigated. The measurements were performed with magnetic fields up to 13 T applied in the two main crystal directions. The results were interpreted in terms of the Ginzburg–Landau approach for three-dimensional materials under a total-energy cutoff. The analysis shows that properly-tuned proton irradiation does not appreciably affect fundamental superconducting parameters like the Tc value, the upper critical fields or the anisotropy. This has important consequences from the point of view of possible applications due to the enhancement of vortex pinning induced by irradiation. YSK was supported by the NRF grant funded by the Ministry of Science, ICT and Future Planning (No. NRF-2015M2B2A9028507 and NRF-2016R1A2B4012672). TP was supported by the NRF grant funded by the Ministry of Science, ICT and Future Planning of Korea (No. 2012R1A3A2048816). JM acknowledges support by project FIS2016-79109-P (AEI/FEDER, UE) and by the Xunta de Galicia (project AGRUP 2015/11). SL was supported by the Global Research Network program through the NRF funded by the Ministry of Science and ICT & Future Planning (NRF-2014S1A2A2028361) SI

This paper reports the superconducting behavior of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and the magnetic moment measurements confirmed its superconductivity with T zero c and T mag c of 9.4 K under ambient pressure. Electron probe macro analysis indicated that the sample had a stoichiometric Fe:Se ratio of 1:1. The Seebeck coefficient, which was 12.3 μV/K at room temperature, changed to a negative value near 200 K, indicating it to be a two-carrier material. Above Tc, the ρ(T ) curve revealed an ‘S’ shape, and dρ(T )/dT and dρ(T )/dT 2 showed two anomalous features, one near Tan = 30 K and the other near T ∗ = 110 K. The Seebeck coefficient, S(T ), also displays peculiar behavior near the Tan and T ∗. We also report a rather smaller critical current density, Jc = 10 2 ∼ 10 A/cm, compared to those observed for the other pnictides and doped chalcogenides.

The magnetic properties and the magnetic entropy change of La ( Fe 0.89 Si 0.11 ) 13 compounds under proton irradiation were investigated. With increasing proton fluence, the atomic displacement increased, which gave rise to increase of the lattice constant and Curie temperature. The magnetic entropy change slightly decreased with increasing proton fluence, but still remained large. Among the demonstrated attractive features of the materials, the maximum hysteresis loss almost disappeared and the relative cooling power increased slightly under proton irradiation.

We report the superconducting properties of Y-doped semimetallic Bi3O2S3 superconductors. From electrical resistivity measurements, the Tc,zero of the optimally Y-doped Bi3O2S3 was higher than that of pure Bi3O2S3. This Y content dependence of the superconducting properties in Bi3O2S3 is explained by the chemical pressure effect.

Thermoelectric materials, which convert heat into electricity, are much studied for their potential in energy-saving applications — for example as a way of recovering waste heat in cars. At present, though, these materials are inefficient, with very few of them achieving a thermoelectric figure of merit (ZT) above one in the mid-temperature range (500–900 K). Now a figure of merit of 1.48, notably high for a bulk material, is reported for indium selenide crystals (In4Se3–δ) at 705 K. The high thermoelectric performance of this material is related to a Peierls distortion of the crystal lattice at 710 K. This work suggests a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves. Thermoelectric materials, which can convert heat into electricity, are of great interest for energy sustainability. The problem is the low efficiency of these materials, quantified by a coefficient, ZT, which for mid-temperature materials is usually around 1. The realization of a material, In4Se3–δ, which achieves the ZT value of 1.48 at 705 K, could open up a new avenue in the research to generate high ZT materials. Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit1. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the ${d}_{yz}$ hole band at the $Z$ point and the ${d}_{xz/yz}$ electron band at the $A$ point shift to a higher binding energy side. However, at the high-symmetry points $Z,\phantom{\rule{0.16em}{0ex}}A,\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Gamma}}$, and $M$, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

Cerium (Ce)-based heavy-fermion materials have a characteristic double-peak structure (mid-IR peak) in the optical conductivity [$\sigma(\omega)$] spectra originating from the strong conduction ($c$)--$f$ electron hybridization. To clarify the behavior of the mid-IR peak at a low $c$-$f$ hybridization strength, we compared the $\sigma(\omega)$ spectra of the isostructural antiferromagnetic and heavy-fermion Ce compounds with the calculated unoccupied density of states and the spectra obtained from the impurity Anderson model. With decreasing $c$-$f$ hybridization intensity, the mid-IR peak shifts to the low-energy side owing to the renormalization of the unoccupied $4f$ state, but suddenly shifts to the high-energy side owing to the $f$-$f$ on-site Coulomb interaction at a slight localized side from the quantum critical point (QCP). This finding gives us information on the change in the electronic structure across QCP., Comment: 6 pages, 4 figures. To appear in JPSJ (Letters)

