Michael E. Manley

Metallization of vanadium dioxide driven by large phonon entropy

Phase competition underlies many remarkable and technologically important phenomena in transition metal oxides. Vanadium dioxide (VO2) exhibits a first-order metal–insulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. Ongoing attempts to explain this coupled structural and electronic transition begin with two alternative starting points: a Peierls MIT driven by instabilities in electron–lattice dynamics and a Mott MIT where strong electron–electron correlations drive charge localization. A key missing piece of the VO2 puzzle is the role of lattice vibrations. Moreover, a comprehensive thermodynamic treatment must integrate both entropic and energetic aspects of the transition. Here we report that the entropy driving the MIT in VO2 is dominated by strongly anharmonic phonons rather than electronic contributions, and provide a direct determination of phonon dispersions. Our ab initio calculations identify softer bonding in the tetragonal phase, relative to the monoclinic phase, as the origin of the large vibrational entropy stabilizing the metallic rutile phase. They further reveal how a balance between higher entropy in the metal and orbital-driven lower energy in the insulator fully describes the thermodynamic forces controlling the MIT. Our study illustrates the critical role of anharmonic lattice dynamics in metal oxide phase competition, and provides guidance for the predictive design of new materials.

Einstein modes in the phonon density of states of the single-filled skutterudite Yb0-2Co4Sb12

Measurements of the phonon density of states by inelastic neutron time-of-flight scattering and specific-heat measurements along with first-principles calculations, provide compelling evidence for the existence of an Einstein oscillator (rattler) at {omega}{sub E1} {approx} 5.0 meV in the filled skutterudite Yb{sub 0.2}Co{sub 4}Sb{sub 12}. Multiple dispersionless modes in the measured density of states of Yb{sub 0.2}Co{sub 4}Sb{sub 12} at intermediate transfer energies (14 {le} {omega} {le} 20 meV) are exhibited in both the experimental and theoretical density of states of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at {omega}{sub E2} {approx} 12.8 meV obtained from heat-capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb{sub 3} cage as a result of Yb filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at {omega}{sub E1} {approx} 5.0 meV. Our observations offer a plausible explanation for the significantly higher dimensionless figures of merit of filled skutterudites, compared to their parent compounds.

Giant electromechanical coupling of relaxor ferroelectrics controlled by polar nanoregion vibrations.

Polar nanoregion vibrations control the ultrahigh piezoelectric response of relaxor-based ferroelectrics used in applications. Relaxor-based ferroelectrics are prized for their giant electromechanical coupling and have revolutionized sensor and ultrasound applications. A long-standing challenge for piezoelectric materials has been to understand how these ultrahigh electromechanical responses occur when the polar atomic displacements underlying the response are partially broken into polar nanoregions (PNRs) in relaxor-based ferroelectrics. Given the complex inhomogeneous nanostructure of these materials, it has generally been assumed that this enhanced response must involve complicated interactions. By using neutron scattering measurements of lattice dynamics and local structure, we show that the vibrational modes of the PNRs enable giant coupling by softening the underlying macrodomain polarization rotations in relaxor-based ferroelectric PMN-xPT {(1 − x)[Pb(Mg1/3Nb2/3)O3] – xPbTiO3} (x = 30%). The mechanism involves the collective motion of the PNRs with transverse acoustic phonons and results in two hybrid modes, one softer and one stiffer than the bare acoustic phonon. The softer mode is the origin of macroscopic shear softening. Furthermore, a PNR mode and a component of the local structure align in an electric field; this further enhances shear softening, revealing a way to tune the ultrahigh piezoelectric response by engineering elastic shear softening.

Lattice Vibrations Boost Demagnetization Entropy in Shape Memory Alloy

Magnetocaloric (MC) materials present an avenue for chemical-free, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material Ni45Co5Mn36.6In13.4. Upon heating across TC, the material exhibits an anomalous increase in phonon entropy of 0.17 0.04 k_B/atom, which is nine times larger than expected from conventional thermal expansion. We find that the phonon softening is focused in a transverse optic phonon, and we present the results of first-principle calculations which predict a strong coupling between lattice distortions and magnetic excitations.

Symmetry and Correlations Underlying Hidden Order in URu2Si2

In this paper, we experimentally investigate the symmetry in the hidden order (HO) phase of intermetallic URu2Si2 by mapping the lattice and magnetic excitations via inelastic neutron and x-ray scattering measurements in the HO and high-temperature paramagnetic phases. At all temperatures, the excitations respect the zone edges of the body-centered tetragonal paramagnetic phase, showing no signs of reduced spatial symmetry, even in the HO phase. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose features are corroborated by the phonon measurements. Due to a large hybridization energy scale, a full uranium moment persists in the HO phase, consistent with a lack of observed crystal-field-split states. Our results are inconsistent with local order-parameter models and the behavior of typical density waves. Finally, we suggest that an order parameter that does not break spatial symmetry would naturally explain these characteristics.

