Anomalous Phonon Softening Research Articles

Structural distortion-induced low-temperature dielectric dispersion in lanthanide titanate pyrochlores

Rare-earth titanate pyrochlores have attracted significant attention for their unique magnetic frustration; however, research on the origin of low-temperature dielectric dispersion and the relationship between dielectric properties and structure lags far behind. Here, by systematically investigating the dielectric properties of representative rare-earth titanates R2Ti2O7 (R = La, Nd, Sm, Er, Yb, and Lu), we demonstrate that R2Ti2O7 with a cubic pyrochlore structure exhibits low-temperature dielectric dispersion behavior, while the other compounds with a monoclinic perovskite-like layered structure possess no dispersion behavior but excellent temperature-stable dielectric property. The dielectric dispersion in cubic pyrochlores arises from the structural distortion. Furthermore, the existence of structural distortion is affirmed by the anomalous phonon softening of A1g Raman mode around the dielectric dispersion temperature, and the origin of the structural distortion is attributed to anharmonic phonon–phonon interactions induced by intrinsic vacant oxygen at Wyckoff 8a sites. In addition, with increasing ionic radius from R = Lu to Sm, the increased lattice parameter leads to varied bond length and bond angle of Ti-O(1)-Ti, which strengthens the local lattice distortion of TiO(1)6 octahedra and thus enhances diffusion degree of dielectric dispersion. On the other hand, the absence of intrinsic vacant oxygen site hardly gives rise to the local structural distortion and thus no dielectric dispersion in monoclinic R2Ti2O7. Our work not only clarifies the mechanism of dielectric dispersion but also gives a comprehensive perspective on the structure–property relationship of rare-earth titanates R2Ti2O7, and thus lays a solid foundation for further work on related materials.

Unconventional multiferroicity induced by structural distortion and magnetostriction effect in the layered spin-1/2 ferrimagnet Bi2Cu5B4O14

Complex systems with strongly entangled microscopic degrees of freedom often host a plethora of exotic properties. Herein, we report the unconventional and interesting multiferroic behavior of a layered noncentrosymmetric ferrimagnetic spin-1/2 system, ${\mathrm{Bi}}_{2}{\mathrm{Cu}}_{5}{\mathrm{B}}_{4}{\mathrm{O}}_{14}$. Neutron powder-diffraction study deciphers a robust ferrimagnetic ($\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) ordering below a temperature ${T}_{C}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which remains unaltered even under a high magnetic field of $H=10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Interestingly, two successive ferroelectric transitions are observed, one of which is triggered by the magnetostriction effect and emerges concomitantly with the ferrimagnetic ordering at ${T}_{C}={T}_{E2}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This is accompanied by a pronounced dielectric peak and a prominent magnetodielectric coupling at ${T}_{C}$. By contrast, other ferroelectric transition commences in the paramagnetic state at ${T}_{E1}=32\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is solely caused by a prominent structural distortion. This strong structural distortion is revealed by the synchrotron x-ray-diffraction study, which is further supported by a concurrent, sharp, anomalous phonon softening observed in the Raman spectra.

Anomalous phonon softening of G-band in compressed graphitic carbon nitride due to strong electrostatic repulsion

Graphitic carbon nitride (C2N and C3N) with various π electron distributions on layers have been studied under pressure through a combined theoretical and experimental approach and a comparison with graphite. It is found that as these materials transform into low compressibility phases in the pressure range from 15 to 45 GPa, strong electrostatic repulsion between π electrons and in-plane sp2 electrons may distort and soften the sp2 bonds, leading to anomalous pressure evolutions of the intralayer phonon vibrations, such as a plateau-like behavior of E2g mode (G-band) in C2N and C3N. This also causes a slow increase in the resistivity/resistance of C2N and C3N as pressure increases, and the gradual interlayer bonding leads to an abrupt increase in resistance of the materials but with different pressure responses due to their different π electron distributions. Moreover, the intensity enhancement of the G band in both CN materials may be related to their electronic structure changes. The results deepen our understanding of the effects of π electron distribution on the structural transition of graphitic materials and may explain some unexplained in previous studies.

Electron-phonon interactions and two-phonon modes associated with charge density wave in single crystalline1T−VSe2

Charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides (TMDCs). Here, we report on Raman spectroscopy of single crystal 1T-VSe2 and observed signature of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition, in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A1g mode ~ 206 cm-1 along with two new weak and broad Raman modes associated to Eg mode ~ 257 cm-1 and two-phonon mode (2ph) ~ 332 cm-1. The onset of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ~ 115 K, a significant enhancement in the intensity of weak Eg mode is observed along with emergence of a doubly degenerate Eg(2) mode ~ 144 cm-1. Below 70 K, a weak A1g mode ~ 170 cm-1 emerged signifying the onset of C-CDW. Anomalous phonon softening of Eg mode ~ 257 cm-1 and 2ph process mode below I-CDW has been also observed, which is signifying the involvement of electron-phonon coupling.

