Search results for "Condensed Matter - Other Condensed Matter"
showing 10 items of 195 documents
Efficient Approach for Simulating Distorted Materials
2010
The operation principles of nanoscale devices are based upon both electronic and mechanical properties of materials. Because these properties can be coupled, they need to be investigated simultaneously. At this moment, however, the electronic structure calculations with custom-made long-range mechanical distortions are impossible, or expensive at best. Here we present a unified formalism to solve exactly the electronic structures of nanomaterials with versatile distortions. We illustrate the formalism by investigating twisted armchair graphene nanoribbons with the least possible number of atoms. Apart from enabling versatile material distortions, the formalism is capable of reducing computa…
Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds
2008
The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]$^{4-}$). The basis for this is that [AlSi]$^-$ (with the same electron count as Si$_2$) and [CoSb]$^{4-}$ (the same electron count as GaSb), are both structurally and electronically, zinc-blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be d…
High-pressure structural phase transitions in CuWO4
2010
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 the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experim…
Domain-Enhanced Interlayer Coupling in Ferroelectric/Paraelectric Superlattices
2004
We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlatti…
Direct and inverse spin-orbit torques in antiferromagnetic and ferromagnetic FeRh/W(001)
2021
We use \textit{ab-initio} calculations to investigate spin-orbit torques (SOTs) in FeRh(001) deposited on W(100). Since FeRh undergoes a ferromagnetic-antiferromagnetic phase transition close to room temperature, we consider both phases of FeRh. In the antiferromagnetic case we find that the effective magnetic field of the even torque is staggered and therefore ideal to induce magnetization dynamics or to switch the antiferromagnet (AFM). At the antiferromagnetic resonance the inverse SOT induces a current density, which can be determined from the SOT. In the ferromagnetic case our calculations predict both even and odd components of the SOT, which can also be used to describe the ac and dc…
Determination of the origin of the spin Seebeck effect - bulk vs. interface effects
2013
The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained by any effects originating from the interface, such as magnetic proximity effects in the spin detec…
In- and out-of-equilibrium {\em ab initio} theory of electrons and phonons
2023
We lay down the {\em ab initio} many-body quantum theory of electrons and phonons in- and out-of-equilibrium at any temperature. We begin by addressing a fundamental issue concerning the {\em ab initio} Hamiltonian in the harmonic approximation, which we show must be determined {\em self-consistently} to avoid inconsistencies. After identifying the most suitable partitioning into a ``noninteracting'' and an ``interacting'' part we embark on the Green's function diagrammatic analysis. We single out key diagrammatic structures to carry on the expansion in terms of dressed propagators and screened interaction. The final outcome is the finite-temperature nonequilibrium extension of the Hedin eq…
Controlling magnetism with light in zero orbital angular momentum antiferromagnet
2023
Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin …
Study of the thermochromic phase transition in CuMo$_{1-x}$W$_x$O$_4$ solid solutions at the W L$_3$-edge by resonant X-ray emission spectroscopy
2020
Polycrystalline CuMo$_{1-x}$W$_x$O$_4$ solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L$_3$-edge to follow a variation of the tungsten local atomic and electronic structures across thermochromic phase transition as a function of sample composition and temperature. The experimental results were interpreted using ab initio calculations. The crystal-field splitting parameter for the 5d(W)-states was obtained from the analysis of the RXES plane and was used to evaluate the coordination of tungsten atoms. Temperature-dependent RXES measurements were successfully employed to determine the hysteretic behaviour of the structural phase transition between the $��…
Accurate calculation of the transverse anisotropy in perpendicularly magnetized multilayers
2015
The transverse anisotropy constant and the related D\"oring mass density are key parameters of the one-dimensional model to describe the motion of magnetic domain walls. So far, no general framework is available to determine these quantities from static characterizations such as magnetometry measurements. Here, we derive a universal analytical expression to calculate the transverse anisotropy constant for the important class of perpendicular magnetic multilayers. All the required input parameters of the model, such as the number of repeats, the thickness of a single magnetic layer, and the layer periodicity, as well as the effective perpendicular anisotropy, the saturation magnetization, an…