With the current interest in the rare-earth tellurides as high temperature charge density wave materials, a greater understanding of the physics of these systems is needed, particularly in the case of the ditellurides. We report a detailed study of the band structure of LaTe_2 in the charge density wave state using high-resolution angle resolved photoemission spectroscopy (ARPES). From this work we hope to provide insights into the successes and weaknesses of past theoretical study as well as helping to clear up prior ambiguities by providing an experimental basis for future work in the tellurides., Comment: 6 pages, 7 figures, Proceedings of the ICESS 10(August 2006), Journal of Electron Spectroscopy and Related Phenomena - Published May 2007

We report on the magnetic susceptibility, specific heat and electrical resistivity of the heavy fermion compounds CeNiGe2−xSix (). Compounds with x

We investigate the doping dependence of flux pinning in superconducting NaFeCo x As (x = 0.01, 0.03, 0.05 and 0.07) single crystals grown by the Bridgman method. The electronic specific heat displays a pronounced anomaly in a sample series at superconducting transition temperature, which hardly shows any residual part at low temperature. We found that Co doping plays an important role in signifying the secondary peak in the magnetic hysteresis of optimally doped (x = 0.03) and heavily doped (x = 0.05, 0.07) crystals. Furthermore, the dependence of the relaxation rate S = d ln M/d ln t on magnetic field and temperature exhibits a decreasing trend within a certain range corresponding to the secondary peak effect in the optimally and heavily doped samples. The magnetic relaxation rate combined with the Maley analysis of the current-dependent creep energy shows a single-vortex pinning in the lightly doped sample dominant at low applied fields, and plastic pinning at high applied fields, without showing a secondary peak. However, in the optimally and heavily doped samples, the magnetic relaxation rate and U(J) isothermal analysis show that the collective pinning that dominates below H peak crosses over to plastic pinning for fields above H peak.

We have studied the transport, magnetic and thermal properties of Pr5Ni2Si3 with complex triangular lattices under various magnetic fields. The ferromagnetic transitions in the basal plane were observed at T(C1) = 52 K and T(C2) = 65 K. A decrease in magnetization below 30 K and the reduced paramagnetic Curie temperature θP are indicative of the development of antiferromagnetic correlation. These features are well understood by the frustration effect of the magnetic moments of Pr ions which constitute the triangular structural unit. The frustration caused the rapid rise of electrical resistivity below 30 K and an enormous entropy in low-temperature regions. The antiferromagnetic correlation acting between the frustrated Pr ions never causes any long-range order down to 0.6 K.

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya, 464-8603, Japan School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan Department of Emerging Materials Science, DGIST, Daegu 711-873, Republic of Korea

We report a study of the lattice dynamics in superconducting NaFeAs (${T}_{c}$ = 8 K) and doped ${\text{NaFe}}_{0.97}$${\text{Co}}_{0.03}$As (${T}_{c}$ = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on isostructural and isoelectronic LiFeAs, anomalous broadening of ${E}_{g}$(As) and ${A}_{1g}$(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the ${B}_{1g}$(Fe) vibration is observed. This renormalization cannot be understood within a single band and simple multiband approaches. A theoretical model that includes the effects of spin-density wave correlations along with sign-changing $s$-wave pairing state and interband scattering has been developed to explain the observed behavior of the ${B}_{1g}$(Fe) mode.

Highly anisotropic properties of CeTe 2- x with the layered Cu 2 Sb-type structure have been found by the measurements of electrical resistivity ρ( T ), magnetic susceptibility χ( T ) and magnetization M ( B ) on single crystals for x =0.15–0.18. Both M ( B ∥ c ) and M ( B ⊥ c ) at 2 K exhibit a metamagnetic transition at 0.04 T. M ( B ∥ c ) saturates rapidly to a value of 1.06 µ B /f.u. at around B =0.2 T, whereas M ( B ⊥ c ) gradually increases to a value of 0.75 µ B /f.u. at 5.5 T. The absence of hysteresis and remanence in M ( B ) indicates an antiferromagnetic ground state. The resistivity ratio ρ ∥ c /ρ ⊥ c increases from 3 to 150 on cooling from 300 K to 1.5 K. Both ρ ∥ c and ρ ⊥ c exhibit a sharp peak at 6 K before the onset of long-range magnetic order at T N =4.3 K, where χ( T ) exhibits a sharp peak. Measurements of the Hall coefficient R H ( T ), specific heat C ( T ), magnetoresistance ρ( B ) and electron tunneling have been done for polycrystals with x =0.13 and 0.18. Electron-tunneling spec...