Anomalous phonon softening in the topological insulator Sn-doped Bi1.1Sb0.9Te2S

Investigations into the phonon behavior provide important information on interactions between different excitations in quantum materials. We perform a detailed study of the phonon behavior in the topological insulator Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S using infrared spectroscopy. We observe two IR-active phonon modes at about 64 and 165 cm$^{-1}$, which are labeled with $\\alpha$ and $\\beta$, respectively. While the evolution of the $\\beta$ mode with temperature can be well described by the expected anharmonic decay process, the $\\alpha$ mode shifts to lower frequencies with decreasing temperature. Such an anomalous softening of the $\\alpha$ mode may arise from the charged-phonon effect due to the coupling between this mode and the topological surface states in Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S.

Hybridization Dynamics in CeCoIn_{5} Revealed by Ultrafast Optical Spectroscopy.

We investigate the quasiparticle dynamics in the prototypical heavy fermion CeCoIn_{5} using ultrafast optical pump-probe spectroscopy. Our results indicate that this material system undergoes hybridization fluctuations before the establishment of heavy electron coherence, as the temperature decreases from ∼120 K (T^{†}) to ∼55 K (T^{*}). We reveal that the anomalous coherent phonon softening and damping reduction below T^{*} are directly associated with the emergence of collective hybridization. We also discover a distinct collective mode with an energy of ∼8 meV, which may be experimental evidence of the predicted unconventional density wave. Our findings provide important information for understanding the hybridization dynamics in heavy fermion systems.

Geometrically frustrated GdInO3 : An exotic system to study negative thermal expansion and spin-lattice coupling

In this article, we report negative thermal expansion and spin frustration in hexagonal GdInO$_{3}$. Rietveld refinement of the XRD patterns reveal that the negative thermal expansion in the temperature range of 50-100K stems from the triangular lattice of Gd$^{3+}$ ions. At low temperature, the downward deviation of the inverse susceptibility ($\chi^{-1}$) vs. $T$ plot from the Curie-Weiss law indicates spin frustration which inhibits long-range magnetic ordering down to 2K. Magnetostriction measurements clearly demonstrate a strong spin-lattice coupling. Low temperature anomalous phonon softening, as obtained from temperature dependent Raman measurements, also reveals the same. Our experimental observations are supported by first principles density functional theory calculations of the electronic and phonon dispersion of GdInO$_3$. The calculations suggest that the GdInO$_3$ lattice is highly frustrated at low temperature. Further, the calculated normal mode frequencies of the Gd related $\Gamma$ point phonons are found to depend on the magnetic structure of the lattice, suggesting significant magneto-elastic coupling.

Monitoring electrostatically-induced deflection, strain and doping in suspended graphene using Raman spectroscopy

Electrostatic gating offers elegant ways to simultaneously strain and dope atomically thin membranes. Here, we report on a detailed in situ Raman scattering study on graphene, suspended over a Si/SiO2 substrate. In such a layered structure, the intensity of the Raman G- and 2D-mode features of graphene are strongly modulated by optical interference effects and allow an accurate determination of the electrostatically-induced membrane deflection, up to irreversible collapse. The membrane deflection is successfully described by an electromechanical model, which we also use to provide useful guidelines for device engineering. In addition, electrostatically-induced tensile strain is determined by examining the softening of the Raman features. Due to a small residual charge inhomogeneity, we find that non-adiabatic anomalous phonon softening is negligible compared to strain-induced phonon softening. These results open perspectives for innovative Raman scattering-based readout schemes in two-dimensional nanoresonators.

Extraordinary Dynamic Mechanical Response of Vanadium Dioxide Nanowires around the Insulator to Metal Phase Transition

Nanomechanical resonators provide a compelling platform to investigate and exploit phase transitions coupled to mechanical degrees of freedom because resonator frequencies and quality factors are exquisitely sensitive to changes in state, particularly for discontinuous changes accompanying a first-order phase transition. Correlated scanning fiber-optic interferometry and dual-beam Raman spectroscopy were used to investigate mechanical fluctuations of vanadium dioxide (VO2) nanowires across the first order insulator to metal transition. Unusually large and controllable changes in resonator frequency were observed due to the influences of domain wall motion and anomalous phonon softening on the effective modulus. In addition, extraordinary static and dynamic displacements were generated by local strain gradients, suggesting new classes of sensors and nanoelectromechanical devices with programmable discrete outputs as a function of continuous inputs.

Phonon softening in [formula omitted] studied by inelastic X-ray scattering

The phonon dispersions in the CuO2 planes are measured in La1.74Eu0.1Sr0.16CuO4 single crystals using high-resolution inelastic X-ray scattering at BL35XU of SPring-8. The oxygen bond-stretching mode shows anomalous phonon softening (a dip in the dispersion with phonon peak broadening) at Q∼1/3, which coincides with the wave vector of stripe charge modulation. The result indicates that strong electron–electron correlations, in the form of charge-ordering, create a strong electron–phonon coupling in high-temperature superconductors.

