Lattice fluctuation induced pseudogap in quasi-one-dimensional Ta2NiSe5

In conventional solid-state systems, the development of an energy gap is often associated with a broken symmetry. However, strongly correlated materials can exhibit energy gaps without any global symmetry breaking -- the so-called pseudogap, most notably in the Mott insulating state1 and the fluctuating superconducting or charge density wave states. To date, lattice induced pseudogap remains elusive. With angle-resolved photoemission spectroscopy (ARPES) and single crystal x-ray diffraction, we identify a pseudogap in the quasi-1D excitonic insulator candidate Ta2NiSe5. Strong lattice contribution is revealed by the pervasive diffuse scattering well above the transition temperature and the negative electronic compressibility in the pseudogap state. Combining first-principles and microscopic model calculations, we show that inter-band electron-phonon coupling can create fluctuating phonon-mediated electron-hole pairing or hybridization. This suppresses the spectral weight on the Fermi surface, causing a metal-to-insulator-like transition without breaking the global symmetry. Our work establishes the precedence of a pseudogap with a lattice origin, highlighting Ta2NiSe5 as a room-temperature platform to study lattice-induced charge localization and low dimensional fluctuations.

Specific Heat on Single-crystalline YVO3

The specific heat of single-crystalline YVO3 was measured from 2 K up to 250 K at zero field. The results reveal three transitions, at around 75, 115, and 200 K. The transitions at around 115 K and 200 K show that the phase transition is of the second-order type, whereas at around 75 K, unusual features of the specific heat are found. These unusual features are attributed to the effect of a large change in the volume. The specific heat data were analyzed in terms of a lattice contribution, a Schottky contribution and an excess magnetic contribution at high temperature. The magnetic contribution well above the magnetic ordering temperature is ascribed to short-range interactions due to the presence of strong magnetocrystalline anisotropy. The magnetic entropy considered by using this approach is 9.13 J/mole K which is close to the theoretical estimate for the S = 1 system.

Теплоемкость скандобората NdSc-=SUB=-3-=/SUB=-(BO_3)-=SUB=-4-=/SUB=-

Single crystals of trigonal neodymium scandoborate NdSc3(BO3)4 were grown by the group method from a solution-melt based on bismuth trimolybdate. The molar heat capacity C(T) was studied in the temperature range 2-300 K and magnetic fields up to 9 T. The experimental curve was approximated by the combined Debye-Einstein model. The lattice contribution was determined from ab-initio calculations. Schottky anomaly was observed in the low-temperature region C(T) with the applied magnetic field.

The Quasicrystal Model as a Framework for Order to Disorder Transitions in 2D Systems

Order to disorder transitions are important for 2D objects such as oxide films with a cellular porous structure, honeycomb, graphene, and Bénard cells in liquid and artificial systems consisting of colloid particles on a plane. For instance, solid films of the porous alumina represent an almost regular quasicrystal structure (perfect aperiodic quasicrystals discovered in 1991 is not implied here). We show that, in this case, the radial distribution function is well described by the quasicrystal model, i.e., the smeared hexagonal lattice of the two-dimensional ideal crystal by inserting a certain amount of defects into the lattice. Another example is a system of hard disks in a plane, which illustrates the order to disorder transitions. It is shown that the coincidence with the distribution function, obtained by the solution of the Percus-Yevick equation, is achieved by the smoothing of the square lattice and injecting the defects of the vacancy type into it. However, a better approximation is reached when the lattice is a result of a mixture of the smoothened square and hexagonal lattices. Impurity of the hexagonal lattice is considerable at short distances. Dependences of the lattices constants, smoothing widths, and impurity on the filling parameter are found. Transition to the order occurs upon an increasing of the hexagonal lattice contribution and decreasing of smearing.

Magnetic phase transitions and magnetic entropy in the XY antiferromagnetic pyrochlores (Er 1− x Y x ) 2 Ti 2 O 7

We present the results of experimental determination of the heat capacity of the pyrochlore Er 2 Ti 2 O 7 as a function of temperature (0.35–300 K) and magnetic field (up to 9 T), and for magnetically diluted solid solutions of the general formula (Er 1− x Y x ) 2 Ti 2 O 7 ( x ≤0.471). On either doping or increase of magnetic field, or both, the Néel temperature first shifts to lower temperature until a critical point above which there is no well-defined transition but a Schottky-like anomaly associated with the splitting of the ground state Kramers doublet. By taking into account details of the lattice contribution to the heat capacity, we accurately isolate the magnetic contribution to the heat capacity and hence to the entropy. For pure Er 2 Ti 2 O 7 and for (Er 1− x Y x ) 2 Ti 2 O 7 , the magnetic entropy as a function of temperature evolves with two plateaus: the first at R ln ⁡ 2 , and the other at R ln ⁡ 16 . When a very high magnetic field is applied, the first plateau is washed out. The influence of dilution at low values is similar to the increase of magnetic field, as we show by examination of the critical temperature versus critical field curve in reduced terms.

Analysis of Thermal Conductivity of LaFeAsO at Low Temperature

Thermal conductivity κ (T) of LaFeAsO is theoretically investigated below the spin density wave (SDW) anomaly. The lattice contribution to the thermal conductivity (κph) is discussed within the Debye-type relaxation rate approximation in terms of the acoustic phonon frequency and relaxation time below 150 K. The theory is formulated when heat transfer is limited by the scattering of phonons from defects, grain boundaries, charge carriers, and phonons. The lattice thermal conductivity dominates in LaFeAsO and is an artifact of strong phonon-impurity and-phonon scattering mechanism. Our result indicates that the maximum contribution comes from phonon scatters and various thermal scattering mechanisms provide a reasonable explanation for maximum appeared in κ (T).

Synthesis and Thermoelectric Properties of Nd-Doped Uddlesden-Popper Phase SrO(SrTiO3)n (N=1,2) Oxides

The Nd-doped SrO(SrTiO3)n (n=1,2) bulk samples were prepared by combining a sol-gel method and spark plasma sintering (SPS). The microstructures of the precursor powders were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM), thermogravimetric (TG) and differential scanning calorimetry (DSC). The oxides of (Sr1-xNdx)n+1TinO3n+1(n=1,2;x=0.05, 0.1) were prepared by solid-state reaction of the precursor powders with post-spark plasma sintering for the first time and the thermoelectric properties showed that electrical resistivity ρ and the absolute values |S| of Seebeck coefficient increased with temperature and depended on the dopant concentration, indicating a n-type degenerate semiconductor behavior. Compared with the total thermal conductivity κ (4.1-5.2 Wm-1K-1) at room temperature, the estimated electronic thermal conductivity κe(0.2-0.7 Wm-1K-1) were very small, indicating that lattice contribution was predominant in the RP phase compounds. The largest dimensionless figure of merit ZT, 0.13 at 905K, was obtained the 10 at.% Nd-doped Sr3Ti2O7. This synthetic method provides a simple way to prepare thermoelectric oxides.