arXiv subjects

C. Deparis

Publications and source records attributed to C. Deparis.

7 recordsLinked to original sources

arXiv2018

Edge-emitting polariton laser and amplifier based on a ZnO waveguide

We demonstrate edge-emitting exciton-polariton (polariton) lasing from 5 to 300 K and amplification of non-radiative guided polariton modes within ZnO waveguides. The mode dispersion below and above the lasing threshold is directly measured using gratings present on top of the sample, fully demonstrating the polaritonic nature of the lasing modes. The threshold is found to be similar to that of radiative polarions in planar ZnO microcavities. These results open broad perspectives for guided polaritonics by allowing an easier and more straightforward implementation of polariton integrated circuits exploiting fast propagating polaritons.

cond-mat.mes-hall↗

arXiv2011

Nature of resonant Raman scattering in ZnCoO films under sub-band excitation

Using the methods of scanning electron (SEM) and atom force microscopy (AFM) as well as photoluminescence (PL) and Raman micro-spectroscopy, we investigated $Zn_{1-x}Co_xO$ films ($x=5$ and $15\%$) grown by molecular beam epitaxy on sapphire substrates. It is found that the films have a nanocrystalline structure with the grain size decreased from $\sim150$ down to $28$ nm at changing the $Co$ concentration from $5$ to $15\%$. High-resolution SEM images of $Zn_{0.85}Co_{0.15}O$ film have been interpreted as inhomogeneous lateral distribution of $Co$ atoms. Two broad emission bands observed in spectra of band-to-band PL are ascribed to emission from $Zn_{1-x}Co_xO$ nano-regions enriched and depleted with $Co$. In low-temperature PL spectra under sub-band excitation of $Zn_{1-x}Co_xO$ films, there observed are intra-center optical transitions due to $Co^{2+}$ center: $^2E(G)$, $^2A_1(G)$, $^2T_1(G)$, $^4T_1(P)$, $^2T_2(G)$ $\longrightarrow$ $^4A_2(F)$. Offered is a new approach to interpretation of resonant enhancement observed in multi-phonon scattering by $LO$ phonons in $Zn_{1-x}Co_xO$ under sub-bandgap excitation combined with the extrinsic Fröhlich interaction mediated via a localized exciton bound to the isoelectronic impurity in which the electron is strongly localized at the magnetic $Co^{2+}$ ion. This conclusion is confirmed by the dependence of Raman spectra on the quantum energy of exciting radiation. It assumes formation of intermediated sub-band electron excited states of isoelectron $Co$ dopant, they are also referred to as charge transfer processes.

cond-mat.mtrl-sci↗

arXiv2011

Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O

This work presents results of near-band gap magnetooptical studies on (Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence, that shift towards higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what points to small and positive N_0 beta. It is shown that both the increase of the gap with x and the small positive value of the exchange energy N_0 beta corroborate recent theory describing the exchange splitting of the valence band in a non-perturbative way, suitable for the case of a strong p-d hybridization.

cond-mat.mtrl-sci↗

arXiv2011

Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies

We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co

We report on the magnetic properties of thoroughly characterized Zn1-xCoxO epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.

cond-mat.mtrl-sci↗

arXiv2007

Antiferromagnetic interactions in single crystalline Zn1-xCoxO thin films

In a rather contradictory situation regarding magnetic data on Co-doped ZnO, we have succeeded in fabricating high-quality single crystalline Zn1-xCoxO (x=0.003-0.07) thin films. This gives us the possibility, for the first time, to examine the it intrinsic magnetic properties of ZnO:Co at a quantitative level and therefore to address several unsolved problems, the major one being the nature of the Co-Co interaction in the ZnO structure.

cond-mat.mtrl-sci↗

arXiv2005

Effect of the s,p-d exchange interaction on the excitons in (Zn,Co)O epilayers

We present a spectroscopic study of (Zn,Co)O layers grown by molecular beam epitaxy on sapphire substrates. (Zn,Co)O is commonly considered as a promising candidate for being a Diluted Magnetic Semiconductor ferromagnetic at room temperature. We performed magneto-optical spectroscopy in the Faraday configuration, by applying a magnetic field up to 11 T, at temperatures down to 1.5 K. For very dilute samples (less than 0.5% Co), the giant Zeeman splitting of the A and B excitons is observed at low temperature. It is proportional to the magnetization of isolated Co ions, as calculated using the anisotropy and g-factor deduced from the spectroscopy of the d-d transitions. This demonstrates the existence of spin-carrier coupling. Electron-hole exchange within the exciton has a strong effect on the giant Zeeman splitting observed on the excitons. From the effective spin-exciton coupling, _X=0.4 eV, we estimate the difference of the exchange integrals for free carriers, N0|Alpha-Beta|=0.8 eV. The magnetic circular dichroism observed near the energy gap was found to be proportional to the paramagnetic magnetization of anisotropic Co ions even for higher Co contents.

cond-mat.mtrl-sci↗