Anisotropy of transport properties of approximants to the decagonal quasicrystals

U radu je proučavana anizotropija transportnih koeficijenata (električne i toplinske vodljivosti, termostruje i Hallovog koeficijenta) monokristala aproksimanata dekagonalnih kvazikristala. Aproksimanata dekagonalnih kvazikristala. zbog natjecanja uređenja kratkog i periodičnog uređenja dugog dosega posjeduju posebno zanimljiva fizikalna svojstva od kojih su neka slična onima kod dekagonalnih kvazikristala. Dekagonalni kvazikristali u dvije dimenzije posjeduju kvazikristalno uređenje dok se u trećoj dimenziji kvazikristalne ravnine periodički slažu. Eksperimentalno je utvrđeno da je anizotropija fizikalnih svojstava dekagonalnih kvazikristala ovisna o broju kvazikristalnih ravnina unutar jedne periodičke jedinice. Stoga se postavilo pitanje je li izražena anizotropija fizikalnih svojstava, mjerenih u, i okomito na ravninu, posljedica kvazikristalnog uređenja unutar dvodimenzionalnih atomskih slojeva i periodičkog uređenja u smjeru okomitom na te slojeve ili je anizotropija posljedica kompleksnog lokalnog atomskog uređenja na skali susjednih atoma bez direktne povezanosti s kvaziperiodičnošću. Zbog nepostojanja translacijske invarijantnosti kvazikristali predstavljaju vrlo složene sisteme za teorijska proučavanja, te se aproksimiraju dekagonalnom kvazikristalnom fazom. Struktura aproksimanata dekagonalnih kvazikristala se može opisati pseudodekagonalnim kristalnim ravninama koje odgovaraju kvazikristalnim ravninama u dekagonalnim kvazikristalima, što se periodički slažu u smjeru okomitom na njih. Lokalno atomsko uređenje aproksimanata slično je onome u kvazikristalima, no kako je kod aproksimanata kristalna struktura periodička, moguće je provesti teorijske proračune za ovu strukturu i usporediti ih s eksperimentima. Kod dekagonalnih kvazikristala transportni koeficijenti mjereni u periodičnom smjeru i kvaziperiodičnoj ravnini pokazuju anizotropiju koja se smanjuje s porastom broja dekagonalnih atomskih slojeva unutar jedne periodične jedinice. Eksperimentalno je opaženo da i kod aproksimanata dekagonalnih kvazikristala koji su periodični duž sva tri smjera veličina anizotropije transportnih koeficijenata pada s pora-stom broja atomskih ravnina koje po lokalnoj atomskoj strukturi odgovaraju dekagonalnim ravninama kod kvazikristala. Utvrđeno je i da je stupanj strukturne savršenosti također faktor koji utječe na iznos anizotropije.Transport coefficients (electrical and thermal conductivity, thermopower and Hall effect) of the monocrystals of approximants to the decagonal quasicrystals have been explored in the thesis. Because of the competition of the short range order and the periodic long range order, they have especially interesting physical properties, some of them resembling those of the decagonal quasicrystals. Decagonal quasicrystals possess two dimensional quasiperiodic arrangements, while in the third spatial direction these quasiperiodic planes are periodically stacked. It has been experimentally determined that the anisotropy of the physical properties of the decagonal quasicrystals depends on the number of the quasiperiodic layers along one periodic unit. The basic question here is whether the anisotropy of the physical properties between the in-plane and the stacking direction is the consequence of the quasiperiodic structural order within the 2D atomic layers versus the periodic order in the perpendicular direction or is it rather a conse-quence of complex local atomic order on the scale of near-neighbor atoms with no direct relationship to the quasiperiodicity. Due to the lack of translational periodicity within the quasiperiodic planes, quasicrystals are very complex systems for the theoretical analysis. The problem can be overcome by considering approximant phases to the decagonal phase. The structure of the approximants to the decagonal quasicrystals can be described by the pseudo decagonal atomic layers that correspond to the quasiperiodic layers in the decagonal quasicrystals, which are periodically stacked along the direction perpendicular to the layers. Local atomic order of the approximant phases is similar to that of the quasicrystals, but, since approximant phases are periodic solids in three dimensions; the theoretical simulations are straightforward to perform. The anisotropy of the transport coefficients of the decagonal quasicrystals measured along the stacking direction and in the quasiperiodic plane is decreasing with the increasing number of the decagonal layers in one periodic unit. It has been experimentally observed that in the approximants to the decagonal quasicrystals, which are 3D periodic solids, the amount of the transport coefficient anisotropy also decreases with the increasing number of the atomic layers that corre-spond to the decagonal layers in decagonal quasicrystals. It has been determined that the degree of structural perfection is also a factor influencing anisotropy amount

Anisotropy of transport properties of approximants to the decagonal quasicrystals

