Low Hole Concentration Research Articles

Breakdown of the Nagaoka phase in the two-dimensionalt−Jmodel

In the limit of weak exchange J at low hole concentration $\ensuremath{\delta}$ the ground state of the two-dimensional $t\ensuremath{-}J$ model is believed to be ferromagnetic. We study the leading instability of this Nagaoka state, which emerges with increasing J. Both exact diagonalization of small clusters, and a semiclassical analytical calculation of larger systems show that above a certain critical value of the exchange, ${J}_{\mathrm{cr}}\ensuremath{\sim}t{\ensuremath{\delta}}^{2},$ Nagaoka's state is unstable to phase separation. In a finite-size system a bubble of antiferromagnetic Mott insulator appears in the ground state above this threshold. The size of this bubble depends on $\ensuremath{\delta}$ and scales as a power of the system size N.

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr 1-x CaxMnO3 system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr 0.8 Ca0.2MnO3 single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr3+ ions.

Preparation of ferromagnetic (In,Mn)As with relatively low hole concentration and Curie temperature 50 K

Ferromagnetic (In,Mn)As/GaSb and (In,Ga,Mn)As/GaSb heterostructures with Mn composition x=0.03–0.09 have been prepared by molecular beam epitaxy. It is found that the relation between maximum Curie temperature T C and Mn content x can be expressed experimentally by T C=800 x up to about T C=50 K within the limit of this study. The hole concentration sufficient for T C=50 K is found to be several times lower than the value shown in the existing theory (Science 287 (2000) 1049). Formation of structural pairs of Mn atoms (or few-atomic aggregates) in some close lattice sites is discussed to explain the observed results.

The effect of p-GaN : Mg layers on the turn-on voltage of p–n junction LED

We have studied the effect of p-GaN : Mg cap layers which were grown using MOCVD with various flow rates of Cp 2Mg on the turn-on voltage of p–n junction LED. Before fabricating the LEDs, the p-GaN : Mg/sapphire (0 0 0 1) epilayers grown with various flow rates of Cp 2Mg were evaluated in order to investigate the characteritics of each layer. We investigated the dependence of acceptor concentration on the flow rate of dopant source. The Van der Pauw technique, double crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN/sapphire (0 0 0 1) epitaxy increases, the resistivity of the epilayers increases abruptly without discontinuity due to the increase of the much uncracked Mg–H complex. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers increases at first and then decreases from a certain amount of Mg incorporation. As a result of the increase of the hole concentration, the emission intensity of the layers in the PL spectra at room temperature increases at first and then decreases from a certain flow rate of Cp 2Mg. After full evaluation of each p-GaN : Mg epitaxial layer, LED structures of the p–n junction type were grown by the growth of these cap layers and fabricated. Next, they were characterized by I– V measurement in order to investigate the effect of p-GaN : Mg cap layers on the turn-on voltage of the LED structures. The current of the LEDs at forward bias mode becomes low as the resistivity of p-GaN : Mg cap layer increases. However, the LED fabricated by the cap layer which has the highest hole concentration shows the greatest current value. While the turn-on voltage of p–n junction LED fabricated by p-GaN : Mg having high hole concentration above 1×10 17/cm 3 depends on the hole concentration of each p-GaN : Mg cap layer, the voltage of the devices fabricated by the cap layer having low hole concentration below 1×10 17/cm 3 is nearly dependent on the electrical resistivities of each p-GaN : Mg.

Orbital polarons in the metal-insulator transition of manganites

The metal-insulator transition in manganites is strongly influenced by the concentration of holes present in the system. Based upon an orbitally degenerate Mott-Hubbard model, we analyze two possible localization scenarios to account for this doping dependence: First, we rule out that the transition is initiated by a disorder-order crossover in the orbital sector, showing that its effect on charge itineracy is only small. Second, we introduce the idea of orbital polarons originating from a strong polarization of orbitals in the vicinity of holes. Considering this direct coupling between charge and orbital degrees of freedom in addition to lattice effects we are able to explain well the phase diagram of manganites for low and intermediate hole concentrations.

