82 research outputs found
Nonlinear electron-phonon coupling in doped manganites
We employ time-resolved resonant x-ray diffraction to study the melting of
charge order and the associated insulator-metal transition in the doped
manganite PrCaMnO after resonant excitation of a
high-frequency infrared-active lattice mode. We find that the charge order
reduces promptly and highly nonlinearly as function of excitation fluence.
Density functional theory calculations suggest that direct anharmonic coupling
between the excited lattice mode and the electronic structure drive these
dynamics, highlighting a new avenue of nonlinear phonon control
Pore network model of primary freeze drying
[EN] he pore scale progression of the sublimation front during primary freeze drying depends on the local vapor transport and the local heat transfer as well. If the pore space is size distributed, vapor and heat transfer may spatially vary. Beyond that, the pore size distribution can substantially affect the physics of the transport mechanisms if they occur in a transitional regime. Exemplarily, if the critical mean free path is locally exceeded, the vapor transport regime passes from viscous flow to Knudsen diffusion. At the same time, the heat transfer is affected by the local ratio of pore space to the solid skeleton. The impact of the pore size distribution on the transitional vapor and heat transfer can be studied by pore scale models such as the pore network model. As a first approach, we present a pore network model with vapor transport in the transitional regime between Knudsen diffusion and viscous flow at constant temperature in the dry region. We demonstrate the impact of pore size distribution, temperature and pressure on the vapor transport regimes. Then we study on the example of a 2D square lattice, how the presence of micro and macro pores affects the macroscopic progression of the sublimation front.Vorhauer, N.; Först, P.; Schuchmann, H.; Tsotsas, E. (2018). Pore network model of primary freeze drying. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 221-228. https://doi.org/10.4995/IDS2018.2018.7284OCS22122
Разработка программной системы визуализации многомерных данных на основе кривых Эндрюса
Методы представления многомерных данных с помощью кривых на плоскости, например, метод кривых Эндрюса, являются мощным инструментом визуализации многопараметрических данных. Они отлично подходят для первичного анализа, так как, благодаря их свойствам, визуализировав набор данных с их помощью, исследователь может понять, какова структура распределения этих данных, присутствуют ли в них кластеры, содержат ли они аномалии и так далее. В данной работе описана разработка программной системы визуализации данных, основанной на кривых Эндрюса.Methods of multidimensional data representation with curves on plane, for example Andrews curves, are a powerful tool of multivariate data visualization. These methods can be successfully applied at the stage of primary data analysis, as due to their properties they may help researcher understand what is the structure of a data distribution, does it split into any clusters, does it contain any outliers or anomalous data entries, and so on. This paper describes development of a data visualization software system, which uses Andrews curves method to represent data on plane
An optically stimulated superconducting-like phase in K3C60 far above equilibrium Tc
The control of non-equilibrium phenomena in complex solids is an important
research frontier, encompassing new effects like light induced
superconductivity. Here, we show that coherent optical excitation of molecular
vibrations in the organic conductor K3C60 can induce a non-equilibrium state
with the optical properties of a superconductor. A transient gap in the real
part of the optical conductivity and a low-frequency divergence of the
imaginary part are measured for base temperatures far above equilibrium Tc=20
K. These findings underscore the role of coherent light fields in inducing
emergent order.Comment: 40 pages, 23 figure
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Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO<sub>3</sub>
In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO3, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, Tc = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to Tneq> 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti t2g levels. Notably, light-induced high temperature ferromagnetism in YTiO3 is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities
Ultrafast strain engineering in complex oxide heterostructures
We report on ultrafast optical experiments in which femtosecond mid-infrared
radiation is used to excite the lattice of complex oxide heterostructures. By
tuning the excitation energy to a vibrational mode of the substrate, a
long-lived five-order-of-magnitude increase of the electrical conductivity of
NdNiO3 epitaxial thin films is observed as a structural distortion propagates
across the interface. Vibrational excitation, extended here to a wide class of
heterostructures and interfaces, may be conducive to new strategies for
electronic phase control at THz repetition rates
The interface between silicon and a high-k oxide
The ability to follow Moore's Law has been the basis of the tremendous
success of the semiconductor industry in the past decades. To date, the
greatest challenge for device scaling is the required replacement of silicon
dioxide-based gate oxides by high-k oxides in transistors. Around 2010 high-k
oxides are required to have an atomically defined interface with silicon
without any interfacial SiO2 layer. The first clean interface between silicon
and a high-K oxide has been demonstrated by McKee et al. Nevertheless, the
interfacial structure is still under debate. Here we report on first-principles
calculations of the formation of the interface between silicon and SrTiO3 and
its atomic structure. Based on insights into how the chemical environment
affects the interface, a way to engineer seemingly intangible electrical
properties to meet technological requirements is outlined. The interface
structure and its chemistry provide guidance for the selection process of other
high-k gate oxides and for controlling their growth. Our study also shows that
atomic control of the interfacial structure can dramatically improve the
electronic properties of the interface. The interface presented here serves as
a model for a variety of other interfaces between high-k oxides and silicon.Comment: 10 pages, 2 figures (one color
Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO
In quantum materials, degeneracies and frustrated interactions can have a
profound impact on the emergence of long-range order, often driving strong
fluctuations that suppress functionally relevant electronic or magnetic phases.
