A detailed high pressure study is carried out on 1T$^\prime$ MoTe$_{2}$ using X-ray diffraction(XRD) and Raman spectroscopy measurements upto about 30.5 GPa. High pressure XRD measurements show no structural transition. All the lattice parameters exhibit anomalous changes in the pressure region 8.4 to 12.7 GPa. Compressibility of the sample is found to be reduced by almost four times above 12.7 GPa with respect to that below 8.4 GPa. The anomalies in the Raman mode corresponding to the out of plane vibrations of Mo atoms sitting in the unit cell surface indicate a strong electron phonon coupling possibly mediated by differential strain inside the unit cell.
We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) MoTe2. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML MoTe2 based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe2. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe2 (NL MoTe2; N is an integer…
Transition metal dichalcogenide MoTe$_2$ is an important candidate for realizing the newly predicted type-IIWeyl fermions, for which the breaking of the inversion symmetry is a prerequisite. Here we present direct spectroscopic evidence for the inversion symmetry breaking in the low temperature phase of MoTe$_2$ by systematic Raman experiments and first principles calculations. We identify five lattice vibrational modes which are Raman active only in noncentrosymmetric structure at low temperature. A hysteresis is also observed in the peak intensity of inversion symmetry activated Raman modes,…
Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic lay…
We study the second-order Raman process of mono- and few-layer MoTe$_2$, by combining {\em ab initio} density functional perturbation calculations with experimental Raman spectroscopy using 532, 633 and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the electron-photon resonance process occurs at the high-symmetry M point in the Brillouin zone, where a strong optical absorption occurs by a logarithmic Van-Hove singularity. Double resonance Raman scattering with inter-valley electron-phonon coupling connects two of the three inequivalent M…
We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe$_{2}$) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with recent study reported by G. Froehlicher et al, (2015 $Nano$ $Lett.$ 15 6481) we observe a complex structure of the out-of-plane vibrational modes (A$_{1g}$/A$^{'}_{1}$), which can be explained in terms of interlayer interactions between single atomic planes of MoTe$_{2}$. In the case of low-energy shear and breathing modes of rigid interlayer vibrations it is shown that their en…
Raman scattering from the out-of-plane vibrational modes (A$_{1\text{g}}$/A'$_{1}$), which originate from the bulk-inactive out-of-plane B$^{1}_{2\text{g}}$ mode, are studied in few-layer MoTe$_{2}$. Temperature-dependent measurements reveal a doublet structure of the corresponding peaks in the Raman scattering spectra of tetralayer and pentalayer samples. A strong enhancement of their lower energy components is recorded at low temperature for 1.91 eV and 1.96 eV laser excitation. We discuss the attribution of the peaks to the inner modes of the respective Raman-active vibrations. The temperat…
We report on Raman experiments performed on a single crystal MoTe$_2$ sample. The system belongs to the wide family of Transition Metal Dichalcogenides which includes several of the most interesting two dimensional materials for both basic and applied physics. Measurements were performed in the standard basal plane configuration, by placing the $ab$ plane of the crystal perpendicular to the wave vector $k_i$ of the incident beam to explore the in plane vibrational modes, and in the edge plane configuration with $k_i$ perpendicular to the crystal $c$ axis, thus mainly exciting out-of-plane mode…
High pressure Raman spectroscopy of bulk 2H-MoTe$_2$ upto $\sim$ 29 GPa is shown to reveal two phase transitions (at $\sim$ 6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A$_{1g}$ and E$^{1}_{2g}$ Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at $\sim$ 16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in t…
Since double resonance Raman (DRR) spectra are laser-energy dependent, the first-principles calculations of DRR for two-dimensional materials are challenging. Here, the DRR spectrum of monolayer MoTe$_2$ is calculated by home-made program, in which we combine {\em ab-initio} density-functional-theory calculations with the electron-phonon Wannier (EPW) method. Within the fourth-order perturbation theory, we are able to quantify not only the electron-photon matrix elements within the dipole approximation, but also the electron-phonon matrix elements using the Wannier functions. The reasonable ag…
To develop highly sensitive, stable and repeatable surface-enhanced Raman scattering (SERS) substrates is crucial for analytical detection, which is a challenge for traditional metallic structures. Herein, by taking advantage of the high surface activity of 1T' transition metal telluride, we have fabricated high-density gold nanoparticles (AuNPs) that are spontaneously in-situ prepared on the 1T' MoTe2 atomic layers via a facile method, forming a plasmonic-2D material hybrid SERS substrate. This AuNP formation is unique to the 1T' phase, which is repressed in 2H MoTe2 with less surface activit…
We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, between the Td and 1T' phases in type-II Weyl semimetal MoTe2. Thinning down the MoTe2 crystal from bulk material to about 25nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from 40K to more than 300K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the…
The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T$^\prime$ semimetallic phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy barrier between these two phases, while also allowing access to the T$_d$ Weyl semimetal phase. The MoWTe$_2$ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessi…
We performed comparable polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes (10Ag+5Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were…
The layered structure of tetragonal Ni(CN)2, consisting of square-planar Ni(CN)4 units linked in the a-b plane, with no true periodicity along the c-axis, is expected to show anisotropic compression on the application of pressure. High-pressure neutron diffraction (elastic) and inelastic neutron scattering experiments have been performed on polycrystalline Ni(CN)2 to investigate its compressibility and stability. The intralayer a lattice parameter does not show any appreciable variation with increase of pressure up to 2.7 kbar. Above this pressure value, a decrease in a is observed. The c latt…
We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In-situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature dependent electrical measurements…
The compression behavior of delafossite compound CuCrO2 has been investigated by in-situ x-ray diffraction and Raman spectroscopic measurements upto 23.2 and 34 GPa respectively. X-ray diffraction data shows the stability of ambient rhombohedral structure upto ~ 23 GPa. Material shows large anisotropy in axial compression with c-axis compressibility, c= 1.2610-3(1) GPa-1 and a-axis compressibility, a= 8.9010-3(6) GPa-1. Our XRD data shows an irreversible broadening of diffraction peaks. Pressure volume data when fitted to 3rd order Birch-Murnaghan equation of state gives the value of bulk modu…
In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8 observed from high-pressure angle-dispersive x-ray diffraction and Raman spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble distortion in the lattice. Pressure evolution of unit-cell volume showed a change in compressibility above 5 GPa. The unit-cell parameters of the high-pressure rhombohedral phase at 12.1 GPa are ah = 14.6791(3) Å, ch = 17.9421(5) Å, V = 3348.1(1) Å3. The structure relation between the parent cubic …
Pressure induced structural and electronic transitions of Se helical chains confined inside nano-channels are studied. Raman scattering and optical absorption experiments show strong evidence of band gap reduction under high pressure. Ab initio calculations reveal that under hydrostatic compression, the Se chains should elongate and the change in morphology leads to a softening of phonons and narrowing of band gaps, and these signatures are observed in experiments. Our investigation demonstrates a negative compressibility in one dimension.