Observation of Anomalous Phonon Softening in Bilayer Graphene

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W. Kohn, Phys. Rev. Lett. 2, 393 (1959)], the observed phonon anomaly exhibits a logarithmic singularity in the optical-phonon energy. Arising from a resonant electron-phonon coupling effect, the anomaly was also expected, albeit not observed, in monolayer graphene. We propose an explanation for why it is easier to observe in bilayer samples.

Interplay of structural instability and lattice dynamics inNi2MnAl

We report on the structural instability of ${\text{Ni}}_{2}\text{MnAl}$ from calorimetry measurements and inelastic neutron scattering. The acoustic and optical phonon dispersions along the high-symmetry $[\ensuremath{\xi}00]$, $[\ensuremath{\xi}\ensuremath{\xi}0]$, and $[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]$ directions have been interpolated from the normal modes of vibration using the Born--von K\'arm\'an model. The tendency of ${\text{Ni}}_{2}\text{MnAl}$ to undergo a martensitic transformation shows up in the anomalous phonon softening of the particular ${\text{TA}}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ phonons in the $\ensuremath{\xi}$ range 0.1--0.25 rlu of two different crystals. The phonon frequencies of this branch scale inversely with the valence electron concentration in good agreement with ab initio calculations. Contrary to the prediction of first-principles calculations in the Heusler $\text{L}{2}_{1}{\text{-Ni}}_{2}\text{MnAl}$, no anomaly is seen in the optical phonons measured in $\text{B}2{\text{-Ni}}_{2}\text{MnAl}$. The anomalous ${\text{TA}}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ phonon softening is not enhanced below room temperature when ${\text{Ni}}_{2}\text{MnAl}$ orders in the antiferromagnetic state.

Q-Dependence of the giant bond-stretching phonon anomaly in the stripe compound La 1.48Nd 0.4Sr 0.12CuO 4 measured by IXS

Inelastic X-ray scattering (IXS) was used to study the Cu–O bond-stretching vibrations in the static stripe phase compound La 1.48Nd 0.4Sr 0.12CuO 4. It was found that the intrinsic width in Q-space of the previously reported huge anomalous phonon softening and broadening is approximately 0.08 r.l.u. HWHM. A detailed comparison was also made to inelastic neutron scattering (INS) studies, which indicate a two-peak lineshape (with superimposed broad and narrow peaks) in the vicinity of the anomaly. The high resolution IXS data show that the narrow peak is mostly an artifact of the poor transverse Q-resolution of INS. Otherwise, the agreement between the INS and IXS was excellent.

Anomalous phonon softening in intermediate-valence CeNi

Inelastic neutron scattering measurements of the phonon dispersion in CeNi have been performed in the [1 0 0]/[0 1 0] plane at temperatures between 10 and 295 K. A general softening of the acoustic phonon frequencies with respect to T=295 K is observed at low temperatures for all measured longitudinal and transverse branches. The largest effect occurs at the zone boundary. Anomalies are observed in the temperature dependencies of the energies of several modes, showing either a minimum or a maximum in the range 100 K< T<200 K. This behaviour is indicative of an unusually strong coupling of valence fluctuations and lattice vibrations.

Spectroscopic characterization of phase transformations in KC 60

We have performed inelastic neutron scattering experiments on fullerene samples of KC 60 stoichiometry in the temperature range from 30 to 500 K. The KC 60 polymer phase transforms with a strongly temperature dependent rate to intermediate (int)KC 60. This transition is direct, i.e. in contrast to RbC 60 there are no indications for equilibrated phase separated polymer-monomer states. The dynamic response of int-KC 60 can be expressed as a linear superposition of C 60 and K 3C 60 confirming the picture of an inhomogeneous C 60 K 3C 60 phase separated system. The spectral changes of the internal pressures caused by the phase separation turn out to be small. Apart from a lowering of the orientational ordering temperature the vibrational spectrum of the mixed phase shows no temperature variations going beyond those of the constituent parent phases. The transition towards the homogeneous rotator phase at about 440 K is evidenced by the disappearance of the K 3C 60 libron peak in the high temperature spectra. This transition shows a hysteresis indicating its first order character and is not preceeded by anomalous phonon softening.

Pseudoelastic behavior of hypostoichiometric NiAl alloys: A simple model.

We studied the pseudoelastic behavior of hypostoichiometric NiAl alloys using a very simple model, with interplanar interaction parameters extracted from recent neutron-scattering experiments. The behavior of the system under shear is studied numerically both in the static (quasiequilibrium) limit and with molecular dynamic simulations. Our results indicate that anomalous phonon softening can facilitate the nucleation of the martensitic phase near boundaries or defects as the system approaches the transformation temperature. For our simple model, we found that the criical stress for the nucleation of the martensitic phase is reduced by almost a factor of 2 as a result of anomalous phonon softening near the martensitic transformation temperature.