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Vpliv dopiranja na kvantno spinsko tekočino 1T-TaS$_2$
ID Benedičič, Izidor (Author), ID Arčon, Denis (Mentor) More about this mentor... This link opens in a new window

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Abstract
Plastoviti kristal 1T-TaS$_2$ je pri temperaturah $\textit{T}$ < 180 K Mottov izolator. Raziskovalci so nedavno predlagali, da je magnetno osnovno stanje v tej fazi kvantna spinska tekočina. Lastnosti kvantne spinske tekočine v 1T-TaS$_2$ še niso jasne in zahtevajo dodatne poskuse. Eden od možnih pristopov k raziskovanju je podiranje spinske tekočine z dopiranjem. S preučevanjem nizkoenergijskih vzbuditev v 1T-TaS$_{2-x}$Se$_x$ želimo opazovati, kako faza spinske tekočine znotraj stanja Mottovega izolatorja prehaja v kovinsko oziroma superprevodno fazo. V magistrskem delu smo z jedrsko magnetno resonanco (NMR) na jedrih $^{77}$Se in jedrsko kvadrupolno resonanco (JKR) na jedrih $^{181}$Ta preučevali magnetne lastnosti s selenom dopiranega 1T-TaS$_2$. Opazovali smo obliko in položaj spektralnih črt, za določanje lastnosti nizkoenergijskih vzbuditev (spinonov) pa smo pomerili temperaturno odvisnost spinsko-mrežnega relaksacijskega časa. JKR spektri $^{181}$Ta v nizko dopiranem vzorcu 1T-TaS$_{1.94}$Se$_{0.06}$ so v primerjavi s čistim vzorcem 1T-TaS$_2$ znatno razširjeni, kar kaže na povečanje lokalnega nereda zaradi vnašanja nemagnetnih selenovih nečistoč. Temperaturna odvisnost spinsko-mrežnega relaksacijskega časa je pri tako nizkem dopiranju še vedno potenčna, in sicer $1/T_1 \propto T^{2.7}$ na temperaturnem intervalu 10 K < $\textit{T}$ < 160 K, kar kaže na spinonsko disperzijsko relacijo, ki je v neki točki Brillouinove cone linearna v energiji in brez reže. Pri $\textit{T}$ < 10 K se temperaturna odvisnost $1/T_1$ spremeni v $1/T_1 \propto T^\eta$, $\eta \approx 0.6$, kar nakazuje, da ima spinonska disperzija tudi kvadratičen pas, značilen za spinone s Fermijevo površino. Primerjava z meritvami na nedopiranem vzorcu nakazuje, da je disperzija v 1T-TaS$_2$ podobna, le da se pri $\textit{T}$ = 50 K v linearnem delu spektra odpre energijska špranja reda velikosti 1 meV. Vzrok nastanka te energijske reže trenutno še ni jasen, verjetno pa odraža zamrzovanje nabojnih ali strukturnih prostostnih stopenj. Po drugi strani močno dopirani vzorec 1T-TaSSe v preiskovanem temperaturnem območju ni več Mottov izolator, temveč kovina. Temperaturna odvisnost spinsko-mrežnega relaksacijskega časa je v tem primeru približno linearna, kot pričakujemo v normalnem kovinskem stanju. Kljub temu Moriyev faktor ojačitve, ki meri vpliv spinskih korelacij v kovinah, znatno odstopa od vrednosti 1. To nakazuje, da so v kovinski fazi antiferomagnetne korelacije še vedno zelo pomembne. V svojem magistrskem delu sem torej pokazal, kako se spinonska disperzijska relacija spreminja z dopiranjem v modelnem sistemu 1T-TaS$_2$.

Language:Slovenian
Keywords:jedrska magnetna resonanca, kvantne spinske tekočine, kvantna spinska frustracija
Work type:Master's thesis/paper
Typology:2.09 - Master's Thesis
Organization:FMF - Faculty of Mathematics and Physics
Year:2018
PID:20.500.12556/RUL-103828 This link opens in a new window
COBISS.SI-ID:3258980 This link opens in a new window
Publication date in RUL:27.09.2018
Views:1675
Downloads:280
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Secondary language

Language:English
Title:Effect of doping on a quantum spin liquid 1T-TaS$_2$
Abstract:
At temperatures $\textit{T}$ < 180 K, layered compound 1T-TaS$_2$ is a Mott insulator. Magnetic ground state of this phase was recently proposed to be a quantum spin liquid. Properties of this state are yet unclear and further experiments are required. One possible approach is a destabilization of quantum spin liquid with doping. With investigation of low-temperature excitations we want to observe how a quantum spin liquid progresses into the metalic or the superconducting phase across the 1T-TaS$_{2-x}$Se$_x$ phase diagram. In this master's thesis, we report a $^{77}$Se nuclear magnetic resonance (NMR) and $^{181}$Ta nuclear quadrupole resonance (NQR) study of magnetic properties of selenium-doped 1T-TaS$_2$. Shapes and positions of spectral lines were measured to follow changes in static local magnetic fields, while temperature dependence of spin-lattice relaxation rate was explored to determine low-energy excitations. $^{181}$Ta NQR spectra for the lightly doped 1T-TaS$_{1.94}$Se$_{0.06}$ sample exhibit significant line broadening in comparison to pristine samples, indicating Se doping-induced increase of local disorder. Temperature dependence of spin-lattice relaxation rate follows a power law. $1/T_1 \propto T^{2.7}$ over the broad temperature range 10 K < $\textit{T}$ < 160 K and indicates a linear gapless spinon dispersion relation at some point of the Brillouin zone. For the lowest temperatures $\textit{T}$ < 10 K, temperature dependence of spin-lattice relaxation rate changes to $1/T_1 \propto T^\eta$, $\eta \approx 0.6$, which is consistent with a quadratic spinon dispersion band - a hallmark for spinons forming a Fermi surface. Comparison with the data on pristine samples suggests that 1T-TaS$_2$ has similar dispersion relation, but with linear part opening an energy gap on the order of 1 meV for $\textit{T}$ < 50 K. The origin of such gap is still unclear, but it may reflect freezing of charge or structural degrees of freedom. In the investigated temperature range, the heavily doped 1T-TaSSe is a conductor rather than Mott insulator. The temperature dependence of spin-lattice relaxation rate is roughly linear in temperature, as it is expected for metallic samples. However, the Moriya enhancement factor, which measures the effect of spin correlations in metals, is significantly smaller than 1, implying that antiferromagnetic correlations remain very important even deeply in the metallic regime. In my master's thesis, I demonstrated how spinon dispersion relation changes with doping in model system 1T-TaS$_2$.

Keywords:nuclear magnetic resonance, quantum spin liquids, quantum spin frustration

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