Atmospheric Pressure Chemical Vapor Deposition Growth Window for Undoped Gallium Antimonide

Metalorganic chemical vapor deposition (MOCVD) GaSb growth using trimethylgallium and trimethylantimony as a function of substrate temperature and V/III ratio was examined. These parameters were found to have a significant effect on the growth rate and surface morphology of the GaSb films. A phase diagram is used to interpret the effect of these growth parameters on the GaSb film growth. The region of single-phase growth was found to be narrow, falling between 540 and 560 °C. The optimum growth conditions for the MOCVD growth of GaSb have been determined for a TMGa flow rate of 20 sccm and a carrier gas flow of 8 l/min. The optimum substrate temperature and V/III ratio were found to be 540 °C and 0.72, respectively. In these conditions the lowest hole concentration of 5 × 1016 cm-3 and the highest room temperature mobility of 500 cm2 V-1 s-1 were achieved, accompanied by a steep, well-resolved band edge at 0.72 eV.

MOVPE of AlAsSb using tritertiarybutylaluminum

We investigated the growth of AlAs 0.16Sb 0.84 by using tritertiarybutylaluminum (TTBAl), tertiarybutylarsine (TBAs) and triethylantimony (TESb) as precursors. AlAs 0.16Sb 0.84 layers, which are lattice-matched to InAs substrates, are used as cladding layers for Sb-based IR-lasers. The use of a dilution line for TBAs reduces inhomogenities of the As content in the solid. X-ray diffraction results show that layers have good structural qualities. No gas phase prereactions are observed among the precursors. The precursor combination is suitable to reduce C and O incorporation in AlAsSb. We achieved undoped AlAs 0.16Sb 0.84 with low hole concentrations between 5×10 16 and 3×10 17 cm −3 and very high mobilities up to 500 cm 2/(V s) at 77 K and 300 cm 2/(V s) at room temperature. P- and n-type doping of 2×10 19 and 5×10 16 cm −3 are obtained by using diethylzinc (DEZn) and diethyltellurium (DETe), respectively.

Phase diagram and phase separation of cuprate oxides in decorated Ising model

In application to lanthanum cuprates the Ising model with random antiferromagnetic (AF) and ferromagnetic (FM) interactions between Cu atoms in CuO 2 layers is investigated by the mean-field approximation. The effective interaction between holes as well as their participation in a formation of the intermediate oxygen valency of CuO 2 layer are taken into account. In the phase diagram temperature–hole concentration which exhibits a range of phase separation and contains the critical point AF phase with low hole concentration, AF phase with rich hole concentration is found. Depending on doping level the suggested model describes both the absence of phase separation and the separation into paramagnetic (PM)–AF, PM–FM, and AF–FM phases.

Hole Distribution in Underdoped and Overdoped Y(Ba(2-y)Sr(y)())Cu(3)O(6+delta) Compounds Studied by X-ray Absorption Spectroscopy.

The hole distribution of underdoped and overdoped states in the Y(Ba(2)(-)(y)()Sr(y)())Cu(3)O(6+)(delta) (delta approximately 0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully oxygenated Y(Ba(2)(-)(y)()Sr(y)())Cu(3)O(6+)(delta) (delta approximately 0.9) compounds gives rise to high hole concentrations within both the CuO(2) planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y = 0 to 84 K for y = 0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba(2)(-)(y)()Sr(y)())Cu(3)O(6+)(delta) (delta approximately 0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit underdoped state due to the low hole concentration.

Superconductivity Within the Pair-Tunneling Model Close to the Metal-Insulator Transition

We have considered the problem of s-wave and d-wave superconductivity in the two-layer Hubbard model close to the metal-insulator transition. To mimic the formation of the insulating gap, the Coulomb correlations have been taken into account within the Hubbard I approximation. The interlayer momentum-conserving Josephson tunneling between the layers has been included on the mean-field level. We demonstrate that the interlayer tunneling may contribute to the occurrence of mixed d + s wave symmetry of the superconducting state with a dominating d-wave component at a low concentration of holes. The problem of the validity of the pair-tunneling model is also briefly discussed.

