Influence of europium doping on magnetic properties of 3D topological semimetal Cd<Subscript>3</Subscript>As<Subscript>2</Subscript> from ESR data

Influence of weak doping by rare-earth elements on magnetic and transport properties of a 3D topological semimetal Cd₃As₂ was studied experimentally. In particular, the doping by europium causes to change of the magnetoresistance sign from positive to negative. First measurements of electron spin resonance and magnetic susceptibility have shown that there are two types of Eu²⁺ magnetic ions that occupy the positions of cadmium ions and tetrahedral vacancies and form the ferromagnetic and antiferromagnetic phases, respectively. These facts give an evidence of small-scaled phase separation and a transformation of the Dirac semimetal to a Weyl semimetal induced by magnetic impurities.     

Topological invariants for phase transition points of one-dimensional Z<Subscript>2</Subscript> topological systems

We study topological properties of phase transition points of two topologicallynon-trivial Z₂ classes (D and DIII) in one dimension byassigning a Berry phase defined on closed circles around the gap closing points in theparameter space of momentum and a transition driving parameter. While the topologicalproperty of the Z₂ system is generally characterized by aZ₂topological invariant, we identify that it has a correspondence to the quantized Berryphase protected by the particle-hole symmetry, and then give a proper definition of Berryphase to the phase transition point. By applying our scheme to some specific models ofclass D and DIII, we demonstrate that the topological phase transition can be wellcharacterized by the Berry phase of the transition point, which reflects the change ofBerry phases of topologically different phases across the phase transition point.     

Infrared signature of the charge-density-wave gap in ZrTe<Subscript>3</Subscript>

The chain-like ZrTe₃ compound undergoes a charge-density-wave (CDW) transition at T_CDW=63 K, most strongly affecting the conductivity perpendicular to the chains. We measure the temperature (T) dependence of the optical reflectivity from the far infrared up to the ultraviolet with polarized light. The CDW gap Δ(T) along the direction perpendicular to the chains is compatible for T<T_CDW with the behavior of an order parameter within the mean-field Bardeen-Cooper-Schrieffer (BCS) theory. Δ(T) also persists well above T_CDW, which emphasizes the role played by fluctuation effects.     

Ab initio study of 2DEG at the surface of topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

By means of ab initio DFT calculation, we analyze the mechanism that drives the formation and evolution of the 2D electron gas (2DEG) states at the surface of Bi₂Te₃ topological insulator (TI). As it has been proved earlier it is due to an expansion of the van der Waals (vdW) spacing produced by intercalation of adsorbates. We will show that the effect of this expansion, in this particular surface, leads to several intriguing phenomena. On one hand we observe a different dispersion of the Dirac cone with respect to the ideal surface and the formation of Parabolic Bands (PB) below the conduction band and M-shaped bands in the valence band, the latters have been observed recently in photoemission experiments. On the other hand the expansion of the vdW gaps changes the symmetry of the orbitals forming the Dirac cone and therefore producing modifications in the local spin texture. The localization of these new 2DEG-states and the relocalization of the Dirac cone will be studied as well.     

Lacking Raman spectroscopic evidence for a structural phase transition in ZrTe5 at 141 K

The Raman spectra of ZrTe₅ are recorded at various temperatures between 10 and 500 K. The data give no evidence for a structural phase transition, in spite of appreciable temperature dependent effects in some of the Raman modes. The lack of Raman spectroscopic evidence for a low temperature structural phase transition in ZrTe₅ is also confirmed by powder X-ray and neutron diffraction data collected over 100 to 295 and 10 to 295 K, respectively. The present findings strongly suggest that the reported anomaly in the electrical resistivity at 141 K must reflect change(s) in the electronic band structure of ZrTe₅, and the temperature dependences of the Raman spectra could be caused by variations in the first order susceptibility.     

Anomalous Magneto-Transport Behavior in Transition Metal Pentatelluride HfTe_5

There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe_5.Several mechanisms,such as the formation of charge density wave or polaron,have been proposed,but so far no conclusive evidence has been presented.In this work,we investigate the unusual temperature dependence of magneto-transport properties in HfTe_5.It is found that a three-dimensional topological Dirac semimetal state emerges only at around T_p(at which the resistivity shows a pronounced peak),as manifested by a large negative magnetoresistance.This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe_5.Our work not only provides the first evidence of a temperature-induced critical topological phase transition in HfTe_5 but also gives a reasonable explanation on the long-lasting question.     

