Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe2 film

As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning—tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the ($sqrt{7}$ × $sqrt{3}$) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe₂} film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts Δ₁ and Δ₂. The gap part Δ₁ that closes around ～ 150 K is accompanied with the vanish of the ($sqrt{7}$ × $sqrt{3}$) CDW phase. While another momentum-dependent gap part Δ₂ can survive up to ～ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure Δ₁ + Δ₂, which suggests different forming mechanisms between the ($sqrt{7}$ × $sqrt{3}$) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe₂} film as a two-dimensional (2D) material.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature T _c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, Δ_Γ = 5.1–6.5 meV, Δ_L = 3.8–4.8 meV, and Δ_S = 0.9–1.9 meV (at T ? T _c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ～12, and ～20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming Δ_Γ and Δ_L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ_ΓL ≈ λ_LΓ ? λ_SΓ, λ_SL.     

Powder EPR of Gd3+ in the superconductors SnMo6S8 and PbMo6S8

EPR spectra at 35 and 60 GHz of Gd³⁺ diluted (～1%) in powdered SnMo₆S₈ and PbMo₆S₈ are presented and interpreted. The crystal field is found to be axial, with D = b₂⁰=-760 gauss and -740 Gauss for the tin and lead compounds respectively. Values are obtained for <J(q)> and J(q = 0). Below T_c the impurity to conduction electron relaxation rate is no longer proportional to temperature but decreases more rapidly. From this temperature dependence we extract a value for $\text{2Δ}\text{kT}{}_{\mathrm{c}}$ of roughly 5 for both compounds.     

An EPR study of the temperature dependence of the energy gap in ytterbium dodecaboride

An EPR study of ytterbium dodecaboride (YbB₁₂) showed the presence of an energy gap with a width of 2Δ=12 meV in the energy spectrum of this Kondo insulator. The temperature dependence of the energy gap was determined by interpreting the experimental data within the framework of the exciton dielectric model: Δ(T)=72 K at an absolute zero and Δ(T)=0 at ～115 K. The temperature dependence of the EPR line-width exhibits a feature at 13–15 K, which is indicative of a finite density of states inside the gap. This can be related to the presence of impurity states or bound polaron excitations in the electron spectrum of YbB₁₂.     

On the nature of the energy gap in ytterbium dodecaboride YbB12

The YbB₁₂ Kondo insulator was studied by EPR. The energy spectrum of YbB₁₂ was established to have a gap of width 2Δ≈12 meV. The experimental data are interpreted in terms of the excitonic-dielectric model. The temperature behavior of the gap was determined; it was found that Δ(T)=72 K at absolute zero and vanishes at ～115 K.     

Scanning tunneling spectroscopy on the novel superconductor CaC6

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.     

Order-parameter coupling in the improper ferroelectric lawsonite

Low-temperature specific heat and thermal expansion measurements are used to study the hydrogen-based ferroelectric lawsonite over the temperature range 1.8 K ≤ T ≤ 300 K. The second-order phase transition near 125 K is detected in the experiments, and the low-temperature phase is determined to be improper ferroelectric and co-elastic. In the ferroelectric phase T ≤ 125 K, the spontaneous polarization P(s) is proportional to (1) the volume strain e(s), and (2) the excess entropy ΔS(e). These proportionalities confirm the improper character of the ferroelectric phase transition. We develop a structural model that allows the off-centering of hydrogen positions to generate the spontaneous polarization. In the low-temperature limit we detect a Schottky anomaly (two-level system) with an energy gap of Δ ～ 0.5 meV.     

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO_0.88F_0.12FeAs samples with bulk critical temperature T _c = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are Δ _L = 10.5 ± 2 meV and Δ _s = 2.3 ± 0.4 meV, respectively.     

Scanning tunneling spectroscopy of Bi(2)Sr(2)CuO(6+delta): new evidence for the common origin of the pseudogap and superconductivity

Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi(2)Sr(2)CuO(6+delta) between 275 mK and 82 K. Below T(c) = 10 K, the spectra show a gap with well-defined coherence peaks at +/-Delta(p) approximately 12 meV, which disappear at T(c). Above T(c), the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T(*) approximately 68 K. The comparison with Bi(2)Sr(2)CaCu(2)O(8+delta) demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin.     

Spin dynamics of the spin-1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2

We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagomé lattice antiferromagnet ZnCu3(OH)6Cl2. The susceptibility indicates a Curie-Weiss temperature of theta CW approximately = -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system.     

Hyperfine structure of the <Emphasis Type="Italic">S</Emphasis> levels of the muonic helium ion

Corrections of the α⁵ and α⁶ orders to the energy spectrum of the hyperfine splitting of the 1S and 2S levels of the muonic helium ion are calculated with the inclusion of the electron vacuum polarization effects, nuclear-structure corrections, and recoil effects. The values ΔE ^hfs(1S) = ?1334.56 meV and ΔE ^hfs(2S) = ?166.62 meV obtained for hyperfine splitting values can be considered as reliable estimates for comparison with experimental data. The hyperfine structure interval Δ₁₂ = 8ΔE ^hfs(2S) ? ΔE ^hfs(1S) = 1.64 meV can be used to verify QED predictions.     

