arXiv subjects

O. Tegus

Publications and source records attributed to O. Tegus.

5 recordsLinked to original sources

arXiv2019

Insulator -- half metallic transition by the tetragonal distortion: A first-principles study of strain-induced perovskite RbMnF$_3$

From the spin polarized density functional total energy calculations, we shown that the ground state of cubic perovskite RbMnF$_3$ is an antiferromagnetic (AFM) insulator due to the super-exchange mechanism, in agreement with the other theoretical and experimental results. After tetragonal distortion along the c-axis, keeping the predicted volume, our results indicated that the strain-induced magnetic phase transition from an AFM insulator to a half metallic ferromagnetic (HM-FM) state is available by the tetragonal distortion due to the insulator -- half metallic transition. The predicted electronic and magnetic properties of strain-induced RbMnF$_3$ show the HM-FM nature, making strain-induced RbMnF$_3$ suitable for spintronic application.

cond-mat.mtrl-sci↗

arXiv2016

Microscopic mechanism of the giant magnetocaloric effect in MnCoGe alloys probed by XMCD

One important aspect of the magneto-structural transition in MnCoGe and related materials is the reduction in saturation magnetization from the orthorhombic to the hexagonal phase. Here, by combining an element specific magnetization probe such as x-ray magnetic circular dichroism and band structure calculations, we show that the magnetic moment instability between orthorhombic and hexagonal structures originates from a reduction in the Mn sub-lattice magnetization. The consequences of the moment instability for the magnetocaloric effect are discussed.

cond-mat.str-el↗

arXiv2013

Kondo hybridisation and the origin of metallic states at the (001) surface of SmB6

SmB6, a well-known Kondo insulator, has been proposed to be an ideal topological insulator with states of topological character located in a clean, bulk electronic gap, namely the Kondo hybridisation gap. Seeing as the Kondo gap arises from many body electronic correlations, this would place SmB6 at the head of a new material class: topological Kondo insulators. Here, for the first time, we show that the k-space characteristics of the Kondo hybridisation process is the key to unravelling the origin of the two types of metallic states observed directly by ARPES in the electronic band structure of SmB6(001). One group of these states is essentially of bulk origin, and cuts the Fermi level due to the position of the chemical potential 20 meV above the lowest lying 5d-4f hybridisation zone. The other metallic state is more enigmatic, being weak in intensity, but represents a good candidate for a topological surface state. However, before this claim can be substantiated by an unequivocal measurement of its massless dispersion relation, our data raises the bar in terms of the ARPES resolution required, as we show there to be a strong renormalisation of the hybridisation gaps by a factor 2-3 compared to theory, following from the knowledge of the true position of the chemical potential and a careful comparison with the predictions from recent LDA+Gutzwiler calculations. All in all, these key pieces of evidence act as triangulation markers, providing a detailed description of the electronic landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES experiments to achieve a direct measurement of the Dirac cones in the first topological Kondo insulator.

cond-mat.mtrl-sci↗

arXiv2003

Divergence of the Grueneisen Ratio at Quantum Critical Points in Heavy Fermion Metals

We present low-temperature volume thermal expansion, $β$, and specific heat, $C$, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si$_{0.95}$Ge$_{0.05}$)$_2$ which are located very near to quantum critical points. For both systems, $β$ shows a more singular temperature dependence than $C$, and thus the Grueneisen ratio ${Γ\propto β/C}$ diverges as T --> 0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin-fluctuations. By contrast, the observed singularity in YbRh2$(Si$_{0.95}$Ge$_{0.05}$)$_2$ cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

cond-mat.str-el↗

arXiv2003

Thermal expansion of the magnetorefrigerant Gd5(Si,Ge)4

We report thermal expansion measurements carried out on a single-crystal of the giant magnetocaloric effect material Gd5(Si0.43Ge0.57)4. At the magneto-structural phase transition at T_0 ~ 240 K, large steps in the relative length change DELTAL/L along the principle crystallographic axes are observed. The single-step behaviour in DELTAL/L indicates that the magnetic and structural phase transition occur at one and the same temperature. The specific heat was measured in order to determine the latent heat. Using the Clausius-Clapeyron relation we extract a hydrostatic pressure dependence dT_0/dp of 3.2+-0.2 K/kbar. The pressure effect is strongly anisotropic as follows from the anisotropy in the linear thermal expansion. Uniaxial pressure along the a-axis enhances T_0, while uniaxial pressure in the bc-plane suppresses T_0.

cond-mat.mtrl-sci↗