Search

The electric field-dependence of structural dynamics in a tetragonal ferroelectric lead zirconate titanate thin film is investigated under sub-coercive and above-coercive fields using time-resolved X-ray diffraction. During the application of an external field to the pre-poled thin film capacitor, structural signatures of domain nucleation and growth include broadening of the in-plane peak width of a Bragg reflection concomitant with a decrease of the peak intensity. This disordered domain state is remanent and can be erased with an appropriate voltage pulse sequence.

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe2/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses when the excited TbFe2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, when a SiO2 glass capping layer covers the excited TbFe2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

We measure the transient strain profile in a nanoscale multilayer system composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast X-ray diffraction. The strain propagation through each layer is determined by transient changes of the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Neel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress, and a second contractive stress contribution rising on a 12 ps timescale. These two timescales have recently been measured for the spin-disordering in Ho [Rettig et al, PRL 116, 257202 (2016)]. As a consequence we observe an unconventional bipolar strain pulse with an inverted sign travelling through the heterostructure.

Monitoring structural changes in ferroelectric thin films during electric field-induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms and domain walls. We combine standard ferroelectric test-cycles with time-resolved x-ray diffraction to investigate the response of a nanoscale ferroelectric oxide capacitor upon charging, discharging and switching. Piezoelectric strain develops during the electronic RC time constant and additionally during structural domain-wall creep. The complex atomic motion during ferroelectric polarization reversal starts with a negative piezoelectric response to the charge flow triggered by voltage pulses. Incomplete screening limits the compressive strain. The piezoelectric modulation of the unit cell tweaks the energy barrier between the two polarization states. Domain wall motion is evidenced by a broadening of the in-plane components of Bragg reflections. Such simultaneous measurements on a working device elucidate and visualize the complex interplay of charge flow and structural motion and challenges theoretical modelling.

We study gadolinium thin films as a model system for ferromagnets with negative thermal expansion. Ultrashort laser pulses heat up the electronic subsystem and we follow the transient strain via ultrafast X-ray diffraction. In terms of a simple Grueneisen approach the strain is decomposed into two contributions proportional to the thermal energy of spin and phonon subsystems. Our analysis reveals that upon femtosecond laser excitation phonons and spins can be driven out of thermal equilibrium for several nanoseconds.

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around 1000 cm-1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T-dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm-1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response., Comment: 5 pages, 3 figures

We have measured the temperature dependence of the direct band gap, $E_g$, in SrTi$^{16}$O$_3$ and BaTiO$_3$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that $E_g$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fr\"ohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. We demonstrate that these characteristic changes of $E_g$ can be readily detected even in very thin films of SrTi$^{16}$O$_3$ with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a relatively simple but sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices., Comment: 6 pages, 3 figures, and supplementary material; accepted for publication in Phys. Rev. B

With infrared ellipsometry, x-ray diffraction, and electric transport measurements we investigated the electric-field-effect on the confined electrons at the LaAlO3/SrTiO3 interface. We obtained evidence that the localization of the electrons at low temperature and negative gate voltage is induced, or at least strongly enhanced, by a pyroelectric phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent Coulomb screening. In particular, we show that the charge localisation and the polar order of SrTiO3 both develop below about 50 K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage. Our findings suggest that the pyroelectric order also plays an important role in the quantum phase transition at very low temperatures where superconductivity is suppressed by an electric field., Comment: 5 pages, 4 figures, supplementary material

Using muon spin rotation (\muSR) and infrared spectroscopy we investigated the recently discovered superconductor K0.73Fe1.67Se2 with Tc = 32 K. We show that the combined data can be consistently described in terms of a macroscopically phase segregated state with a matrix of ~88% volume fraction that is insulating and strongly magnetic and inclusions with a ~12% volume fraction which are metallic, superconducting and non-magnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors., Comment: 22 pages, 8 figures. (citation list correction.)

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions., Comment: 5 pages, 3 figures

We report muon spin rotation (muSR) and infrared (IR) spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below Tmag \approx 32 K and becomes reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly fluctuating precursor of the SDW seems to develop alrady below the structural transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T << Tc.

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$\Delta/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm.

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm., Comment: 4 pages, 3 figures, 1 EPAPS file (3 figures)

We present a muon spin rotation (\muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we observe some interesting parallels with the electron doped SmFeAsO1-xFx 1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and arXiv:0807.4876]. In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below Tc\approx25K. To the contrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed AF order and others that are non magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et al., arXiv:0811.2224] and thus seems to be fairly common in these hole doped systems., Comment: 17 pages, 6 figures

The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0.30), which shows that static magnetism persists well into the superconducting regime. The analogy with the cuprates is quite surprising since the parent compounds appear to have different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings suggest that proximity to magnetic order and associated soft magnetic fluctuations, rather than the strong electronic correlations in the vicinity of a Mott-Hubbard-metal-to-insulator transition, may be the key ingredients of HTSC., Comment: Accepted in Nature Materials

The domain wall induced reversal dynamics in compressively strained GaMnAs was studied employing the magneto-optical Kerr effect and Kerr microscopy. Due to the influence of an uniaxial part in the in-plane magnetic anisotropy (90+/-Delta) domain walls with considerably different dynamic behavior are observed. While the (90+Delta) reversal is identified to be propagation dominated with a small number of domain walls, the case of (90-Delta) reversal includes the nucleation of many domain walls. The domain wall nucleation/propagation energy for both transitions are estimated using model calculations from which we conclude that single domain devices can be achievable using the (90+Delta) mode., Comment: 4 figures

We present ellipsometric measurements of the far-infrared dielectric response of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar as in the cuprate high-temperature superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis superconducting plasma frequency of $\omega_{{\rm pl},c}^{\rm SC}\leq 260\cm$ which corresponds to a lower limit of the c-axis magnetic penetration depth of $\lambda_c\geq6\mum$ and an anisotropy of $\lambda_c/\lambda_{ab}\geq 30$ as compared to $\lambda_{ab}=185$ nm from muon spin rotation [A. Drew {\it et al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra exhibit the signatures of a gap-like suppression of the conductivity in the superconducting state with a large gap magnitude of $2\Delta\approx300\cm$ (37 meV) and a ratio of $2\Delta/k_{\rm B}\tc\approx8$ that is suggestive of strong coupling.

Muon-spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth $\lambda_{ab}(0)=190(5)$\thinspace nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between $T_{\mathrm{c}}$ and $\lambda_{ab}(0)^{-2}$., Comment: To appear in Physical Review Letters. Changes to title, figures 1 and 4, references and (minor changes to) the text