Chemical Control of Underdoped and Overdoped States in Y(Ba 2 − y Sr y )Cu 3 O 6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba2 − y Sr y )Cu3O6 + δ (δ∼0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba2−y Sr y )Cu3O6 + δ (δ∼0.9) compounds gives rise to high hole concentrations within both the CuO2 planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba2 − y Sr y )Cu3O6 + δ (δ∼0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.

Two-dimensional Hubbard model close to half-filling

Abstract The physical properties of two-dimensional interacting electron systems are of great current interest. An open question in the physics of low-dimensional fermionic models is whether their properties are described by standard Fermi-liquid theory or whether instead they show unconventional behaviour in strongly correlated electron systems. As the two-dimensional Hubbard model is the typical example of a strongly correlated electron system, we solve for this model the two-body problem in the vicinity of a half-filled Fermi sea. The results obtained show that for a low hole concentration the well known Fermi-liquid picture fails.

Dependence of electrical properties on the crystallographic orientation of CBr 4-doped GaAs epilayers grown on GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition

Carbon incorporation into GaAs epilayers has been performed by atmospheric pressure metalorganic chemical vapor deposition using CBr4. The electrical properties of CBr 4-doped GaAs epilayers grown on the GaAs substrates with various surface crystallographic orientations between (100) and (111)A were investigated. The electrical properties of the epilayers showed a strong crystallographic orientation dependence. On increasing the surface offset angle, the hole concentration of CBr4-doped GaAs epilayers rapidly decreased with a hump at (311)A. The lower hole concentration at the high offset angle can be explained by its higher desorption rate than that of the (100) surface. This hole concentration dependence on the offset angle was not changed in spite of the growth temperature and the V/III ratio variation given in this work. The above behaviors indicate that the surface kinetics plays an important role in the C incorporation into the non-planar GaAs epilayers.

A study of the upper limit of borate substitution in LnSr 2Cu 3− xB xO 7 materials

Compositions LnSr 2Cu 3− x B x O 7 (LnY, La, Nd, Gd, Yb) have been investigated at 1000°C. Almost single phase samples are obtained for LnGd and Y up to x ≈ 0.8. Powder X-ray structure refinement for a nominal YSr 2Cu 2.25B 0.75O 7 sample shows the basic cell to be orthorhombic Pmmm; a = 3.8176(2) A ̊ , b = 3.8345(2) A ̊ , c = 10.9708(5) A ̊ , but electron microscopy shows a well-defined 4Cu:1B ordering in the chain sites producing an a×5 b×2 c supercell. This confirms that the upper limit of borate substitution is x = 0.8. The oxygen contents of the x = 0.8 materials can be increased slightly by annealing, but they are not superconducting due to low hole concentrations within the copper oxide sheets.

X-ray Characterization of PbTe/SnTe Superlattices

AbstractPbTe/SnTe superlattices have been proposed many years ago for use as base material for infrared detectors. However, many difficulties have prevented its use, mainly the ones related to obtaining low concentration SnTe. Recently we have shown that SnTe layers with relatively low hole concentration can be grown by molecular beam epitaxy at low temperature using stoichiometric charges. In this work we investigate the structural properties of PbTe/SnTe superlattices grown by molecular beam epitaxy on (111) BaF2 substrates. Sample characterization has been done by high resolution x-ray diffraction. Information on strain was obtained from reciprocal space maps of asymmetric Bragg reflections and used as input parameters for dynamical simulation of the diffraction spectra.

NMR and transport studies of Y 2Ba 4Cu 7O 15−δ having Tc values from different synthesis conditions

Y 2Ba 4Cu 7O 15−δ superconductors with similar oxygen contents but different T c values have been produced by varying the reaction conditions. Resistivity, 89Y NMR and thermopower measurements were used to ascertain the origin of the different T c's. We find that the sample with the lowest T c has a lower hole concentration on the CuO 2 planes and the normal-state energy gaps are similar to those found in oxygen-depleted YBa 2Cu 3O 7−δ superconductors. Thus the electronic behaviour of Y 2Ba 4Cu 7O 15−δ is similar to that of YBa 2Cu 3O 7−δ and different T c values can be attributed to different hole concentrations on the CuO 2 planes and not to different pairing potentials or pair-breaking. The difference in the temperature dependence of the thermopower between these samples can be understood in terms of the different relative CuO chain and CuO 2 plane contributions.