U radu je proučavana anizotropija transportnih koeficijenata (električne i toplinske vodljivosti, termostruje i Hallovog koeficijenta) monokristala aproksimanata dekagonalnih kvazikristala. Aproksimanata dekagonalnih kvazikristala. zbog natjecanja uređenja kratkog i periodičnog uređenja dugog dosega posjeduju posebno zanimljiva fizikalna svojstva od kojih su neka slična onima kod dekagonalnih kvazikristala. Dekagonalni kvazikristali u dvije dimenzije posjeduju kvazikristalno uređenje dok se u trećoj dimenziji kvazikristalne ravnine periodički slažu. Eksperimentalno je utvrđeno da je anizotropija fizikalnih svojstava dekagonalnih kvazikristala ovisna o broju kvazikristalnih ravnina unutar jedne periodičke jedinice. Stoga se postavilo pitanje je li izražena anizotropija fizikalnih svojstava, mjerenih u, i okomito na ravninu, posljedica kvazikristalnog uređenja unutar dvodimenzionalnih atomskih slojeva i periodičkog uređenja u smjeru okomitom na te slojeve ili je anizotropija posljedica kompleksnog lokalnog atomskog uređenja na skali susjednih atoma bez direktne povezanosti s kvaziperiodičnošću. Zbog nepostojanja translacijske invarijantnosti kvazikristali predstavljaju vrlo složene sisteme za teorijska proučavanja, te se aproksimiraju dekagonalnom kvazikristalnom fazom. Struktura aproksimanata dekagonalnih kvazikristala se može opisati pseudodekagonalnim kristalnim ravninama koje odgovaraju kvazikristalnim ravninama u dekagonalnim kvazikristalima, što se periodički slažu u smjeru okomitom na njih. Lokalno atomsko uređenje aproksimanata slično je onome u kvazikristalima, no kako je kod aproksimanata kristalna struktura periodička, moguće je provesti teorijske proračune za ovu strukturu i usporediti ih s eksperimentima. Kod dekagonalnih kvazikristala transportni koeficijenti mjereni u periodičnom smjeru i kvaziperiodičnoj ravnini pokazuju anizotropiju koja se smanjuje s porastom broja dekagonalnih atomskih slojeva unutar jedne periodične jedinice. Eksperimentalno je opaženo da i kod aproksimanata dekagonalnih kvazikristala koji su periodični duž sva tri smjera veličina anizotropije transportnih koeficijenata pada s pora-stom broja atomskih ravnina koje po lokalnoj atomskoj strukturi odgovaraju dekagonalnim ravninama kod kvazikristala. Utvrđeno je i da je stupanj strukturne savršenosti također faktor koji utječe na iznos anizotropije.Transport coefficients (electrical and thermal conductivity, thermopower and Hall effect) of the monocrystals of approximants to the decagonal quasicrystals have been explored in the thesis. Because of the competition of the short range order and the periodic long range order, they have especially interesting physical properties, some of them resembling those of the decagonal quasicrystals. Decagonal quasicrystals possess two dimensional quasiperiodic arrangements, while in the third spatial direction these quasiperiodic planes are periodically stacked. It has been experimentally determined that the anisotropy of the physical properties of the decagonal quasicrystals depends on the number of the quasiperiodic layers along one periodic unit. The basic question here is whether the anisotropy of the physical properties between the in-plane and the stacking direction is the consequence of the quasiperiodic structural order within the 2D atomic layers versus the periodic order in the perpendicular direction or is it rather a conse-quence of complex local atomic order on the scale of near-neighbor atoms with no direct relationship to the quasiperiodicity. Due to the lack of translational periodicity within the quasiperiodic planes, quasicrystals are very complex systems for the theoretical analysis. The problem can be overcome by considering approximant phases to the decagonal phase. The structure of the approximants to the decagonal quasicrystals can be described by the pseudo decagonal atomic layers that correspond to the quasiperiodic layers in the decagonal quasicrystals, which are periodically stacked along the direction perpendicular to the layers. Local atomic order of the approximant phases is similar to that of the quasicrystals, but, since approximant phases are periodic solids in three dimensions; the theoretical simulations are straightforward to perform. The anisotropy of the transport coefficients of the decagonal quasicrystals measured along the stacking direction and in the quasiperiodic plane is decreasing with the increasing number of the decagonal layers in one periodic unit. It has been experimentally observed that in the approximants to the decagonal quasicrystals, which are 3D periodic solids, the amount of the transport coefficient anisotropy also decreases with the increasing number of the atomic layers that corre-spond to the decagonal layers in decagonal quasicrystals. It has been determined that the degree of structural perfection is also a factor influencing anisotropy amount

Anisotropic Transport Properties of the Al13Fe4 Decagonal Approximant

We have investigated electrical resistivity, thermoelectric power and thermal conductivity of the Al13Fe4 monoclinic approximant to the decagonal quasicrystal. The crystallographic-direction-dependent measurements were performed along the a*, b and c directions of the monoclinic unit cell, where (a*,c) atomic planes are stacked along the perpendicular b direction. The electronic transport exhibit significant anisotropy. The stacking b direction is the most conducting direction for the electricity and heat. The anisotropic thermopower reflects complicated structure of the anisotropic Fermi surface that contains electron-like and hole-like contributions.</p

Hall Effect of the Al13Fe4 Decagonal Approximant and Its Ternary Extension Al13(Fe,Ni)4

We have measured Hall coefficient and electrical resistivity of the Al13Fe4 and Al13(Fe,Ni)4 monoclinic approximants to the decagonal quasicrystal. While the Al13Fe4 crystals are structurally well ordered, the ternary extension Al13(Fe,Ni)4 contains quenched disorder and can be viewed as a disordered version of Al13Fe4. The crystallographic-direction-dependent Hall effect measurements were performed along the a*, b and c directions of the monoclinic unit cell, where (a*,c) atomic planes are stacked along the perpendicular b direction. The stacking b direction is the most conducting direction for the electricity. The effect of quenched disorder in Al13(Fe,Ni)4 is manifested in the large residual resistivity ρ(T → 0) as compared to the ordered Al13Fe4. The Hall coefficient, RH, values for all combinations of directions, are typical metallic. The anisotropic Hall coefficient reflects complicated structure of the anisotropic Fermi surface that contains electron-like and hole-like parts. Depending on the combination of directions of the current and magnetic field electron-like (RH 0) contributions may dominate, or the two contributions compensate each other (RH ≈ 0).</p