Low-dimensional Effects Research Articles

Superconducting transitions from the pseudogap state:d-wave symmetry, lattice, and low-dimensional effects

We investigate the behavior of the superconducting transition temperature within a previously developed BCS-Bose Einstein crossover picture. This picture, based on a decoupling scheme of Kadanoff and Martin, further extended by Patton, can be used to derive a simple form for the superconducting transition temperature in the presence of a pseudogap. We extend previous work which addressed the case of s-wave pairing in jellium, to explore the solutions for T_c as a function of variable coupling in more physically relevant situations. We thereby ascertain the effects of reduced dimensionality, periodic lattices and a d-wave pairing interaction. Implications for the cuprate superconductors are discussed.

Magnetic properties of SrRuO 3/SrTiO 3 superlattices

We have fabricated n-UCT SrRuO 3/SrTiO 3 superlattices, [SrTiO 3 5-UCT/SrRuO 3 n-UCT] m ( n=1∼20), where UCT is a unit-cell thickness, on SrTiO 3(001) substrates to study a low-dimensional structural effect on the magnetic properties of SrRuO 3. We measured temperature-dependent and field-dependent magnetization. The ferromagnetic transition temperature decreased with decreasing n, and the transition vanished for n=1. A strong anisotropy of magnetization with an easy axis perpendicular to the film plane was observed for all the samples, as seen in SrRuO 3 thin films on SrTiO 3(001) substrate. The in-plane magnetization was extremely suppressed as n decreased. For 3-UCT superlattice, the in-plane magnetization became 1/10 of the out of plane magnetization at T=5 K and H=1 T. The role of low-dimensionality between SrRuO 3 and SrTiO 3 on magnetism of SrRuO 3 is discussed.

TCSuppression of SrRuO3/SrTiO3Superlattices

The transverse and longitudinal magnetoresistivities of SrRuO 3 /SrTiO 3 superlattices on SrTiO 3 (001) substrates were measured to study low-dimensional effects on the magnetic properties of SrRuO 3 . Both the differential resistivity, dρ( T )/ d T , and magnetoresistivity, Δρ( H )=ρ( H )-ρ(0), become maximum at the ferromagnetic transition temperature, T C . The transition appears when the SrRuO 3 layer is as thick as 2 unit-cells or more. The T C decreases quickly with decreasing thickness, especially for less than 4 unit-cell thickness.

Multiwafer MOVPE technology for low dimensional Ga-Al-In-N structures

State-of-the-art GaN devices more and more make use of low-dimensional effects. Both light emitting diodes (LEDs) and laser diodes consist of quantum wells in their active regions. This requires an extremely precise control of layer thickness and uniformity of alloy composition during MOVPE growth. For mass production of these structures the requirements are even tougher. Wafer uniformity within one run and reproducibility from run to run are most critical issues here. In this paper we present a class of multiwafer MOVPE-reactors which have proven to produce high quality GaN based material. We will also present data on bulk layer uniformity as well as the excellent interface sharpness of quantum well structures.

Temperature evolution of the magnetic structures in R 2BaNiO 5 oxides

The magnetic structures of R 2BaNiO 5 (R  Nd, Tb, Dy, Ho, Er, Tm, Yb and Y) have been characterized by means of neutron powder diffraction from 1.5 K to room temperature. Three dimensional antiferromagnetic ordering is established for both R and Ni sublattices at relatively high temperatures. The propagation vector of the magnetic structures of the whole family is k = [ 1 2 0 1 2 ]. At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. The strong magnetic moment reduction of Ni 2+ cations is probably due to the combination of low-dimensional quantum effects and covalency. For the compounds with the biggest rare earth ions, the Ni moments rotate from nearly parallel to the a-axis toward the c-axis with decreasing temperature. A smooth re-orientation, governed by the magnetic anisotropy of R 3+, seems to occur below and close to T N.

1H-NMR studies of (DMe-DCNQI-d 7) 2Cu 1 − xLi x

Systematic measurements of 1H-NMR relaxation rate, T 1 −1, in the organic conductor, (DMe-DCNQI-d 7) 2Cu 1 − x Li x ( x = 0.04, 0.5 ∼ 0.6, 1), have been carried out, to clarify the effect of Li doping on the electronic properties of pπ-band. Within the stable metallic region (up to about x = 0.5), T 1 −1 exhibits a Korringa-like behavior, ( T 1 T) −1 = const. In a highly doped sample, a nonmetallic phase appears at low temperatures. The absolute value of T 1 −1 in the metallic region increases remarkably with the Li concentration and becomes the largest in the pure Li-salt, (DMe-DCNQI-d 7) 2Li. This behavior is analyzed as a low-dimensional spin-diffusion effect.