Fourfold structure of vortex-core states in Bi2Sr2CaCu2O8+delta

We present a detailed study of vortex-core spectroscopy in slightly overdoped Bi2Sr2CaCu2O8+delta using a low-temperature scanning tunneling microscope. Inside the vortex core, we observe a fourfold symmetric modulation of the local density of states with an energy-independent period of (4.3 +/- 0.3)a0. Furthermore, we demonstrate that this square modulation is related to the vortex-core states which are located at +/-6 meV. Since the core-state energy is proportional to the superconducting gap magnitude , our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core.     

6,8-二甲基-1,2,4-三唑并嘧啶-3-丁二酮基硫醚酮-烯醇互变异构平衡的1H NMR研究

用¹H NMR方法研究了不同溶剂和温度对6,8-二甲基-1,2,4-三唑并嘧啶-3-丁二酮基硫醚酮-烯醇互变异构平衡的影响,发现温度升高有利于平衡向酮式转变,而溶剂极性的改变对平衡的影响并不规律,获得了四种溶剂中的ΔH和ΔS.     

Tunneling spectroscopy of layered superconductors: intercalated Li0.48(C4H8O)xHfNCl and De-intercalated HfNCl0.7

Tunneling measurements have been carried out on layered superconductors of the β(SmSI)-type – Li_0.48(THF)_xHfNCl (THF?=?C₄H₈O) and HfNCl_0.7 – by means of break-junction and scanning tunneling spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) – like gap structures with typical gap values of 2Δ (4.2 K) = 11–12 meV for Li_0.48(THF)_xHfNCl with the highest T_c = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2Δ (4.2 K) ≈ 17–20 meV closing at T_c. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2Δ/k_BT_c ~ 8 substantially exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2Δ/k_BT_c ratios are rather unusual for conventional superconductors but quite common to high-T_c cuprates, as well as to organic superconductors. Our studies allowed to collect much more evidence concerning the huge pairing energy in those materials and to investigate in detail the complexity of their superconducting gap spectra. An origin of the observed phenomena still remains to be clarified.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a hundred voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis-oriented MgB₂ thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with single-gap features result in a more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps: Δ_S=2.45±0.15 meV and Δ_L=7.0±0.45 meV in MgB₂ with the gap ratio Δ_L/Δ_S=2.85±0.15. Our observations further prove a widely discussed multigap scenario for MgB₂, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.     

YBa2Cu3O6+δ的高频模及其随氧含量δ的变化

在中国原子能科学研究院新建的宽角(～30°)Be过滤探测器中子非弹性散射谱仪上,在入射中子能量从10直至150meV的范围内,测量了几种不同氧含量的YBa₂Cu₃O_6+δ样品的中子非弹性散射能谱。结果表明:(1)在δ=0和0.2时,在60至150meV能量范围内,发现有强的高频模存在,当其温度高到Nel点(～410K)以上直至466K时,没有观察出高频模强度的减弱。而δ=0.78和0.97时,在75至150meV能量范围内,散射强度在测量误 关键词：     

Spin S = 1 centers: a universal type of paramagnetic defects in nanodiamonds of dynamic synthesis

Intrinsic paramagnetic defects in ～5 nm sized nanodiamonds, produced by various dynamic synthesis (DySND) techniques (detonation, shock-wave, pulsed laser ablation of solid carbon containing targets), have been studied by multi-frequency electron paramagnetic resonance (EPR). X-band (9-10 GHz) EPR spectra of DySND, in addition to the main intensive singlet Lorentzian-like EPR signal, reveal a low intensity doublet pattern within the half-field (HF) region (g ～ 4). On transferring spectra to the Q-band (34 GHz) the shape of the HF pattern changes and splitting between doublet components is reduced from 10.4 to 2.6 mT. The HF patterns observed are attributed to the 'forbidden' ΔM(S) = 2 transitions between the Zeeman levels of some spin-triplet (S = 1) centers. The model of two triplet centers with g ～ 2.003 and zero-field splitting parameters D(1) = 0.095 cm(-1) (TR1) and D(2) = 0.030 cm(-1) (TR2) satisfactorily describes experimental results at both microwave frequencies. The spin-triplet-type defects are observed in a wide variety of DySND samples irrespective of industrial supplier, cooling and carbon soot refinement methods, initial purity, disintegration, or subsequent targeted chemical modification. This indicates that the intrinsic defects with S = 1 in DySND systems are of universal origin.     

Theory of the hyperfine structure of the <Emphasis Type="Italic">S</Emphasis> states of muonic tritium

The hyperfine structure of the energy spectrum of the S levels of muonic tritium has been calculated using the quasi-potential method in quantum electrodynamics (QED). The α⁵- and α⁶-order effects on the polarization of vacuum, the structure and recoil of the nucleus, and relativistic corrections have been taken into account. The obtained numerical values of hyperfine splittings of 239.819 meV (1S state) and 29.965 meV (2S state) can be treated as reliable estimates for comparing with future experimental data of the CREMA collaboration, and hyperfine structure interval Δ₁₂ = 8ΔE ^hfs (2S)–ΔE ^hfs (1S) =–0.100 meV can be used for verifying the QED predictions. The resultant precision values of hyperfine splitting are also important for calculating the rates of formation of (μ dt) mesomolecules in muonic catalysis reactions.