Dissimilarity between metallic-like transport in the dielectric spin density wave and field-induced spin density wave states in (TMTSF)<Subscript>2</Subscript>PF<Subscript>6</Subscript>

We report similarities and differences of the transport features in the spin density wave (SDW) and in the field-induced SDW (FISDW) phases of the quasi-one-dimensional compound (TMTSF)₂PF₆. As temperature decreases below ≈2 K, the resistance in both phases exhibits a maximum and a subsequent strong drop. However, the characteristic temperature of the R(T) maximum and its scaling behavior in different magnetic fields B are evidence that the nonmonotonic R(T) dependences have different origin in SDW and FISDW regions of the phase diagram. We also found that the borderline T₀(B, P) which divides the FISDW region of the P-B-T phase diagram into the hysteresis and nonhysteresis domains terminates in the N=1 subphase; the borderline thus has no extension to the SDW N=0 phase.     

Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semifluxon oscillations of Josephson flux-flow in Bi-2212 stacked structures near T_c as a probe of melting of a Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of the oscillations is found to decrease gradually with the temperature and to turn to zero without any jump at T = T₀ (3.5 K below the resistive transition temperature T_c), thus, indicating a phase transition of the second order. This characteristic temperature T₀ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, T_BKT, in the elementary superconducting layers of Bi-2212 at zero magnetic field. On the basis of these facts, we infer that melting of a triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and antivortices. The B-T phase diagram of the BKT phase found from our experiment is consistent with theoretical predictions.     

Magnetoinduced structural,Jahn–Teller,dielectric, and transport effects in La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript>

The temperature and magnetic field dependences of elastic moduli and resistance are studied in a La_0.875Sr_0.125MnO₃ single crystal. In addition to the metal–insulator transition, the structural transition from the cooperative J–T strongly distorted orthorhombic phase to the charge ordering phase is studied at T _J–T = 150 K in magnetic fields of up to 2 T. Results show it is possible to control the acoustic parameters in the vicinity of T _J–T via magnetic fields.     

Phase separation in strained cation- and anion-deficient Nd0.52Sr0.48MnO3 films

The magnetic and transport properties of anion- and cation-deficient Nd_0.52Sr_0.48MnO₃ films with different thicknesses, as well as of two films from this system grown on different SrTiO₃ and LaAlO₃ substrates, are studied. Below Curie temperature T _C, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > T _N, where T _N is the Néel temperature) or antiferromagnetic (T < T _N) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1–2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO₃ (compressed by 0.9%), the characteristic Néel and Curie temperatures are lower than in the film grown on LaAlO₃. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.     

Does the “quantized nesting model” properly describe the magnetic-field-induced spin-density-wave transitions?

Theoretical reinvestigation of a so-called field-induced spin-density-wave (FISDW) phase diagram in a magnetic field in quasi-one-dimensional compounds (TMTSF)₂X (X=PF₆, ClO₄, AsF₆, etc.) has revealed some novel qualitative features. Among them are (1) the FISDW wave vector is never strictly quantized; and (2) the FISDW phase diagram consists of two regions: (a) “Quantum FISDW,” where there exist jumps of the FISDW wave vectors between different FISDW subphases and (b) “Quasiclassical FISDW,” where the jumps disappear above some critical points and only one FISDW phase (characterized by a wave vector oscillating with a magnetic field) exists. Both these features are due to taking account of the breaking of an electron-hole symmetry. They contradict the previous textbook theoretical results (including the calculations of the “Three Dimensional Quantum Hall Effect”) performed by means of the “Quantized Nesting Model” which explicitly assumes the existence of the electron-hole symmetry. We stress that some effects related to the phenomena described above were experimentally observed but not properly interpreted.     

Regularities in temperature,magnetic field and pressure effect on the resistive properties of magnetic semiconductors

The influence of hydrostatic pressure, magnetic field and temperature on resistivity behaviour of bulk and film samples La_0.9Mn_1.1O₃ and La_0.56Ca_0.24Mn_1.2O₃ at action of magnetic field and temperature has been analysed. It is established that the maximum of magnetoresistive and the revealed baroresistive, magnetobaroresistive effects coincide at the same temperature T_pp. This temperature is equal to the “metal-semiconductor” phase transition temperature T_ms. “Cooling” and “heating” effects of pressure and magnetic field have been revealed. A mutual correspondence of T–P–H (6.2 K, 1 kbar, 2.7 kOe) influence on polycrystalline sample La_0.9Mn_1.1O₃ resistivity has been determined. The linear change of T_ms(P) and T_ms(H) in La_0.9Mn_1.1O₃, La_0.56Ca_0.24Mn_1.2O₃ resistivity have been found. An importance of the regularities of elastic-deforming correspondence of T–H–P influence on magnetic, resistivity properties, phase transitions and effects was elucidated and explained. An alternating influence of T–H–P and its role in resistivity has been pointed. A correlation between structural, elastic and resistive properties is specified.     