Optical studies on epitaxy films and superlattices of diluted magnetic semiconductor Zn 1− xMn xSe

Zn 1− x Mn x Se films and Zn 1−x Mn x Se ZnSe superlattices with different composition x have been grown by molecular beam epitaxy (MBE). Superlattice structure transference from ZnSe as a well to Zn 1− x Mn x Se as a well has been observed for the first time by photoluminescence measurements at small values of x when the temperature increases. It is due to the anomalous behavior of the energy gap at low x in Zn 1− x Mn x Se, which forms a bow region and the shape of this bow and the dip in energy gap are dependent strongly on temperature. It has been found by time-resolved photoluminescence spectra that the relaxation time of Mn ++ ions in excited states in superlattices is much longer than that in thin films. This may be attributed to a low-dimensional scaling effect on the exchange interaction between Mn ++ ions.

Effects of low-dimensionality in solid-state protonic conductors

An analytic expression for the resolution-broadened neutron scattering function of two-dimensional translational diffusion for polycrystalline samples is derived. We show that this expression exhibits a typical behaviour as a function of the energy resolution width, which can be used in order to distinguish between two- and three-dimensional diffusion. Using experimental data of the protonic conductor, CsOH · H 2O, and numerical calculations, it is demonstrated to what extent such an analysis can yield information on the question of dimensionality, which otherwise would only be obtainable from the study of single-crystalline specimens.

Dielectric function and collective modes of two-dimensional interacting bosons.

We present a theoretical study of the dielectric response and collective excitations of a two-dimensional system of bosons interacting via a dipolar interaction. The model is designed to simulate the situation occurring in semiconductor double quantum well systems under strong electric fields perpendicular to the layers. The field produces a net polarization of photoexcited electron and hole carriers and favors the appearance of long-lived polarized excitons. This gas of interacting point dipoles shows interesting features and we study the effects of low dimensionality and statistics. Well-defined density-fluctuation excitations appear at low temperatures with a linear dispersion relation at long wavelengths. The low dimensionality of the system gives rise to an effective long-ranged potential with only a faster power-law decay.

Ferromagnetic resonance study of Co/Pt multilayers

In this paper the results of ferromagnetic resonance study of sputtered Co/Pt multilayers, with a fixed Co layer thickness of 15 Å, are presented. For these multilayers with small Pt layer thickness, in addition to a uniform resonance mode, a spin wave resonance mode is found to exist, which confirms the existence of an interlayer coupling between the neighboring Co layers. Simultaneously, the effective magnetization 4π M eff and the resonance linewidth Δ H are found to change correlatively with varying Pt layer thickness. This phenomenon can be explained as a result of an interplay between the interlayer coupling and the low-dimensional effect.

High-resolution tests of low-dimensionality effects in photoemission.

High-energy-resolution photoemission spectra, taken on three quasi-one-dimensional crystals, ${\mathrm{TaTe}}_{4}$, ${\mathrm{NbTe}}_{4}$, and (${\mathrm{NbSe}}_{4}$${)}_{3}$I confirm the general phenomenon of the suppression of the photoemission intensity near the Fermi level in one dimension. We analyze the possible causes of this phenomenon in light of previous results on (${\mathrm{TaSe}}_{4}$${)}_{2}$I.

Magnetic structure of Ho 2BaNiO 5

The magnetic behaviour of Ho 2BaNiO 5 has been characterized by means of neutron powder diffraction from 1.5 K to Room Temperature (RT). This orthorhombic compound (S.G. Immm, a≈3.764 Å, b≈5.761 Å, c≈11.336 Å) is structurally characterized by the presence of NiO 6 octahedra forming chains along the a-axis. Three Dimensional Antiferromagnetic ordering (3D-AF) is established for both Ho and Ni sublattices at T N≈53 K. The propagation vector of the magnetic structure is k=[ 1 2 0 1 2 ] . At 1.5 K the magnetic moments lie in the ac-plane and have values of 9.0 μ B (Ho) and 1.4 μ B (Ni), the moment of Ho 3+ is nearly parallel to the c-axis and the smallest angle of the Ni-moment direction with the c-axis is 26°. The strong magnetic moment reduction of Ni 2+ cations is probably due to a combination of low-dimensional quantum effects and covalency. A smooth re-orientation, governed by the magnetic anisotropy of Ho 3+, seems to occurs below and very close to T N; i.e. the Ni moments rotate from nearly parallel to the a-axis toward the c-axis following the Ho moments. The thermal evolution of the magnetic structure from T N until 1.5 K is studied in detail.

