Charge density wave transport in NbSe<Subscript>3</Subscript> at low temperatures under high magnetic field

Charge density wave (CDW) depinning and sliding regimes have been studied in NbSe₃ at low temperatures down to 1.5 K under magnetic field of 19 T oriented along the c-axis. We found that the threshold field for CDW depinning becomes temperature independent below T ₀ ≈ 15 K. Also CDW current to frequency ratio characterizing CDW sliding regime increases by factor 1.7 below this temperature. The results are discussed as a crossover from thermal fluctuation to tunneling CDW depinning at T < T ₀. Besides, we found that CDW sliding strongly suppresses the amplitude of Shubnikov-de Haas oscillations of magnetoresistance.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Surface impedance of <Emphasis Type="Italic">k</Emphasis>-(BEDT-TTF)<Subscript>2</Subscript>Cu[N(CN)<Subscript>2</Subscript>]Br crystals

Precision measurements of the real and imaginary parts of the microwave surface impedance Z _ac(T) = R _ac(T) + iX _ac(T) of the conducting ac layers of the k-(BEDT-TTF)₂Cu[N(CN)₂]Br crystals in the temperature interval of 0.5 < T < 100 K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear Δλ_ac(T)∞ΔX _ac(T) in the superconducting state at T T _c ∼ 11.5 K; (ii) the curves R _ac(T) = X _ac(T) coincide at T _c < T < 40 K; (iii) the X _ac(T) value at T > 40 K increases in comparison with R _ac(T); (iv) the dependence R _ac(T) at T > 40 K is nonmonotonic in thin crystals. These features of the impedance Z _ac(T) with increasing T are interpreted in terms of (i) the d-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of k-(BEDT-TTF)₂Cu[N(CN)₂]Br have been determined.     

Evidence of an intermediate phase in ternary Ge<Subscript>7</Subscript>Se<Subscript>93-x</Subscript>Sb<Subscript>x</Subscript> glasses

The compositional dependence of thermal properties, such as glass transition temperature (T_g), non-reversing enthalpy change (ΔH_NR) and the specific heat capacity change (ΔC_p) of melt quenched Ge₇Se_93-xSb_x (21 ≤ x ≤ 31) glasses, has been studied using alternating differential scanning calorimetry (ADSC) which is analogous to modulated differential scanning calorimetry (MDSC). The glass transition temperature, T_g, which is a measure of global connectivity of the glass, has been found to increase with the addition of Sb. In addition, a change in slope has been observed in the composition dependence of T_g at an average coordination 〈r〉 = 2.40. The experimentally observed compositional variation of glass transition temperature, has been compared with the theoretical predictions from the stochastic agglomeration theory (SAT) and has been found to be consistent. Further, a narrow thermally reversing window is seen in the compositional variation of the relaxation enthalpy (ΔH_NR), which is centered around 〈r〉 = 2.40. The change in specific heat capacity (ΔC_p) at T_g is also found to exhibit a distinct minima at 〈r〉 = 2.40, suggesting that the structural rearrangements for the liquid in the glass transition region are minimized around 〈r〉 = 2.4.     

Interaction of both charge density waves in NbSe3 from interlayer tunneling experiments

An interlayer tunneling technique has been used for spectroscopy of charge density wave (CDW) energy gaps (Δ_1,2) in NbSe₃ subsequently opened at the Fermi surface on decreasing temperature at T _p1 = 145 K (CDW1) and at T _p2 = 60 K (CDW2). We found that the CDW2 formation is accompanied by an increase of the CDW1 gap below T _p2. The maximum enhancement of Δ₁, δΔ₁ is about 10%. The effect observed has been predicted theoretically as resulting from the joint phase locking of both CDWs with the underlying crystalline lattice below T _p2. The text was submitted by the authors in English.     

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> (R = Y,Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E ₀ ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature T _s and hysteresis ΔT _s increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.     

Charge density wave and superconductivity in 2H- and 4H-NbSe<Subscript>2</Subscript>: A revisit

Good-quality hexagonal NbSe₂ single crystals were prepared. In 2H-NbSe₂, superconducting and charge density wave (CDW) transitions were found at T _s = 7.4 K and T _c = 35 K respectively as reported previously. We have noticed that these two transitions are changed to T _c = 42 K and T _s = 6.5 K, in 4H-NbSe₂. Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as ~3 and 6.3 for 2H- and 4H-single crystals around t = 0.81, where t = T/T _s, from resistivity and explained in terms of coherence length. From the relation, H_c2 (T)=H_c2 (0)[1-t²]H_{\rm c2} (T)=H_{\rm c2} (0)[1-t^2], H_c2^l (0)H_{\rm c2}^l (0) was calculated as ~8.15 T and 16.98 T at t = 0.84 in 2H-NbSe₂ and 4H-NbSe₂ respectively. However, H_c2^t (0) = 2.68H_{\rm c2}^t (0) = 2.68T for both single crystals.     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.     

Effect of Te doping on superconductivity and charge-density wave in dichalcogenides 2H-NbSe2-xTex （x = 0, 0.1, 0.2）

Single crystals of Te-doped dichalcogenides 2H-NbSe2-xTex （x ---- 0, 0.10, 0.20） were grown by vapour transport method. The effect of Te doping on the superconducting and charge-density wave （CDW） transitions has been investigated. The sharp decrease of residual resistance ratio, RRR = R（3OOK）/R（SK）, with increasing Te content was observed, indicating that the disorder in the conducting plane is induced by Te doping. Meanwhile the superconducting transition temperature, Tc, decreases monotonically with Te content. However, the CDW transition temperature, TCDW, shown by a small jump in the temperature dependence of the resistivity near 30 K, increases slightly. The results show that the suppression of superconductivity might be caused by the enhancement of CDW ordering. The disorder has little influence on the CDW ordering.     

