arXiv subjects

Martin Rejhon

Publications and source records attributed to Martin Rejhon.

6 recordsLinked to original sources

arXiv2026

Terahertz spin-current transparency through rough interfaces

Spin transport across interfaces is critical for spintronic devices, yet remains difficult to probe on ultrafast timescales. We use terahertz emission spectroscopy on Co|Pt heterostructures whose interface roughness is tuned through the thickness of an underlying Au buffer layer, while leaving other growth parameters unchanged. From the measured THz electric field, we extract the interface spin-current transparency ts after correcting for the changes in sample impedance and optical absorption of the stack. Surprisingly, we find that ts decreases by only approximately 30% as the interface root-mean-square roughness and the lateral grain size both increase by a factor of three, with no measurable change in the THz spectrum. These results demonstrate that interfacial spin transport is relatively robust against morphological variations on ultrafast timescales, establishing terahertz emission spectroscopy as a reliable probe of spin dynamics across imperfect interfaces.

cond-mat.mes-hall↗

arXiv2026

Unveiling Davydov-Split Excitons in a Template-Engineered Molecular-Graphene Heterostructure

The realization of high-fidelity organic-inorganic quantum emulators is frequently hindered by the interfacial imperfections introduced during device fabrication. Here, we demonstrate a robust nanofabrication protocol that restores the atomic-scale purity of epitaxial graphene on SiC to UHV-equivalent levels, as confirmed by Low-Energy Electron Diffraction, and Microscopy. This pristine interface enables the emergence of macroscopic excitonic coherence in epitaxial overlayers of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), a model molecular system characterized by intense electron-phonon coupling. Through a combination of high-sensitivity Fourier Transform Photo-current Spectroscopy, photoluminescence, and dynamic Raman mapping, we resolve a complex vibronic manifold governed by Davydov splitting. We show that the $P6_3/m$ crystalline symmetry of the HMTP overlayer lifts the degeneracy of the HOMO-LUMO transition, creating discrete bright and dark excitonic branches. Using an analytical tight-binding model parameterized by ARPES-derived intermolecular coupling and Raman vibrational modes validated by molecular dynamics simulations, we quantify the polarization energy, the Huang-Rhys factor, and Herzberg-Teller corrections to the Franck-Condon model. Our results reveal that the dark-state branch dominates the radiative channel, following a polaron-mediated relaxation pathway consistent with Kasha's rule. By reconciling macroscopic device architecture with UHV-level surface science, this work establishes a scalable platform for the study of dark-exciton dynamics and the development of solid-state molecular quantum memories.

cond-mat.mes-hall↗

arXiv2023

Investigation of internal electric fields in graphene/6H-SiC under illumination by Pockels effect

In this paper, we introduce a method for mapping profiles of internal electric fields in birefringent crystals based on the electro-optic Pockels effect and measuring phase differences of low-intensity polarized light. In the case of the studied 6H-SiC crystal with graphene electrodes, the experiment is significantly affected by birefringence at zero bias voltage applied to the crystal and a strong thermo-optical effect. We dealt with these phenomena by adding a Soleil-Babinet compensator and using considerations based on measurements of crystal heating under laser illumination. The method can be generalized and adapted to any Pockels crystal that can withstand sufficiently high voltages. We demonstrate the significant formation of space charge in semi-insulating 6H-SiC under illumination by above-bandgap light.

physics.optics↗

arXiv2023

Plasmon-plasmon interaction and the role of buffer in epitaxial graphene micro-flakes

We investigate the origin of the translational symmetry breaking in epitaxially grown single-layer graphene. Despite the surface morphology of homogeneous graphene films influenced by the presence of mutually parallel SiC surface terraces, the far-infrared magneto-plasmon absorption is almost independent of the angle between the probing light polarization and the orientation of terraces. Based on a detailed analysis of the plasmon absorption lineshape and its behavior in the magnetic field, supported by confocal Raman mapping and atomic force microscopy, we explain this discrepancy by spontaneously formed graphene micro flakes. We further support our conclusions using data collected on artificially created graphene nanoribbons: we recognize similar plasmon origin in artificial ribbons and naturally formed grains. An unexpectedly large plasmon resonance redshift was observed in nanoribbons. In a hydrogen-intercalated sample (which does not contain the buffer), this redshift is quantitatively taken into account by a plasmon-plasmon interaction. In non-intercalated samples featuring a buffer layer, this redshift is due to an interplay between the plasmon-plasmon coupling and Coulomb screening by the buffer-induced interface states. This model determines the density of interface states in good agreement with experimentally reported values.

cond-mat.mes-hall↗

arXiv2018

Intercalation of hydrogen in the SiC/epitaxial graphene interface

We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated epitaxial graphene grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The samples of epitaxial graphene have been intercalated in the temperature range of 790 to 1250$^\circ$C and compared to the reference samples of hydrogen etched SiC. We have found that although the Si-H bonds form at as low temperatures as 790$^\circ$C, the well developed bond order has been reached only for epitaxial graphene intercalated at temperatures exceeding 1000$^\circ$C. We also show that the hydrogen intercalation degradates on a time scale of few days when samples are stored in ambient air. The optical spectroscopy shows on a formation of vinyl and silyl functional groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process.

cond-mat.mtrl-sci↗

arXiv2017

Effect of residual gas composition on epitaxial graphene growth on SiC

In recent years, graphene growth optimization has been one of the key routes towards large-scale, high-quality graphene production. We have measured in-situ residual gas content during epitaxial graphene growth on silicon carbide (SiC) to find detrimental factors of epitaxial graphene growth. The growth conditions in high vacuum and purified argon are compared. The grown epitaxial graphene is studied by Raman scattering mapping and mechanical strain, charge density, number of graphene layers and graphene grain size are evaluated. Charge density and carrier mobility has been studied by Hall effect measurements in van der Pauw configuration. We have identified a major role of chemical reaction of carbon and residual water. The rate of the reaction is lowered when purified argon is used. We also show, that according to time varying gas content, it is preferable to grow graphene at higher temperatures and shorter times. Other sources of growth environment contamination are also discussed. The reaction of water and carbon is discussed to be one of the factors increasing number of defects in graphene. The importance of purified argon and its sufficient flow rate is concluded to be important for high-quality graphene growth as it reduces the rate of undesired chemical reactions and provides more stable and defined growth ambient.

cond-mat.mtrl-sci↗