Low-dimensional effects on frequency dependence of proton NMR relaxation in liquid cyclohexane confined to porous glasses

1H NMR spin—lattice relaxation times were measured for cyclochexane confined to sol—gel porous silica glasses with 17.2 and 20.4 Å pore radii. The relaxation times were measured at five different frequencies: 9.2, 60, 180, 300 and 500 MHz. Additionally the measurements of the spin—lattice relaxation times in the rotating frame, T 1ϱ, were performed with the frequency of the spin-locking pulse 4 X 10 3-8.5 X 10 4 rad/s. At low frequencies a linear dependence of 1/ T 1ϱ versus log ω was found. In the studied frequency range the relaxation rates change with frequency and pore size as predicted by the theory of restricted translational diffusion in two-dimensional systems.

The vibrational mode softening in thin film cds

Lattice softening of the CdS LO phonon mode was observed by resonant Raman spectroscopy (RRS). This phenomenon was interpreted in terms of the combined effects of grain size and low-dimensionality of the film. As the thickness of the film decreases below 800 Å, the influence of the free-surface on the vibrational spectrum becomes more prominent. Higher order harmonics of the LO-mode also are observed. The intensity of the Raman line is enhanced as it approaches the maximum of the fluorescence emission peak.

Systematic studies of low dimensional effects depending on crystallographic overgrowth

The influence of epitaxial overgrowth on the optical properties of dry- etched InGaAs/InP wires is studied with respect to crystallographic orientation of the wires. Wires oriented in [011]- direction display good surface morphology and exhibit a drastic improvement of the quantum efficiency compared to open wires. The photoluminescence of the wires is strongly polarized parallel to the wire. Photoexcitation spectroscopy in the region of the barrier absorption reveals the strong influence of the carrier capture from the barrier on the quantum efficiency of the wires.

NMR deuteron relaxation study of low-dimensional effects in molecular liquids in restricted geometries

The NMR study of the deuteron relaxation rates T 1 −1 and T 1ϱ −1 of liquid nitrobenzene- d 5 confined to porous silica glasses with small pore sizes reveals a logarithmic frequency dependence of the spectral densities for several decades of frequency, thus reflecting the two-dimensional character of molecular motions. In fact, the concept of reduced dimensionality allows one to explain the unique properties of the relaxation processes as well as their dependence on the pore size. The gradual transition from the predominantly two-dimensional logarithmic dependence of the relaxation rates on frequency to the three-dimensional bulk-like behavior with the increasing pore radius is observed. In addition, the experimental data both for nitrobenzene- d 5 and cyclohexane- d 12 confined to porous silica glasses and surface modified porous silica glasses allow us to discuss the relative role of geometrical confinement and the pore surface effects and, in particular, the applicability and limitations of the two-state fast-exchange model generally used to describe the dependence of the relaxation rate upon the pore radius.

Assessing thermal Cl2 etching and regrowth as methods for surface passivation

The damage layer that results from ion beam processing ultimately limits the smallest size structure which will emit light and hence prevents the realization of low-dimensional quantum size effects. Effective surface passivation of etched structures requires both layer removal and compensation of surface states. Ultimately these two processes must be made compatible. To address this problem, we have coupled an ultrahigh-vacuum (UHV) dry-etching system to a molecular beam epitaxy (MBE) growth chamber. Material is patterned and masked outside of the combined etching/regrowth system using electron beam lithography. Samples are then loaded into the combined UHV system and vertical structures are produced by chemically assisted ion beam etching (CAIBE). These structures are then subjected to a thermal Cl2 gas etch to remove the damage layer created by the ion beam etching. Samples are subsequently transferred directly into the MBE growth chamber, where a cladding layer of Al0.3Ga0.7As is grown over the etched structure to provide passivation. In this way both the damage removal and surface passivation are achieved in-situ without the deleterious effects caused by exposure to air. We find, by low temperature cathodoluminescence, that submicrometer structures subjected to this in-situ processing display fivefold improved luminescence efficiencies. Structures etched and regrown without the thermal Cl2 etching step show no comparable improvement, indicating the damage removal is the key to improved efficiencies.

