Študija valov gostote naboja NbSe$_3$ s tunelskim mikroskopom

Niobijev triselenid (NbSe$_3$) je anorganska spojina iz družine kvazi enodimenzionalnih trihalkogenidov prehodnih kovin. Njegovo zgradbo lahko opišemo s kolonami trigonalnih prizem, pri čemer halkogeni atomi zasedajo njihova oglišča, v središču pa najdemo kovinske atome. Ti stolpci so zloženi v plasteh, ki so med seboj povezane z relativno šibkimi van der Waalsovimi vezmi. Zaradi take strukture kažejo ti materiali eno- kot tudi dvo-dimenzionalno naravo, elektroni v njih pa tvorijo valove gostote naboja (VGN). Osnovna struktura NbSe$_3$ je sestavljena iz treh tipov kolon, ki se razlikujejo po dimenzijah osnovnih ploskev prizem, ki kolono sestavljajo. Med ohlajanjem materiala s sobne temperature pride do dveh Peierlsovih prehodov. Pri prehodu pri temperaturi $mathrm{Tp_1=144 K}$ pride do pojava prvega VGN z valovnim vektorjem $q_1 = left(0, 0.241b^{*},0right)$, pri prehodu pri temperaturi $mathrm{Tp_2=59 K}$ pa do pojava drugega VGN z valovnim vektorjem $q_2 = left(0.5a^{*}, 0.260b^{*},0.5c^{*}right)$, kjer so $a^{*}$, $b^{*}$ in $c^{*}$ inverzne vrednosti dimenzij osnovne celice kristala NbSe$_3$. V preteklosti je bilo določeno na katerih kolonah in v kakšnih kombinacijah se dejansko pojavita ta dva VGN, vendar pa so rezultati preliminarnih meritev na Institutu Jožef Stefan z navedbami v literaturi ne skladajo. Pred nadaljnjimi študijami materiala NbSe$_3$ je bilo zato potrebno nedvoumno določiti na katerih kolonah se pojavijo VGN ter ali in na katerih kolonah pride do pojava obeh VGN. Najprimernejše orodje za take preiskave je vrstični tunelski mikroskop (VTM), ki poleg atomske omogoča tudi elektronsko preiskavo površin materialov. Z VTM nam je uspelo posneti dovolj velike slike površine NbSe$_3$ na katerih je hkrati vidna atomska struktura in VGN. S Fourierovo transformacijo smo analizirali slike in identificirali posamezne kolone ter oba VGN. Ugotovili smo, da se na koloni $III$ pojavlja valovni vektor $q_{2b}$, na koloni $I$ pa se pojavljata oba, $q_{1b}$ in $q_{2b}$. Na kolonah III je bilo v nekaterih primerih šibko videti tako valovni vektor $q_{1b}$ kot tudi $q_{2b}$. Ti rezultati so v nasprotju z rezultati, objavljenimi v literaturi. Niobium triselenide (NbSe$_3$) is an inorganic compound from the family of quasi-one-dimensional transition metal trichalcogenides. Its structure can be described by columns of trigonal prisms, with chalcogen atoms occupying their corners and metal atoms in the centre. These columns are stacked in layers that are interconnected by relatively weak van der Waals bonds. Due to this structure, these materials exhibit one and two dimensional nature, and the electrons in them form charge density waves (CDWs). The basic structure of (NbSe$_3$) consists of three types of columns that differ in the dimensions of the basic prism surfaces that make up the column. During the cooling of these material from room temperature, two Peierls transitions take place. The first CDW with a wave vector $q_1 = left(0, 0.241b^{*},0right)$ is formed at the transition at temperature $mathrm{Tp_1=144 K}$, and the second CDW with $q_2 = left(0.5a^{*}, 0.260b^{*},0.5c^{*}right)$ is formed at the transition at temperature $mathrm{Tp_2=59 K}$. Here $a^{*}$, $b^{*}$ and $c^{*}$ are inverse values of unit cell dimensions. In the past, it was determined on which columns and in what combinations these two CDWs actually appear, but the results of preliminary measurements at the Jozef Stefan Institute do not correspond with the data in the literature. Prior to further studies of NbSe$_3$ material, it was therefore necessary to determine unambiguously on which columns the VGNs occur and whether and on which columns the two VGNs occur. The most appropriate tool for such investigations is a scanning tunneling microscope (STM), which can simultaniously reveal atomic as well as electronic structure of surfaces. With STM, we were able to record large enough images of the NbSe$_3$ surface on which both the atomic structure and the CDWs are visible. Using the Fourier transform, we analyzed the images and identified individual columns and both CDWs. We found that only wave vector $q_{2b}$ appears on column $III$ and both $q_{1b}$ and $q_{2b}$ appear on column $I$. In some cases both $q_{1b}$ and $q_{2b}$ wave vectors could be seen on columns $III$. These results are in contrast to the results reported in the literature.