We report the pressure dependence of perovskite distortions in rare-earth (R) orthochromites (RCrO3) probed using Raman scattering in order to investigate the origin of structural transition from orthorhombic Pnma to rhombohedral R-3C phase in LaCrO3. The pressure induced changes in octahedral tilt modes demonstrates that tilt distortions are suppressed in LaCrO3 and are enhanced in the remaining members of RCrO3 family. This crossover between the two opposite pressure behaviors occurs at a critical R-ion radius of 1.20 Å. We attempted to establish the relation between this unusual crossover a…
We report a high-pressure study of tetragonal scheelite-type CaMoO4 up to 29 GPa. In order to characterize its high-pressure behavior, we have combined Raman and optical-absorption measurements with density-functional theory calculations. We have found evidence of a pressure-induced phase transition near 15 GPa. Experiments and calculations agree in assigning the high-pressure phase to a monoclinic fergusonite-type structure. The reported results are consistent with previous powder x-ray-diffraction experiments, but are in contradiction with the conclusions obtained from earlier Raman measurem…
We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below bandgap broadband emission in the photoluminescence measurement indicates the recombination o…
This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. The diffraction experiments show that monoclinic wolframite-type MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 the onset of the monoclinic-triclinic transition has been also found at 15.1 GPa. These findings are suppo…
This joint experimental and theoretical study of the structural, vibrational and electrical properties of rhombohedral SnSb2Te4 at high pressure unveils the internal mechanisms of its compression. The equation of state and the internal polyhedral compressibility, the symmetry and behavior of the Raman-active modes and the electrical behavior of this topological insulator under compression have been discussed and compared with the parent binary alpha-Sb2Te3 and SnTe compounds and with related ternary compounds. Our X-ray diffraction and Raman measurements together with theoretical calculations,…
We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for …
Pressure-induced hypercoordination of iodine and dimerization of I2O6H in strontium di-iodate hydrogen-iodate (Sr(IO3)2HIO3)
Universidad de Valencia（瓦伦西亚大学）；Universitat Politècnica de València（巴伦西亚理工大学）；University of Edinburgh（爱丁堡大学）；CELLS-ALBA Synchrotron Light Facility（CELLS-ALBA 同步辐射光源设施）；Peking University（北京大学）
The optical absorption of CdWO$_4$ is reported at high pressures up to 23 GPa. The onset of a phase transition was detected at 19.5 GPa, in good agreement with a previous Raman spectroscopy study. The crystal structure of the high-pressure phase of CdWO$_4$ was solved at 22 GPa employing single-crystal synchrotron x-ray diffraction. The symmetry changes from space group $P$2/$c$ in the low-pressure wolframite phase to $P2_1/c$ in the high-pressure post-wolframite phase accompanied by a doubling of the unit-cell volume. The octahedral oxygen coordination of the tungsten and cadmium ions is incr…
Comparative high-pressure study on rare-earth entropy fluorite-type oxides
Universitat de Valencia（瓦伦西亚大学）；Institut de Ciència dels Materials de la Universitat de València（瓦伦西亚大学材料科学研究所）；University of Zagreb（萨格勒布大学）；Diamond Light Source Ltd.（钻石光源有限公司）；CELLS-ALBA Synchrotron Light Facility（CELLS-ALBA同步辐射光源设施）
Pressure induced structural transitions, coordination crossover, and optoelectronic response in divalent metavanadate CaV$_2$O$_6$
Indian Institute of Science Education and Research Kolkata（印度科学教育研究学院加尔各答分校）；National Centre for High-Pressure Studies（高压研究中心）；IMPMC, Sorbonne Université（巴黎第六大学地球物理学研究所）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Deutsches Elektronen Synchrotron（德国电子同步辐射中心）
Recently, the possibility of exploiting the phenomenon of spin transition (ST) has been intensively investigated, therefore, it is particularly important to study the behavior of ST under various stimuli. Here, the shape and content of the intermediate phase of ST in Hoffmann-like compounds [Fe(Fpz)2M(CN)4](M = Pt, Pd) under external stimuli are studied. For this purpose, magnetic and Raman spectroscopy measurements were carried out. In pressure-induced spin transition (PIST), a mixture of high-spin and low-spin states appears, while in temperature-induced spin transition (TIST), a homogeneous…
In pursue of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO$_4$ under high pressure conditions, up until 24 GPa. We have found that PrVO$_4$ undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO$_4$-I--type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. Mona…
