Elementary excitations near the boundary of a strongly correlated crystal
Date
2011-10-14
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Abstract
Viimastel aastatel on ilmnenud aktiivne huvi elementaarergastuste käitumise iseärasuste vastu tugevalt korreleeritud kristalli äärelähedases piikonnas ning kahest kristallist koosneva heterostruktuuri eralduspinnalähedases piirkonnas juhul, kui vähemalt üks kristallidest on tugevate elektronkorrelatsioonidega. Tugevate elektronkorrelatsioonidega kristallidele on tavaliselt iseloomulik magnetiline korrastatus, mis mängib suurt rolli laengukandjate käitumises. Doktoriväitekirja eesmärgiks oli uurida kristalli ääre(pinna) mõju elementaarergastustele sellises kristallis. Esiteks, on uuritud äärepinna mõju magnetilistele ergastustele ja lähinaabrite spinnide korrelatsioonidele poollõpmatutes kahe- ja kolmemõõtmelistes Heisenbergi spinn-1/2 antiferromagneetikutes nulltemperatuuril. Ääre(pinna) olemasolu jagab antiferromagneetiku äärelähedaseks piirkonnaks ja kristalli sügavuseks erinevate dominantsete spinn-ergastustega. Äärelähedases piirkonnas koosneb spekter spinni äärepindlainete (d-1)-mõõtmelisest moodist. Siin d=2 või 3 on antiferromagneetiku dimensioonid. Antiferromagneetiku ülejäänud osa ergastused on d-mõõtmelised seisvad spinnlained. Ääre poolt indutseeritud häirituste kirjeldamine spinn-ergastuste spektris on paljudes aspektides sarnane lokaalse defekti probleemiga. Spinni äärepindlainete mood põhjustab spinnide korrelatsioonide kammstruktuuri moodustumist äärelähedases piirkonnas. Teiseks, on uuritud ääre mõju laengukandjatele poollõpmatus kahemõõtmelises t-J mudelis aukude madalate kontsentratsioonide korral nulltemperatuuril. Äärelähedane piirkond -- mõned äärelähedased read -- on aukudest vaesed. Suikekihi moodustumist põhjustab augu ümbritseva magnonpilve deformatsioon äärelähedases piirkonnas. Aukudest vaesustus põhjustab ääre spektraalfunktsiooni keerulisemat kuju võrreldes kristalli sügavusega - peamaksimumil võivad olla lisaõlad. Ääre spektraalfunktsioonis on nähtavad äärelähedaste ridade spektraalfunktsioonide naksimumide kosted.
In the last few years, an active interest is taken in the behavior of elementary excitations near a junction of a strongly correlated crystal with another crystal or vacuum. Crystals with strong electron correlations are usually characterized by magnetic ordering, which plays a large role in the properties of charge carriers. Our study was aimed at the investigation of the influence of a crystal boundary on elementary excitations in such a crystal. Firstly, the influence of the boundary on magnetic excitations and nearest-neighbor spin correlations of a semi-infinite two- and three-dimensional spin-1/2 Heisenberg antiferromagnets was considered at zero temperature. The presence of the boundary divides the antiferromagnet into near-boundary and bulk regions with different dominant spin excitations. The spectrum of the near-boundary region consists of a (d-1)-dimensional mode of near-boundary spin waves. Here d=2 or 3 is the dimensionality of the antiferromagnet. Excitations of the rest of the antiferromagnet are d-dimensional standing spin waves. The description of perturbations introduced by the boundary into the spectrum of spin excitations is in many respects similar to the problem of a local defect in a crystal. Near-boundary mode of spin waves leads to the appearance of comb-like pattern of nearest-neighbor spin correlations in the near-boundary region. Secondly, the influence of the boundary on charge carriers in a semi-infinite two-dimensional t-J model was considered for the case of small hole concentrations and zero temperatures. The near-boundary region is depleted of holes. The depletion layer is caused by a deformation of the magnon cloud, surrounding the hole, near the boundary. The hole depletion leads to a more complicated spectral function in the near-boundary row in comparison with its bulk shape - the main maximum may have additional shoulders. In the boundary spectral function replicas of maxima from near-boundary rows are seen.
In the last few years, an active interest is taken in the behavior of elementary excitations near a junction of a strongly correlated crystal with another crystal or vacuum. Crystals with strong electron correlations are usually characterized by magnetic ordering, which plays a large role in the properties of charge carriers. Our study was aimed at the investigation of the influence of a crystal boundary on elementary excitations in such a crystal. Firstly, the influence of the boundary on magnetic excitations and nearest-neighbor spin correlations of a semi-infinite two- and three-dimensional spin-1/2 Heisenberg antiferromagnets was considered at zero temperature. The presence of the boundary divides the antiferromagnet into near-boundary and bulk regions with different dominant spin excitations. The spectrum of the near-boundary region consists of a (d-1)-dimensional mode of near-boundary spin waves. Here d=2 or 3 is the dimensionality of the antiferromagnet. Excitations of the rest of the antiferromagnet are d-dimensional standing spin waves. The description of perturbations introduced by the boundary into the spectrum of spin excitations is in many respects similar to the problem of a local defect in a crystal. Near-boundary mode of spin waves leads to the appearance of comb-like pattern of nearest-neighbor spin correlations in the near-boundary region. Secondly, the influence of the boundary on charge carriers in a semi-infinite two-dimensional t-J model was considered for the case of small hole concentrations and zero temperatures. The near-boundary region is depleted of holes. The depletion layer is caused by a deformation of the magnon cloud, surrounding the hole, near the boundary. The hole depletion leads to a more complicated spectral function in the near-boundary row in comparison with its bulk shape - the main maximum may have additional shoulders. In the boundary spectral function replicas of maxima from near-boundary rows are seen.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
kristallid, heterostruktuurid, piirpinnad, ergastused, crystals, heterostructures, boundary layers, excitations, , , , ,