Correspondence: Reply to ‘Phantom phonon localization in relaxors’

The Correspondence by Gehring et al.1 mistakes Anderson phonon localization for the concept of an atomic-scale local mode. An atomic-scale local mode refers to a single atom vibrating on its own within a crystal. Such a local mode will have an almost flat intensity profile, but this is not the same as phonon localization. Anderson localization is a wave interference effect in a disordered system that results in waves becoming spatially localized2. The length scale of the localized waves is set by the wavelength2, which is ~2 nm in this case. This larger length scale in real space means narrower intensity profiles in reciprocal space. As described in our original article, because Anderson localization is exponential in real space2, it appears Lorentzian in reciprocal space3. Figure 1a illustrates the phonon localization structure along Q= [2, k, 0], after our original paper3. Figure 1b shows the measured profile along this direction in both reduced lattice units and crystal rotation angle. The localization-profile width, which corresponds to a 2 nm coherence length3, translates to a full width at half maximum of about 1° of crystal rotation. Gehring et al.1, however, tilted the crystal out of the plane along Q≈ [2, 0.35, l]. Figure 1c shows how the Anderson localized phonon is expected to appear in the (2kl) plane assuming an isotropic TO phonon. Using an isotropic model, we calculated the intensity profile for the Gehring path based on our original fit shown in Fig. 1b. As shown in Fig. 1d, this isotropic model captures the basic shape of the intensity profile. An inspection of the TO phonon in our data shows that there is some anisotropy, and accounting for this results in better agreement, Fig. 1d. This analysis does not account for deviations from a vertical path in tilting, or the temperature difference (420 versus 488 K). The 420 K temperature used by Gehring et al. is a concern because it is only ≈10 K above the intermediate tetragonal phase, in which case they may have a mix of phases. They also assume an unrealistic lower limit of detection on the LM intensity. Nevertheless, the measured results of Gehring et al. are consistent with phonon localization at a length scale of ≈2 nm, and with our measurements3. Addressing the second issue mentioned by Gehring et al.1, multiple scattering processes involving Bragg scattering and phonons (called ghostons4) cannot explain the phonon localization features. We previously ruled out multiple scattering using several arguments3. First, there is the strong temperature dependence of the intensity profile in the high-temperature phase (original Supplementary Fig. 63)—which is unexpected for the ghostons since neither the underlying phonons nor Bragg peaks have significant temperature dependence. Second, there is the agreement between triple-axis and time-of-flight neutron scattering measurements—which employ different scattering geometries with quite different multiple scattering conditions (original Supplementary Fig. 33). Third, the scattering symmetry shows that the LM appears the same across different zones, which correspond to different scattering conditions (original Supplementary Fig. 23, and Fig. 1 in our more recent work5). Fourth, we modeled possible multiple scattering paths and could not identify a combination of Bragg plus phonon scattering that can fit the experimental results. Finally, the Bragg scattering intensities for our crystal were far too weak to support ghostons at the phonon localization intensities. The average diffracted beam was only about 0.2% the intensity of the incident beam. At its strongest point the phonon localization intensity reaches 30% of the TO phonon, which is 150 times too strong to be explained by ghostons. We conclude that the claims in the Correspondence by Gehring et al.1 are incorrect because they mistakenly assume that the length scale for Anderson localization is atomic, and because the experimental observations rule out multiple scattering as the origin.

Supersonic propagation of lattice energy by phasons in fresnoite

Controlling the thermal energy of lattice vibrations separately from electrons is vital to many applications including electronic devices and thermoelectric energy conversion. To remove heat without shorting electrical connections, heat must be carried in the lattice of electrical insulators. Phonons are limited to the speed of sound, which, compared to the speed of electronic processes, puts a fundamental constraint on thermal management. Here we report a supersonic channel for the propagation of lattice energy in the technologically promising piezoelectric mineral fresnoite (Ba2TiSi2O8) using neutron scattering. Lattice energy propagates 2.8−4.3 times the speed of sound in the form of phasons, which are caused by an incommensurate modulation in the flexible framework structure of fresnoite. The phasons enhance the thermal conductivity by 20% at room temperature and carry lattice-energy signals at speeds beyond the limits of phonons.Fresnoite has an incommensurate structure that can be described as a nonlinear soliton lattice. Manley et al. show that the additional phason degrees of freedom associated with the solitonic structure can travel faster than more conventional phonon excitations, enabling supersonic energy transport.

Phonon localization transition in relaxor ferroelectric PZN-5%PT

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn1/3 Nb2/3)O3]–0.05PbTiO3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increase in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. We also model the effects of damped mode coupling on the observed p...