Specific features of electrical conductivity of V<Subscript>3</Subscript>O<Subscript>5</Subscript> single crystals

The electrical conductivity of V₃O₅ single crystals has been investigated over a wide temperature range, including the region of existence of the metallic phase and the region of the transition from the metallic phase to the insulating phase. It has been shown that the low electrical conductivity of metallic V₃O₅ is caused, on the one hand, by a lower concentration of electrons and, on the other hand, by a strong electronelectron correlation whose role with decreasing temperature increases as the phase transition temperature is approached. The temperature dependence of the electrical conductivity of the insulating phase of V₃O₅ has been explained in the framework of the theory of hopping conduction, which takes into account the effect of thermal vibrations of atoms on the resonance integral.     

Coulomb interaction and semimetal-insulator transition in graphene

The strong Coulomb interaction between massless Dirac fermions can drive a semimetal-insulator transition in single-layer graphene by dynamically generating an excitonic fermion gap. There is a critical interaction strength λc that separates the semimetal phase from the insulator phase. We calculate the specific heat and susceptibility of the system and show that they exhibit distinct behaviors in the semimetal and insulator phases.     

Effect of deformation on the electronic structure and topological properties of the A<Superscript>II</Superscript>Mg<Subscript>2</Subscript>Bi<Subscript>2</Subscript> (A<Superscript>II</Superscript> = Mg,Ca,Sr,Ba) compounds

The electronic structure and topological properties of the A^IIMg₂Bi₂ (A^II = Mg,Ca,Sr,Ba) compounds are theoretically studied with the use of exact exchange. It is found that the Mg₃Bi₂ compound in the equilibrium state is a semimetal, whereas three other compounds are semiconductors with a direct fundamental band gap. It is predicted that the uniaxial deformation of three-component compounds results in transitions to topologically nontrivial phases: topological insulator and topological and Dirac semimetals. Owing to such a rich variety of topologically nontrivial phases, these compounds may be of interest for further theoretical and experimental studies.     

C<Subscript>60</Subscript> heat capacity anomaly at the orientational phase transition

An orientational phase transition in C60 crystals was studied by differential scanning calorimetry with the highest resolution provided by this method. The temperature dependence of the specific heat ΔC _p (T) was found to have a double peak in the range 250–270 K. An analysis of the temperature dependences of heat capacity in the region of the peaks revealed that the lower temperature peak follows a power law of the type ΔC _p = A/(T?T₀)^1/2 characteristic of order-disorder second-order phase transitions, while the high-temperature peak can be identified with a diffuse Λ-shaped first-order phase transition.     

Broken-symmetry states of Dirac fermions in graphene with a partially filled high Landau level

We report on numerical study of the Dirac fermions in partially filled N=3 Landau level (LL) in graphene. At half-filling, the equal-time density-density correlation function displays sharp peaks at nonzero wave vectors +/-q*. Finite-size scaling shows that the peak value grows with electron number and diverges in the thermodynamic limit, which suggests an instability toward a charge density wave. A symmetry broken stripe phase is formed at large system size limit, which is robust against perturbation from disorder scattering. Such a quantum phase is experimentally observable through transport measurements. Associated with the special wave functions of the Dirac LL, both stripe and bubble phases become possible candidates for the ground state of the Dirac fermions in graphene with lower filling factors in the N=3 LL.     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Fluctuation-induced conductivity and dimensionality in the new Y-based Y<Subscript>3</Subscript>Ba<Subscript>5</Subscript>Cu<Subscript>8</Subscript>O<Subscript>18-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductor

The fluctuation conductivity measurement on the new Y-based Y₃Ba₅Cu₈O_{18- x} superconductor is presented. The dimensional crossovers between different temperature regimes were analyzed with Aslamazov-Larkin (AL) theory and a good quantitative agreement was achieved for the experimental data. For our data, the mean field regime is dominated by 2D AL fluctuations. Our results reveal the occurrence of critical fluctuation regime in consistent with the prediction of the full dynamic 3D XY model. We found the dynamical critical exponent to be z = 3.4 for our data. We analyzed also the excess conductivity data by Hikami-Larkin theory and estimated the phase relaxation time.     