Optical Properties of CdS Microcrystallite-Doped SiO2 Glass Thin Films

The optical properties of SiO2 glass thin films containing CdS microcrystallites have been investigated. The thin films were deposited onto SiO2 and GaAs substrates using an rf-sputtering technique. X-ray photoelectron spectroscopy and X-ray diffraction measurement indicated that the Cd and S atoms were doped as CdS with a hexagonal system in SiO2 glass matrixes. In the optical absorption spectra, the absorption edge of the films with 18.5 at% of CdS clearly exhibited a blue shift by about 0.13 eV in comparison with that of the bulk CdS. In accordance with this result, the photoluminescence spectra of the films exhibited a blue band-edge emission at a higher energy than that of the bulk CdS. The shift to higher energy seems to be a low-dimensional quantum size effect due to confinement of electrons and holes in CdS microcrystallites.

Resonant tunneling of double-barrier quantum wells affected by interface roughness.

Resonant tunneling of double-barrier quantum wells (DBQW's) affected by interface roughness has been investigated. Our results show that interface roughness induces oscillation resonant structure around the principal resonant peak. EA'ects of interface roughness on the resonant bias voltage, peak-to-valley current ratio, and the width of the principal resonant peak are also investigated. Temperature eAect is discussed. The results obtained here may be used to explain the oscillation or intrinsic instability observed in DBQW resonant-tunneling structures. With recent developments in semiconductor artificial structures, such as quantum wells (QW's) and superlattices (SL's), many important physical properties have been explored. In fundamental physics, quantum-well and superlattice structures have been used to explore the physical properties of low-dimensional systems and quantum efFects. Many novel phenomena in the quantum regime have been discovered, such as resonant tunneling of double-barrier quantum wells with negative differential resistance. ' Applications of these quantum wells and superlattices include high-speed electronics, photoelectronics, and photonic devices such as quantum-well lasers, modulation-doped field-effect transistors (MODFET's), photodetectors, etc. Double-barrier quantum-well (DBQW) tunneling structures have recently drawn a great deal of attention because tunneling is the fastest charge-transport phenomenon in semiconductors. Many important properties of resonant tunneling of electrons in DBQW's have been observed, such as negative differential resistance. Although many important properties of DBQW tunneling structures have been discovered, there are still many important questions to be answered, such as those concerning oscillation or instability in DBQW resonanttunneling structures. Although molecular-beam-epitaxy (MBE) can be used to fabricate the quantum-well or superlattice structures on an atomic scale, interface roughness cannot be cornpletely eliminated. It is important to understand how this interface roughness afFects the transport properties of DBQW structures. In some cases the presence of interface roughness simply acts as a small perturbation on effects that already exist in the absence of interface roughness. In other cases it can lead to quantitative changes in phenomena that exist in the absence of interface roughness. An example of this would be the roughness-induced splitting of the surface-plasmon dispersion curve observed experimentally, and discussed theoretically. Overall, roughness can have a significant influence on effects occurring at interfaces. The effects of interface roughness on the optical properties of quantum wells have been studied previously. The dominant effects observed due to interface roughness were the exciton linewidth broadening and the Stokes shift of the emission: the emission of the lowest heavyhole exciton generally is slightly shifted to lower energy (typically a few millielectron-volts) with respect to the absorption- or excitation-spectrum maximum. Recently, dynamic processes of excitonic transitions affected by interface roughness have also been studied. The excitontransition peak shifting towards lower energy with increasing delay time caused by interface roughness has been observed. In this paper, effects of interface roughness on the resonant tunneling in DBQW structures have been investigated. We show that interface roughness can cause multiple resonance peaks in DBQW's, and thus strongly modify the I- V characteristic. The I- V characteristic for different interface-roughness parameters as well as for different DBQW structures are studied. The effects on multiple resonance peaks are discussed. From our results, it is possible that the previously observed oscillations between the low- and high-current states in the DBQW s (Ref. 5) or intrinsic instability is induced by interface roughness.

Horizons for new semiconducting materials*

The properties of existing semiconductors limit performance in a whole range of devices and leave many interesting potential applications (e.g. for lighting, power generation or air‐conditioning) possible in principle but technologically impracticable. Better properties, then, could be valuable. They could be obtained in two ways, by discovering new materials (a long‐range option apt for academic investigation) or by modifying existing materials either chemically (e.g. by forming solid solutions, perhaps of highly unconventional type) and/or physically (e.g. by low dimensional confinement effects). These options are reviewed and some of their implications explored.