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C₆₀ single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at T _c =250–260 K: crystal strengthening is observed for T<T _c , and softening occurs for T>T _c . This indicates a change in the crystal lattice structure in the magnetic field.     

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe_1?x S_x (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe_1?x S_x with x≥0.27 and HgSe are similar in properties.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Disappearance of the heat capacity peak of Sc3In around the curie temperature in high magnetic fields

The low temperature (1.3–20.0 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to ～ 10 T. The heat capacity peak observed around T_c = 6.0 K in zero field becomes smaller with increasing fields and at 9.98 T its magnetic entropy is ≈ 18% of the zero field value. Above T_c, the spin fluctuation contribution to the heat capacity, which is enhanced by the magnetic field at lower fields (?5 T), is quenched at higher fields (?5 T). This depression of the spin fluctuation contibution to the heat capacity by the high magnetic fields occurs at lower magnetic fields than had been considered possible heretofore. Our results suggest that the itinerant ferromagnetism is Sc₃In is completely quenched at 12 T.     

High-field magnetoresistance of polycrystalline ZrZn2

We report magnetoresistance measurements of polycrystalline ZrZn₂ as a function of temperature (4.2–48K) and magnetic field up to 19 T. The results indicate the presence of both positive and negative contributions to the magnetoresistance. The latter is due to spin fluctuations. Below T_c the resistivity varies with temperature like T² over the entire field range (0–19 T). The coefficient of the T² -term decreases with increasing field and fits a $\text{H}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$ dependence above ～ 10 T, in accordance with theoretical predictions. Complex behaviour of the magnetoresistance is found in the paramagnetic regime above ～ 5 T.     

Scaling analysis of phase transition in Hg-based superconductor

With the vibrating-reed technique, the internal friction (IF) Q⁻¹ is measured for sing-phase (Hg_0.66Pb_0.34)Ba₂Ca₂Cu₃O_8+x superconductor as a function of temperature at low applied magnetic field up to 0.5 T and as a function of frequency at normal state temperatures. An IF peak associated with flux motion can be found below T_C. The IF peak becomes higher and shifts towards lower temperature with increasing magnetic field. In addition an IF peak is found near 200 K. By scaling analysis we have demonstrated that the internal friction around the peak temperature can be collapsed into a single curve, indicating that the IF peak below T_C is originated from a phase transition associated with a vortex glass transition and a structural phase transition occurs at around 200 K in the superconductor.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

Finite-temperature correlation functions

Lattice measurements of the Pisa group are analyzed numerically, and the parameters of correlation functions are extracted fromthe data—both below and above the deconfinement temperature T _c. The gluon condensate is found for six temperatures in the interval (0.956–1.131)T _c, and field distributions in the deconfined phase are obtained.     

Fluctuations of the order parameter in R 0.55Sr0.45MnO3 manganites near the metal-insulator phase transition

The magnetic phase transformations induced by changes of the composition, external magnetic field strength, and temperature in manganites with a nearly half-filled conduction band in the vicinity of the metal-insulator phase transition have been investigated experimentally. It has been found that the substitution of rare-earth ions (Sm) for Nd ions with a larger ionic radius in R _0.55Sr_0.45MnO₃ manganites leads to a linear decrease in the Curie temperature T _C from 270 to 130 K and a transformation of the second-order ferromagnetic (FM) phase transition into a first-order phase transition. The results of measurements of the alternating-current (ac) magnetic susceptibility in the (Nd_{1 ? y} Sm _y )_0.55Sr_0.45MnO₃ system indicate the existence of a Griffiths-like phase in samples with a samarium concentration y > 0.5 in the temperature range T _C < T < T* (where T* ～ 220 K). For samples with y > 0.5, the magnetization isotherms at temperatures above T _C exhibit specific features in the form of reversible metamagnetic phase transitions associated with strong fluctuations of the short-range ferromagnetic order in the system of Mn spins in the high-temperature Griffiths phase consisting of ferromagnetic clusters. According to the results of measurements of the ac magnetic susceptibility in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for a gadolinium concentration y = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering T _N ? 48.5 K. An increase in the external static magnetic field at 4.2 K leads to an irreversible induction of the ferromagnetic phase, which is stable in the temperature range 4.2–60 K. In the temperature range 60 K < T < 150 K, there exists a high-temperature Griffiths-like phase consisting of clusters (correlations) with a local charge/orbital ordering. The metastable antiferromagnetic structure is retained in samples with gadolinium concentrations y = 0.6 and 0.7, but it is destroyed with a further increase in the gadolinium concentration upon the transition to the spin-glass state. The magnetization isotherm obtained with variations in the external static magnetic field in the field range ±70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility χ suggest that, in the Gd_0.55Sr_0.45MnO₃ ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of χ(T) near T = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature T _G ? 42 K. The low-temperature state with quantum fluctuations exists in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for y ≥ 0.5.     