Angle-resolved photoemission spectroscopy on a quasi-two-dimensional strongly correlated Ce 4f electron compound CeTe 2 has been performed to elucidate the importance of the three-dimensionality at the electronic structure in understanding its anomalous physical properties such as a complicated charge density wave formation. We have found that the Fermi surface as well as the band structure show the characteristic three-dimensionalities as follows: (1) a systematic reduction of the spectral weight at the hole-like Fermi surface along the Γ M line away from the ZR line suggests the imperfect nesting along the [ 1 1 0 ] axis; (2) the anomalous dispersive feature which is suggestive of the “band-folding” along the Γ Z line. The observed three-dimensionality indicates the existence of the three-dimensional nesting vector on CeTe 2 .

PrInNi 4 is a particularly interesting magnetic system which may be compared with PrNi 5 , a Van-Vleck paramagnet displaying nuclear magnetic ordering below 0.4 mK. Our heat capacity measurements on PrInNi 4 reveal that it undergoes a ferromagnetic transition at T C = 0.75 K and show that T C rises dramatically with field, such that T C = 9 K at 6 T. We have determined the magnetic excitations in PrInNi 4 using the HET spectrometer at ISIS. The lowest energy mode softens as q → 0, consistent with the low temperature ferromagnetic order.

Two anomalous peaks of electrical resistivity around 65 and 5.6 K have already been reported for CeTe2. To make the origin of the anomalies clear, the magnetoresistivity, electrical resistivity and Hall coefficient in CeTe2 are measured. From these measurements, the carrier concentration and the mobilities of the electron and hole are obtained. The former peak around 65 K in the resistivity seems to be due to the competition between a thermal excitation effect in the low carrier system of p-f mixing. The latter peak around 5.6 K seems to be due to a pair Wigner crystal, charge-density wave, magnetic scattering, and so on. In addition, the carrier concentration of CeTe2 as an extremely low carrier system is estimated to be about 1020−1021 m−3.

Reflectivity spectra of ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were measured in a wide photon energy region between 5 meV and 30 eV at room temperature. Using the conductivity spectra obtained from the Kramers-Kronig analysis, electronic structures of the manganese oxides were investigated. In particular, the states near the Fermi energy ${\mathrm{E}}_{\mathrm{F}}$, O 2p and Mn 3d (${\mathrm{e}}_{\mathrm{g}}$,${\mathrm{t}}_{2\mathrm{g}}$), were studied in detail. It was found that the O 2p band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than the Mn ${\mathrm{t}}_{2\mathrm{g}}$ band. For ${\mathrm{LaMnO}}_{3}$, the filled Mn ${\mathrm{e}}_{\mathrm{g}\mathrm{\ensuremath{\uparrow}}}^{1}$ band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than any other valence band. The ${\mathrm{t}}_{2\mathrm{g}}$-${\mathrm{e}}_{\mathrm{g}}$ Hund exchange energy was found to be about 3.4 eV and the Jahn-Teller stabilization energy ${\mathrm{E}}_{0}$ was estimated to be less than 0.5 eV.

PrFe4P12 shows a unique transition at 6.5 K with clear anomalies in its bulk properties. In order to understand the nature of the transition, we have undertaken inelastic neutron scattering studies from 300 to 2 K with varying magnetic fields up to 7 T. The analysis of the present data demonstrates that there is a pronounced change of the spin dynamics at 6.5 K which we attribute to the interplay between the spin and quadrupole degrees of freedom. (c) 2005 Elsevier B.V. All rights reserved.

The optical reflectivity spectra ( R ( ω )) of CeSb have been measured at high pressures to investigate the origin of the physical properties appearing under pressures. At 300 K, the pressure dependence of R ( ω ) revealed that the carrier density proportionally increases with increasing pressure up to 2.5 GPa. At lower temperature, the R ( ω ) spectrum drastically changes with both temperature and pressure due to the complex magnetic phase diagram. Especially, at 2.5 GPa and 30 K, the R ( ω ) intensity below 0.2 eV decreases, on the contrary that at around 0.3 eV increases. This result suggests that the density of states near the Fermi level decreases and shifts to the high-energy side.

We report the pressure response of charge-density-wave (CDW) and ferromagnetic (FM) phases of the rare-earth intermetallic SmNiC${}_{2}$ up to 5.5 GPa. The CDW transition temperature (${T}_{\text{CDW}}$), which is reflected as a sharp inflection in the electrical resistivity, is almost independent of pressure up to 2.18 GPa but is strongly enhanced at higher pressures, increasing from 155.7 K at 2.2 GPa to 279.3 K at 5.5 GPa. Commensurate with the sharp increase in ${T}_{\text{CDW}}$, the first-order FM phase transition, which decreases with applied pressure, bifurcates into the upper (${T}_{M1}$) and lower (${T}_{c}$) phase transitions and the lower transition changes its nature to second order above 2.18 GPa. Enhancement both in the residual resistivity and the Fermi-liquid ${T}^{2}$ coefficient $A$ near 3.8 GPa suggests abundant magnetic quantum fluctuations that arise from the possible presence of a FM quantum critical point.