We report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.
Low-dimensional materials with charge density waves (CDW) are attractive for their potential to exhibit superconductivity and nontrivial topological electronic features. Here we report the two-dimensional (2D) chalcogenide, BaSbTe2S which acts as a new platform hosting these phenomena. The crystal structure of BaSbTe2S is composed of alternating atomically thin Te square-net layers and double rock-salt type [(SbTeS)2]2- slabs separated with Ba2+ atoms. Due to the electronic instability of the Te square net, an incommensurately modulated structure is triggered and confirmed by both single-crystal X-ray diffraction, electron diffraction, and the presence of an energy bandgap in this compound. Our first-principles electronic structure analysis and investigation of structural dynamical instability suggest that the Te network plays a dominant role in its origin. The incommensurate structure is refined with a modulation vector of q = 0.351(1)b* using an orthorhombic cell of a = 4.4696(5) Å, b = 4.4680(5) Å, and c = 15.999(2) Å under superspace group Pmm2(0β0)000 at 293 K. The modulation vector q varies as a function of both occupancy of Te in the square net and temperature, indicating the CDW order can be modulated by local distortions. The CDW can be suppressed by pressure, leading to the emergence of superconductivity with a Tc up to 7.5 K at 13.6 GPa, suggesting a competition between the CDW order and superconductivity. Furthermore, electrical transport under the magnetic field reveals the existence of compensated high mobility electron- and hole-bands near the Fermi surface (μ ∼600-3500 cm2V-1s-1), suggesting Dirac-like band dispersion.
Pyrochlore magnets are candidates for spin-ice behavior. We present theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare earth) supported by magnetothermal measurements on selected systems. By considering long ranged dipole-dipole as well as short-ranged superexchange interactions we get three distinct behaviours: (i) an ordered doubly degenerate state, (ii) a highly disordered state with a broad transition to paramagnetism, (iii) a partially ordered state with a sharp transition to paramagnetism. Thus these competing interactions can induce behaviour very different from conventional ``spin ice''. Closely corresponding behaviour is seen in the real compounds---in particular Ho2Ti2O7 corresponds to case (iii) which has not been discussed before, rather than (ii) as suggested earlier.
in ferroelectric relaxor materials like Pb(Zn1=3Nb2=3)O3 and Pb(Mg1=3Nb2=3)O3 are of pressing applied interest due to their in∞uence on the remarkable piezoelectric properties of their solid solutions with PbTiO3. In the recent literature, x-ray and neutron single-crystal difiuse scattering techniques have been shown to provide qualitative insight into the local structure of these materials. Here, we present a quantitative three-dimensional Huang-scattering analysis of 80 keV x-ray single-crystal difiuse scattering data from PZN-4.5%PT.
The nature of the competing ferromagnetic and incommensurate spin correlations in the spin-glass phase of ${\text{La}}_{0.85}{\text{Sr}}_{0.15}{\text{CoO}}_{3}$ has been investigated by various neutron scattering techniques. Spin-polarized scattering indicates that the observed incommensurate peaks are dominantly magnetic in nature. Magnetic field experiments show that a field applied perpendicular to the short-range ordering wave vector destroys the incommensurate correlations and induces long-range ferromagnetic order. However, even for fields up to 7 T, short-range ferromagnetic correlations still coexist with the long-range ordered regions.
The spin correlations of the bilayer manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ have been studied using neutron scattering. On cooling within the paramagnetic state, we observe purely two-dimensional behavior with a crossover to three-dimensional scaling close to the ferromagnetic transition. Below $T_C$, an effective finite size behavior is observed. The quantitative agreement of these observations with the conventional quasi two-dimensional Kosterlitz-Thouless model indicates that the phase transition is driven by the growth of magnetic correlations, which are only weakly coupled to polarons above $T_C$.
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