Magnetotransport and magnetocaloric effect in Ho<Subscript>2</Subscript>In

The magnetotransport and magnetic properties of the binary intermetallic compound Ho₂In have been investigated. Clear signature of long range ferromagnetic order in the resistivity and the magnetization data at T_C = 85 K is observed. A further spin reorientation type transition is also apparent in our measured data at around T_t = 32 K. The sample exhibits negative magnetoresistance (peak value of –14% at 5 T) over a wide temperature range that extends well above T_C. Substantially large magneto-caloric effect is also observed in the sample (maximum value of –8.5 J kg^-1K^-1 for 0 → 5 T), which peaks around the T_C of the sample. The observed magnetoresistance and magnetocaloric effect are related to the suppression of spin disorder by an external magnetic field. Ho₂In can be an interesting addition to the list of rare-earth based magnetic refrigerant materials showing magneto-caloric effect across a second order phase transition.     

Thermoelectric power of HfTe5 and ZrTe5

The thermoelectric power and the electrical resistance of the two low-dimensional conductors, HfTe₅ and ZrTe₅ have been measured over the temperature range of 7K to 380K. The thermoelectric power for both materials is large and positive at high temperatures and then drops precipitously and crosses zero at the temperature of the anomalous resistivity peak, T_p. At lower temperatures, the thermoelectric power reaches a large negative peak and then decreases to zero in a metallic fashion. The abrupt change in thermopower, which occurs at T_p for both materials, is indicative of a phase transition where the carrier type changes from hole-like to electron-like.     

Resistive switching in β-SrV<Subscript>6</Subscript>O<Subscript>15</Subscript>

We investigate the pressure and temperature behavior of current-dependent resistivity of β-SrV₆O₁₅. We observe a switching between states of different resistivities in the insulating state of β-SrV₆O₁₅. In the low pressure phase, the resistive switching appears at temperatures below the semiconductor-insulator transition. In the high pressure phase, under ~1.6 GPa, the switching appears in the temperature range of the phase transition. The existence of switching may imply an important role of strontium off-stoichiometry for the electrical transport in β-SrV₆O₁₅. No electric-field-induced enhancement of the conductivity is observed. However, the conduction is significantly nonlinear under ~1.6 GPa, indicating that the charge order pattern in the high pressure phase is considerably different from that of the low pressure phase.     

Effect of sintering temperature of Ce<Superscript>3+</Superscript>-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> phosphors on light emission and properties of crystal structure for white-light-emitting diodes

The effect of the sintering temperature of Ce³⁺-doped Lu₃Al₅O₁₂ (Ce-LuAG) phosphors on the emission and properties of the crystal structure was studied. A cathodoluminescence peak at 317 nm, which was assigned to lattice defects, was exhibited in addition to emission peaks at 508 and 540 nm for the Ce-LuAG phosphors. The intensities of the 317 nm emission peak for the phosphors with mean particle diameters of 5.0 and 10.0 µm formed at a low sintering temperature of 1430 °C were higher than those for the phosphors with mean particle diameters of 18.0 and 20.5 µm formed at a high sintering temperature of 1550 °C. In contrast, the electroluminescence spectra for fabricated white-light-emitting diodes (LEDs) using the phosphors revealed that the intensity of the peak at 540 nm was strong for the mean particle diameters of 18.0 and 20.5 µm. The intensity of the 540 nm peak, which is attributed to the 4f→5d transition of the Ce³⁺ activator, showed a dependence on the sintering temperature. The relationship between the optical properties and the lattice defects is discussed.     

Acoustical investigations of a La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> single crystal

The acoustical, resistive, and magnetic properties of a La_0.75Sr_0.25MnO₃ lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.     

Doping dependence of the vortex glass and sublimation transitions in the high-T<Subscript>c</Subscript> superconductor La<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript> as determined from macroscopic measurements

Magnetization and ac-susceptibility measurements are used to characterize the mixed phase of the high-temperature cuprate superconductor La_2-xSr_xCuO₄ over a large range of doping (0.075 0.20). The first order vortex lattice phase transition line H_FOT(T), the upper critical field H_c2(T) and the second peak H_sp(T) have been investigated up to high magnetic fields (8 Tesla applied perpendicular to the CuO₂ planes). Our results reveal a strong doping dependence of the magnetic phase diagram, which can mainly be explained by the increasing anisotropy with underdoping. Within our interpretation, the first order vortex lattice phase transition is due to the sublimation (rather than melting) of the vortex lattice into a gas of pancake vortices, whereas the second peak is related to the transition to a more disordered vortex glass state.