Doping Nature of Native Defects in 1T-TiSe2

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T-CDW approximate to 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.

Spectroscopic Studies on Iron-Chalcogenide Superconductors

Ping-Hui Lin

Superconductivity in F-doped LaFeAsO, Tc= 26 K was first reported by Kamihara et al in 2008. Following that, more iron-based superconductors, e.g., (Ba,K)Fe2As2, LiFeAs, and Fe1+yTe1-xSex were discovered. The new class of high Tc superconductors have attracted a tremendous interest. Iron-chacogenides do not involve arsenic but share several common features with the iron-pnictides. They are characterized by FeCh layers which are isostructural to the FeAs or FeP layers in the iron-pnictides. The electronic structure near the Fermi energy derives from the Fe 3d orbitals. Both families exhibit a spin density wave (SDW) ground state. Se substation for Te in FeTe suppresses the SDW and leads to superconductivity. From a more practical point of view, the iron-chacogenides have a rather simple structure, and they are less toxic and easier to obtain in large single crystals than the arsenide counterparts. For all these properties, the iron-chalcogenides are ideal compounds for angle-resolved photoemission (ARPES) studies. In this thesis, I have investigated by ARPES the normal state electronic structure of Fe1+yTe1-xSex superconductors with various doping level. I have studied the multi-orbital Fermi surface of Fe1+yTe1-xSex, mainly by means of polarized photons from synchrotron radiation sources. Individual bands of different orbital characters could then be distinguished by exploiting ARPES selection rules. I have also investigated the 3D electronic structure of these materials by performing normal emission measurements as a function of the incident photon energy. Namely, by using photon energies as large as 150 eV, I have performed an unusually broad ARPES survey of momentum space. The data reveal two inequivalent Fermi contours at nominally equivalent Gamma points in different Brillouin zones. The observed periodicity corresponds to a unit cell containing only one Fe atom, rather than the standard tetragonal unit cell containing two Fe atoms. I also observe a strong and selective suppression of spectral weight around perpendicular lines at the corner of the Brillouin zone.

EPFL2011

Nature of the Charge Carriers and Conduction Mechanism in Anatase TiO2

Jacim Jacimovic

The anatase phase of TiO2 is a key material in the third generation of photovoltaics and for photo-catalytic devices. The number of promising applications, where one would profit from the electronic degrees of freedom of the material, like in memristors, spintronics, thermoelectrics etc., is constantly increasing. This vivid interest for anatase, beyond the traditionally used nanoparticle form, puts forward other structures like nanowires and thin films. Despite this progress in the demand and processing of anatase the description of the electronic properties is very limited. Few of the relevant questions are: what is the best way to engineer the band-gap; what is the character of the injected charge carriers; is there a polaron formation in this ionic material; what is the best way to stabilize a metallic state; can one observe superconductivity in TiO2 like in SrTiO3; how do the electronic properties evolve on going from bulk crystals, thin films, nanowires to nanoparticles? Bringing answer to some of these questions was the leitmotif of my PhD program. The principle quantities which I have investigated were the electrical transport coefficients: resistivity and thermoelectric power. Both quantities were measured in a broad temperature range (in some cases from 30 mK to 1000 K) and as a function of hydrostatic pressure (up to 4 GPa). With the progressive emergence of a physical model about this material in time, it was necessary to perform other measurements, to go beyond the expertise of our laboratory. I initiated and participated in optical and pumped-probe optical measurements and launched angle resolved photoemission spectroscopy (ARPES) studies. The major results are the following: 1. The easiest and almost inevitable doping of anatase is by oxygen vacancies. They create a low-lying donor level. The electrons are dressed by interactions with the lattice and upon thermal excitation into the conduction band they behave as large polarons. Their lifetime depends on the density of the vacancies. Besides high temperature heat treatment high energy photons (90 eV) can create these vacancies, as well. This offers the way of finely tuning the carrier density and to study polaron-polaron interactions. High pressure transport study confirms the polaronic character of the charge carriers and shows a non-monotonic pressure dependence of their mobility. 2. In Cr, N doped and Cr-N co-doped anatase thin films the dominant doping comes from oxygen vacancies. Surprisingly, with increasing dopant concentration the donor level - conduction band separation decreases considerably while the scattering rate goes up. These measuremnts call for an extreme care in the synthesis if one wants to have high quality films. 3. The best metallicity with doping is achieved in the case of Nb replacement of Ti. The 6% doping leads to 10−4 Ωcm resistivity at room temperature. A slight cation off-stoichiometry gives a weak but noticeable Kondo scattering at sub-Kelvin temperatures which might be at the origin of the absence of superconductivity. 4. In the case of an assembly of nanowires and nanoparticles the transport mechanism is suspected to be of hopping of charge carriers. However, the samples were too resistive to be measured at room temperatures with a d.c. method. Going to high temperatures the samples loose oxygen which precludes the precise measurement of the temperature dependence of the resistivity and the extraction of the hopping parameters. This extensive program of characterizing the electronic properties of anatase in all forms was possible due to the exceptional sample preparation expertise of our laboratory. Unique pristine and Cr, Co, Ir and Nd doped single crystals, nanowires and nanoparticles were prepared in-house; the thin films were provided by Dr. G. Eres (Oak Ridge national Laboratory) and by Prof. T. Hasegawa (Tokyo University) in the frame of a collaboration.

EPFL2012

Scanning tunneling microscopy of the charge density wave in 1T-TiSe2 in the presence of single atom defects

Philipp Aebi, Helmuth Berger

We present a detailed low-temperature scanning tunneling microscopy (STM) study of the commensurate charge density wave (CDW) in 1T-TiSe2 in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defect concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. A systematic analysis of STM micrographs combined with density functional theory modeling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no 2H-polytype inclusions in the CDW phase, as previously found at room temperature [A. N. Titov et al., Phys. Solid State 53, 1073 (2011)]. They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125 (2011)].