ペロブスカイト型Ｍｎ酸化物薄膜の磁気抵抗

La1-xSrxMnO3 (x=0.3) thin films with thicknesses of 100-900Å were grown epitaxially on SrTiO3 (l00) and (110) substrates by magnetron sputtering. The ferromagnetic transition temperatures (Tc) of the films ranged from 140 to 330 K, depending on the growth conditions and film thickness.This variation in the Tc values of the films was possibly due to their oxygen deficiency. As a general trend,larger magnetoresistance (MR) ratios were obtained for films with lower Tc values (lower hole concentrations).Ultra-thin film of 100Å with Tc of 140 K exhibited relatively large magnetoresistance (MR) with an MR ratio (Δρ/ρ0) of 71% at H= 18 kOe.

FORMATION OF INTERFACES AND TEMPLATES IN THE Si(111)-Cr SYSTEM

The literature data and new data on investigation of the Si(111)-Cr system were systematized to the diagram of structural phase transitions. The ranges on this diagram give the formation conditions of various phases and reflect the mechanism of the Cr/Si (111) and CrSi 2/ Si (111) interface formation during room-temperature Cr deposition following annealing. The proofs of the multilayer surface phase formation and the data about transitions between the multilayer surface phases and bulk silicides during formation of the Cr/Si (111) and CrSi 2/ Si (111) interfaces were presented. The investigation of A- and B-type CrSi 2 templates formation was carried out. It was discovered that nucleation conditions and, in particular, the type of surface phases determine the azimuthal orientation of the epitaxial CrSi 2 islands relative to Si (111) under their nucleation. The A- and B-type CrSi 2 epitaxial films were grown by means of the template technique. The A-type CrSi 2 semiconductor film with low concentration and high mobility of holes was obtained.

Evidence for enhanced zinc interstitial concentration in strain-relaxed heteroepitaxial indium phosphide

Heteroepitaxial InP films grown on GaAs substrates by metalorganic chemical vapor deposition are demonstrated to have a higher concentration of zinc occupying interstitial sites than do equivalent homoepitaxial InP layers. A zinc interstitial-related donor-to-acceptor photoluminescence peak is observed in heteroepitaxial InP films which is absent in the homoepitaxial sample spectra. Capacitance–voltage measurements yield a lower hole concentration in the heteroepitaxial layers versus the homoepitaxial layers, which is attributable to a higher fraction of zinc in interstitial sites within the heteroepitaxial layers. Additionally, the hole concentration of the heteroepitaxial layers is found to be lower near the heterojunction as compared with the film surface region, correlating with a higher dislocation density near the heterojunction as seen by transmission electron microscopy. We conclude that the increased zinc interstitial concentration and the reduced hole concentration are due to dislocation-zinc solute interactions.

Annealing behavior of AlxGa1−xAs:C grown by metalorganic molecular beam epitaxy

The effects of rapid thermal annealing on the materials properties of the carbon-doped AlxGa1−xAs system have been systematically studied. The hole concentration in GaAs and AlGaAs layers doped up to 1021 cm−3 drop severely after annealing for 5 min at temperatures above 650 °C. The decrease in hole concentration produces a minimum of 8×1019–1020 cm−3 free carriers. The hole concentration for heavily doped AlAs is relatively stable up to 950 °C in material with p∼1020 cm−3. Unlike GaAs, AlAs with a low hole concentration but a substantially higher total carbon background, exhibits a substantial drop in activation for annealing temperatures above 650 °C. High resolution x-ray diffraction studies show that the lattice parameter of both the AlAs and GaAs becomes less strained relative to the GaAs substrate as the annealing temperature is increased. In both materials, the changes observed with annealing are believed to be due to pairing of the carbon atoms on the As sublattice, resulting in a decrease in the electrically active carbon and the strain in the layer.