Engineering the atomic structure in the bulk or at heterointerfaces has been an
important research strategy to lift these degeneracies, but these equilibrium
methods are limited by thermodynamic, elastic, and chemical constraints. Here,
we show that all-optical, mode-selective manipulation of the crystal lattice
can be used to enhance and stabilize high-temperature ferromagnetism in
YTiO, a material that exhibits only partial orbital polarization, an
unsaturated low-temperature magnetic moment, and a suppressed Curie
temperature, = 27 K. The enhancement is largest when exciting a 9 THz
oxygen rotation mode, for which complete magnetic saturation is achieved at low
temperatures and transient ferromagnetism is realized up to 80 K,
nearly three times the thermodynamic transition temperature. First-principles
and model calculations of the nonlinear phonon-orbital-spin coupling reveal
that these effects originate from dynamical changes to the orbital polarization
and the makeup of the lowest quasi-degenerate Ti levels. Notably,
light-induced high temperature ferromagnetism in YTiO is found to be
metastable over many nanoseconds, underscoring the ability to dynamically
engineer practically useful non-equilibrium functionalities.Comment: 14 pages, 4 figure
Mode-Selective Control of the Crystal Lattice
Driving phase changes by selective optical excitation of specific vibrational modes in molecular and condensed phase systems has long been a grand goal for laser science. However, phase control has to date primarily been achieved by using coherent light fields generated by femtosecond pulsed lasers at near-infrared or visible wavelengths. This field is now being advanced by progress in generating intense femtosecond pulses in the mid-infrared, which can be tuned into resonance with infrared-active crystal lattice modes of a solid. Selective vibrational excitation is particularly interesting in complex oxides with strong electronic correlations, where even subtle modulations of the crystallographic structure can lead to colossal changes of the electronic and magnetic properties. In this Account, we summarize recent efforts to control the collective phase state in solids through mode-selective lattice excitation. The key aspect of the underlying physics is the nonlinear coupling of the resonantly driven phonon to other (Raman-active) modes due to lattice anharmonicities, theoretically discussed as ionic Raman scattering in the 1970s. Such nonlinear phononic excitation leads to rectification of a directly excited infrared-active mode and to a net displacement of the crystal along the coordinate of all anharmonically coupled modes. We present the theoretical basis and the experimental demonstration of this phenomenon, using femtosecond optical spectroscopy and ultrafast X-ray diffraction at a free electron laser. The observed nonlinear lattice dynamics is shown to drive electronic and magnetic phase transitions in many complex oxides, including insulator–metal transitions, charge/orbital order melting and magnetic switching in manganites. Furthermore, we show that the selective vibrational excitation can drive high-T<sub>C</sub> cuprates into a transient structure with enhanced superconductivity. The combination of nonlinear phononics with ultrafast crystallography at X-ray free electron lasers may provide new design rules for the development of materials that exhibit these exotic behaviors also at equilibrium
Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order
We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ∼0.5mJ/cm2. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure
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