Characteristic fields of a Bi2Sr2CaCu2Oy crystal

The field dependences of the transverse resistance of a Bi₂Sr₂CaCu₂O_y (BSCCO) layered superconducting single crystal with T _c0≥92 K are studied in a perpendicular (H⊥(ab)) pulsed magnetic field up to 50 T in a wide temperature range, 4.2–300 K. The temperature dependences of the characteristic fields identified with the “irreversibility curve” and the field corresponding to the nucleation of the superconducting phase are determined. The results obtained for the latter field are compared with the theoretical dependences.     

On the chiral phase transition in hadronic matter

A qualitative analysis of the chiral phase transition in QCD with two massless quarks and nonzero baryon density is performed. It is assumed that, at zero baryonic density, ρ=0, the temperature phase transition is of the second order. Due to a specific power dependence of baryon masses on the chiral condensate, the phase transition becomes of the first order at the temperature T=T_ph(ρ) for ρ>0. At temperatures T_cont(ρ)> _T>_Tph(ρ), there is a mixed phase consisting of the quark phase (stable) and the hadron phase (unstable). At the temperature T=T_cont(ρ), the system experiences a continuous transition to the pure chirally symmetric phase.     

A novel double-peaked spin-glass susceptibility-temperature response in the ternary alloy Fe69Mn26Cr5

We have studied the low-field (B ≦ 10^?2 T) d.c. susceptibility χ of the austenitic stainless-steel alloy Fe₆₉Mn₂₆Cr₅ as a function of the magnetic field B and temperature T. χ(T) shows structure, strong B dependence, and typical irreversible effects. The range of temperatures studied comprises three distinct regions. In the high-temperature region (300 K ≦ T ≦ 380 K) a blunt peak in the susceptibility is noticed at T₂ = 340 K. T₂ was not sensitive to thermal cycling. χ(T) displayed a sharp cusp at T₁ = 200 K. This peak was sensitive to the thermal history of the sample and was strongly suppressed by B. Between T₁ and T₂ a shallow valley with some hysteresis was observed. We interpret this behavior to be due to a low-temperature pure spin-glass phase, a high temperature conventional paramagnetic phase, and coexisting antiferromagnetic and spin-glass phases between T₁ and T₂.     

Step-by-step first order antiferroelectric-paraelectric transition induced by frustration and electric field

We show analytically that even not too strong frustrating next neighbor interaction strongly affects first order antiferroelectric-paraelectric transition in an external electric field. We apply mean-field Landau theory. In the electric field a single phase transition at T ₀ splits into a step-by-step staircase with a series of intermediate phases. Unexpectedly enough we found that the equilibrium structures of the phases differ substantially from structures formed at low temperature both without field and in field. Polarization of intermediate structures decreases with temperature in a stepwise manner. Similar step-by-step transitions can occur also in magnetic materials with frustrating interaction.     

Charge density waves in layer structures: A NMR study on a 2HNbSe2 single crystal

We have studied by pulsed NMR a single crystal of 2HNbSe₂ at ambient pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2–273 K. Our results are consistent with the onset of incommensurate charge density waves (ICDW) at T_CDW = 33 K atP = 1 bar and 26 K at P = 21 kbar. Below T_CDW, the lineshapes of the (m → m ? 1) transitions agree with a local distribution of Knight shift and electric field gradients respecting the symmetry of a triple ICDW, while above T_CDW, pre-transitional broadening is observed. The product T₁T = 500 ± 100 msk was found constant in the temperature range 4.2–77 K and pressure independant.