Utjecaj superstrukturiranja na optička i transportna svojstva odabranih slojevitih materijala

Very special electronic phases brought upon by the superstructuring in layered materials have held the interest of researchers for decades. This thesis deals with the effects of the lattice superstructuring on optical and transport properties of pure and doped samples of 1T-TaS2 and 1T-TiSe2. This work brings the first ever analysis of the optical properties of the nearly-commensurate charge density wave (NCCDW) phase of 1T-TaS2 at temperatures below 200 K. Optical response of the NCCDW phase is for the first time analyzed with the nano-composite-like nature of the phase in mind. Modeling of the optical response using the Bruggeman effective medium theory is suggested and demonstrated. As a result, localized surface plasmon feature is identified in the optical spectrum, as well as the asymmetry of the optical phonon modes brought upon by the coupling of spatially distinct nano-sized domains which comprise the material. A novel analysis of the superstructure-related phonon peaks which allows the determination of the charge redistribution over the star-like superstructure is presented. Lastly, this work also presents the study of the 1T-TiSe2 compound, where the analysis is focused on the properties of the high temperature phase of the material. The contributions of electron and hole pockets are resolved for the first time, as well as the spectral weights and the scattering rates for each channel. Calculated energy ranges of the quasi-2D hole band settle the ongoing debate and determine that 1T-TiSe2 in the high-temperature phase is a semimetal, with scattering which gets stronger over a wide interval as the temperature approaches the phase transition.Vrlo posebne elektronske faze koje nastaju superstrukturiranjem slojastih materijala već desetljećima zaokupljaju pažnju istraživača. Ovaj doktorski rad proučava efekte superstrukturiranja kristalne rešetke na optička i transportna svojstva čistih i dopiranih uzoraka 1T-TaS2 i 1T-TiSe2. Rad donosi prvu analizu optičkih svojstava faze približno sumjerljivog vala gustoće naboja (engl. NCCDW) 1T-TaS2 na temperaturama ispod 200 K. Po prvi put je optički odziv NCCDW faze analiziran uzimajući u obzir njenu nanokompozitnu prirodu, te je predloženo i provedeno modeliranje optičkog odziva pomoću Bruggemanove teorije efektivnog medija. Kao rezultat, u optičkom spektru je identificiran učinak lokalnog površinskog plazmona, kao i asimetrija fononskih modova do koje dolazi zbog vezanja optičkog odziva prostorno razdvojenih nano-domena. Nadalje, predložena je nova analiza intenziteta super-strukturnih fononskih vrhova koja omogućava određivanje redistribucije naboja u zvjezdastoj superstrukturi. Napose, ovaj rad predstavlja i istraživanje 1T-TiSe2, gdje je analiza usredotočena na svojstva visokotemperaturne faze materijala. Po prvi put su razlučeni doprinosi elektronskih i šupljinskih pobuđenja, te njihova spektralna težina i jačina raspršenja. Proračuni energijskog raspona kvazi-dvodimenzionalne šupljinske vrpce omogućuju zaključenje dugotrajne debate u korist zaključka da je 1T-TiSe2 u visokotemperaturnoj fazi “polumetal” (engl. semimetal), s raspršenjem koje jača preko vrlo širokog intervala u kojem se temperatura približava točki prijelaza

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al72.5Mn21.5Fe6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al72.5Mn21.5Fe6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, &radic;T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data.</p

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al_72.5Mn_21.5Fe_6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al_72.5Mn_21.5Fe_6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, √T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data

The Generalization of the Kinetic Equations and the Spectral Conductivity Function to Anisotropic Systems: Case T-Al_72.5Mn_21.5Fe_6 Complex Metallic Alloy

Electrical conductivity, σ, and thermoelectric power, S, of the monocrystalline T-Al_72.5Mn_21.5Fe_6 complex metallic alloy have been investigated in the temperature range from 2 to 300 K. The crystallographic-direction-dependent measurements were performed along the [0 0 1], [0 1 0] and [1 0 0] directions of the orthorhombic unit cell, where the stacking direction is along the [0 1 0] direction. The electrical conductivity exhibits a very small anisotropy, and in all directions shows the non-metallic behaviour with square root, √T, temperature behaviour and finite value in the T = 0 limit. Spectral conductivity function, σS(E), constructed out of measurements, reflects anisotropy of the experimental data and indicate non-analytic square root like singularity at Fermi level. Asymmetry of the spectral conductivity function has been extracted from the thermoelectric power data

Percolative nature of the dc paraconductivity in the cuprate superconductors

We present an investigation of the planar direct-current (dc) paraconductivity of the model cuprate material HgBa$_2$CuO$_{4+\delta}$ in the underdoped part of the phase diagram. The simple quadratic temperature-dependence of the Fermi-liquid normal-state resistivity enables us to extract the paraconductivity above the macroscopic $T_c$ with great accuracy. The paraconductivity exhibits unusual exponential temperature dependence, with a characteristic temperature scale that is distinct from $T_c$. In the entire temperature range where it is discernable, the paraconductivity is quantitatively explained by a simple superconducting percolation model, which implies that underlying gap disorder dominates the emergence of superconductivity

Scattering dominated high-temperature phase of 1T-TiSe2: an optical conductivity study

The controversy regarding the precise nature of the high-temperature phase of 1T-TiSe2 lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of 1T-TiSe2 single crystals. The separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures can be applied in their generic forms as the scattering for both types of carriers is in the vicinity of the Ioffe-Regel limit with decay rates being comparable to or larger than the offsets of band extrema. The nonmetallic temperature dependence of transport properties comes from the anomalous temperature dependence of scattering rates. Near the transition temperature the heavy electrons and the light holes contribute equally to the conductivity. This surprising coincidence is regarded as the consequence of dominant intervalley scattering that precedes the transition. The low-frequency peak in the optical spectra is identified and attributed to the critical softening of the L-point collective mode.Comment: 28 pages, 10 figure

Electronic transport and magnetism in the alternating stack of metallic and highly frustrated magnetic layers in Co1/3_{1/3}NbS2_2

Co$_{1/3}$NbS$_2$ is the only magnetically intercalated layered transition metal dichalcogenide (TMD) suggested to experience the complete suppression of magnetic order under pressure. From elastic neutron scattering we report the direct evidence for the reduction of the antiferromagnetic ordering temperature under pressure, up to complete suppression of magnetic order around 1.7 GPa. The static and ac magnetic susceptibility measurements reveal strong frustration in the magnetic subsystem, and spin canting responsible for the appearance of ferromagnetic (FM) component in dominantly antiferromagnetic (AF) ordered state. The electric transport in directions perpendicular and parallel to layers is explored for the first time in magnetically intercalated TMDs, in the wide temperature and pressure ranges. We show that electric transport reacts differently to magnetic ordering in directions along and perpendicular to layers, with the in-plane conductivity increasing, and the out-of-plane conductivity decreasing in the ordered state. At pressures above 3 GPa, we identify the appearance of the Kondo scattering regime. We use ab-initio calculations to explore the electronic structure in magnetically ordered state, the nature of magnetic interactions, and the mechanism responsible for the changes observed under pressure. The mechanisms of suppression of magnetic order under pressure are scrutinized in the light of these experimental and theoretical findings. We conclude that magnetic couplings beyond nearest-neighbors determine the nature of magnetic ordering. The suppression of ordering under pressure is ascribed to the pressure-induced shift in balance between super-exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic couplings, leading to amplified magnetic frustration.Comment: 34 pages, 17 figure