Spin-orbit coupling in the superconducting phase and DDW states of high-T<Subscript>c</Subscript> cuprates

The effects of the spin-orbit coupling are considered for the high T _c cuprates with asymmetric superconducting gap (SC) and the d-density wave (DDW) phase due to its vital role in the experimental determination of the DDW state. Experiments predict an anisotropy in the DSC gap where |Δ(0,π)|>|Δ(π,0)| and the gap node deviates from the diagonal direction towards the k _x axis. Measurements also demonstrate DDW to be a possible candidate for the pseudogap in the underdoped phase. Due to the spin-orbit (SO) coupling in the low temperature orthorhombic (LTO) phase, the phase diagram of the cuprates suffers a change due to the modification of the T* value, the temperature characteristic of pseudogap, although T _c remains unaltered. Moreover, for a more generalized SO coupling, the DDW gap decreases with the angle but has no effect on the SC gap. We calculate the density of states in the various regimes of doping for the mixed SC+DDW states in the underdoped (UD) phase, SC state in the overdoped phase and also the DDW state in the UD phase and compare them with various theoretical and experimental works. The temperature dependence of the specific heat does not exhibit any qualitative change due to the SO coupling.     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

Shapiro steps in the torsion of a quasi-one-dimensional conductor TaS<Subscript>3</Subscript>

Features have been observed in the current dependences of the torsion angle, φ(I), for samples of a quasi-one-dimensional conductor TaS₃ irradiated by a high-frequency (HF) field. The features appear at the same current values as the Shapiro steps on the current-voltage characteristics, i.e., correspond to the synchronization of the motion of a charge-density wave (CDW) by the HF field. The shape of the features in the φ(I) dependences indicates a decrease in the inhomogeneous deformation of the CDW under the synchronization conditions. The investigation of torsion appears to be a highly sensitive method for determining the spatial coherence of the CDW.     

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ with T _c = 31 K. ρ(T) follows a linear temperature dependence above T _c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu³⁺ moments. The article is published in the original.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

The high-pressure-induced spin-reorientation transition in a ferromagnetic semiconductor Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The dependences of the longitudinal magnetoresistance (Δρ _zz /ρ₀)(P), transverse magnetoresistance (Δρ _xx /ρ₀)(P), and magnetic susceptibility (χ/χ₀(P)) on hydrostatic pressure P ≤ 7 GPa in the ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ at room temperature were investigated.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Spin fluctuations in the skutterudite compound LaFe<Subscript>4</Subscript>Sb<Subscript>12</Subscript>

We report transport, magnetic and thermodynamic properties of the skutterudite compound LaFe₄Sb₁₂. The basic features are a large magnetic susceptibility χ(T), and large electronic coefficient γ of the heat capacity. In particular, a T^1.35, T^1.7, and T^-2/3 temperature dependence of the magnetic susceptibility χ(T), resistivity ρ(T), and Grüneisen parameter Γ(T), respectively, is found at low temperature. An overall understanding of these physical properties is achieved, assuming that LaFe₄Sb₁₂ is a non-Fermi liquid system close to a ferromagnetic quantum critical point, with a spin fluctuation temperature T_sf=50±15 K.     

Kinetic phenomena in zero-gap semiconductors CuFeS<Subscript>2</Subscript> and CuFeTe<Subscript>2</Subscript>: Effect of pressure and heat treatment

Electrical resistivity ρ and Hal coefficient R are measured as a function of the temperature (T = 1.7−310 K) and the magnetic field (up to H = 28 kOe) in zero-gap semiconductor CuFeS₂ samples subjected to hydrostatic compression and under various heat-treatment conditions. At low temperatures, anomalies are observed in the kinetic effects related to the presence of ferromagnetic clusters: the magnetoresistance at T = 4.2 K and T = 20.4 K acquires a hysteretic character and thermopower α changes its sign at T < 15 K. The temperature dependence of conduction-electron concentration n in CuFeS₂ has a power form in the temperature range T = 14−300 K, which is characteristic of the intrinsic conductivity in zero-gap semiconductors. In CuFeS₂, we have n(T) ∝ T ^1.2; in isoelectron compound Cu_1.13Fe_1.22Te₂, we have n(T) ∝ T ^1.93. Heat treatment is found to affect the intrinsic conductivity of CuFeS₂, as the action of hydrostatic compression (carrier concentration changes); that is, the carrier concentration changes. However, a power form of the n(T) and ρ(T) dependences is retained.     

Phase transition and superconductivity high-T<Subscript>c</Subscript> oxides

On the basis of new experimental studies of specific heat, magnetic properties and thermal expansion, it was shown that dielectric-metal phase transitions were the consequence of delocalization of the heavy holes located on metalapical oxygen bonds in the high-T_c oxide systems. The ordered arrays of such holes (CDW) results in the dielectric gaps in electron spectrum and anomalous negative thermal expansion. The free carriers introduced by doping can couple through excitations of these localized holes, (local bosons). The high-T_c superconductivity will possess anomalous H_c2(T) dependence and γ-type specific heat jump at T_c. This local bosons model is the basis for explanation of the obtained experimental data.