Theoretical study of nitride short period superlattices.

Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

Band gap engineering of In(Ga)N/GaN short period superlattices.

Discussion of band gap behavior based on first principles calculations of the electronic band structures for several InN/GaN superlattices (SLs) (free-standing and pseudomorphic) grown along different directions (polar and nonpolar) is presented. Taking into account the dependence on internal strain and lattice geometry mainly two factors influence the dependence of the band gap, E g on the layer thickness: the internal electric field and the hyb wells) is more important. We also consider mIn ridization of well and barrier wave functions. We illustrate their influence on the band gap engineering by calculating the strength of built-in electric field and the oscillator strength. It appears that there are two interesting ranges of layer thicknesses. In one the influence of the electric field on the gaps is dominant (wider wells), whereas in the other the wave function hybridization (narrow wells) is more important. We also consider mIn 0.33 Ga 0.67 N/nGaN SLs, which seem to be easier to fabricate than high In content quantum wells. The calculated band gaps are compared with recent experimental data. It is shown that for In(Ga)N/GaN superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary InGaN alloys.

Evidence for the antiferromagnetic ground state of Zr2TiAl: a first-principles study.

A detailed study on the ternary Zr-based intermetallic compound Zr2TiAl has been carried out using first-principles electronic structure calculations. From the total energy calculations, we find an antiferromagnetic L11-like (AFM) phase with alternating (1 1 1) spin-up and spin-down layers to be a stable phase among some others with magnetic moment on Ti being 1.22 [Formula: see text]. The calculated magnetic exchange interaction parameters of the Heisenberg Hamiltonian and subsequent Heisenberg Monte Carlo simulations confirm that this phase is the magnetic ground structure with Néel temperature between 30 and 100 K. The phonon dispersion relations further confirm the stability of the magnetic phase while the non-magnetic phase is found to have imaginary phonon modes and the same is also found from the calculated elastic constants. The magnetic moment of Ti is found to decrease under pressure eventually driving the system to the non-magnetic phase at around 46 GPa, where the phonon modes are found to be positive indicating stability of the non-magnetic phase. A continuous change in the band structure under compression leads to the corresponding change of the Fermi surface topology and electronic topological transitions (ETT) in both majority and minority spin cases, which are also evident from the calculated elastic constants and density of state calculations for the material under compression.

ZnGeSb2: a promising thermoelectric material with tunable ultra-high conductivity.

First principles calculations predict the promising thermoelectric material ZnGeSb2 with a huge power factor (S2σ/τ) on the order of 3 × 1017 W m-1 K-2 s-1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 1025 Ω-1 m-1 s-1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states.

Calculated high-pressure structural properties, lattice dynamics and quasi particle band structures of perovskite fluorides KZnF3, CsCaF3 and BaLiF3.

A detailed study of the high-pressure structural properties, lattice dynamics and band structures of perovskite structured fluorides KZnF3, CsCaF3 and BaLiF3 has been carried out by means of density functional theory. The calculated structural properties including elastic constants and equation of state agree well with available experimental information. The phonon dispersion curves are in good agreement with available experimental inelastic neutron scattering data. The electronic structures of these fluorides have been calculated using the quasi particle self-consistent [Formula: see text] approximation. The [Formula: see text] calculations reveal that all the fluorides studied are wide band gap insulators, and the band gaps are significantly larger than those obtained by the standard local density approximation, thus emphasizing the importance of quasi particle corrections in perovskite fluorides.

Structural, vibrational, elastic and topological properties of PaN under pressure.

The electronic, structural, vibrational and elastic properties of PaN have been studied at both ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. The high pressure behavior of the acoustic phonon branch along the [1, 0, 0] and [1, 1, 0] directions and the C44 elastic constant are anomalous, which signals a structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition, which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, here it is found that the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). This constitutes an example of GGA and LDA leading to qualitatively different predictions, and therefore it is of great interest to examine experimentally whether this topological transition occurs.

Fermi surface properties of AB3 (A = Y, La; B = Pb, In, Tl) intermetallic compounds under pressure.

The electronic structures, densities of states, Fermi surfaces and elastic properties of AB3 (A =La, Y; B =Pb, In, Tl) compounds are studied under pressure using the full-potential linear augmented plane wave (FP-LAPW) method within the local density approximation for the exchange-correlation functional and including spin-orbit coupling. Fermi surface topology changes are found for all the isostructural AB3 compounds under compression (at V/V0 = 0.90 for LaPb3 (pressure = 8 GPa), at V/V0 = 0.98 for AIn3 (pressure = 1.5 GPa), at V/V0 = 0.80 for ATl3 (pressure in excess of 18 GPa)) apart from YPb3, although its electronic structure at zero pressure is very similar to that of LaPb3. For LaPb3 a softening of the C44 elastic constant under pressure (equivalent to 8 GPa) may be related to the appearance of a new hole pocket around the X point. From the calculated elastic properties and other mechanical properties, all the compounds investigated are found to be ductile in nature with elastic anisotropy. The states at the Fermi level (EF) are dominated by B p states with significant contributions from the A d states. For the La compounds, small hybridizations of the La f states also occur around EF.

Superconductivity in SnO: a nonmagnetic analog to Fe-based superconductors?

We discovered that under pressure SnO with α-PbO structure, the same structure as in many Fe-based superconductors, e.g., ß-FeSe, undergoes a transition to a superconducting state for p≳6 GPa with a maximum Tc of 1.4 K at p=9.3 GPa. The pressure dependence of Tc reveals a domelike shape and superconductivity disappears for p≳16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.

Hubbard-U band-structure methods.

The last decade has seen a large increase in the number of electronic-structure calculations that involve adding a Hubbard term to the local-density approximation band-structure Hamiltonian. The Hubbard term is then determined either at the mean-field level or with sophisticated many-body techniques such as using dynamical mean-field theory. We review the physics underlying these approaches and discuss their strengths and weaknesses in terms of the larger issues of electronic structure that they involve. In particular, we argue that the common assumptions made to justify such calculations are inconsistent with what the calculations actually do. Although many of these calculations are often treated as essentially first-principles calculations, in fact, we argue that they should be viewed from an entirely different point of view, namely, as based on phenomenological many-body corrections to band-structure theory. Alternatively, it may also be considered that they are just based on a Hubbard model that is more complex than the simple one- or few-band models traditionally used in many-body theories of solids.

Predicted superconductive properties of lithium under pressure.

A superconducting state of lithium has not been found at ambient pressure, but the present theoretical work shows that high values of the critical temperature, T(c), may be expected for some high-pressure phases. Ab initio electronic structure calculations are used to calculate the electron-phonon coupling in a "rigid-muffin-tin approximation," and estimates using McMillan's formula suggest that under increasing pressure T(c) in fcc-Li may reach 50--70 K before transitions occur to the rhombohedral (hR1-Li) and subsequently to the cI16-Li phase near 40 GPa. In cI16-Li T(c) may reach a maximum in the range 60--80 K.

New high-pressure phases of lithium.

Lithium is considered a 'simple' metal because, under ordinary conditions of pressure and temperature, the motion of conduction electrons is only weakly perturbed by interactions with the cubic lattice of atomic cores. It was recently predicted that at pressures below 100 GPa, dense Li may undergo several structural transitions, possibly leading to a 'paired-atom' phase with low symmetry and near-insulating properties. Here we report synchrotron X-ray diffraction measurements that confirm that Li undergoes pronounced structural changes under pressure. Near 39 GPa, the element transforms from a high-pressure face-centred-cubic phase, through an intermediate rhombohedral modification, to a cubic polymorph with 16 atoms per unit cell. This cubic phase has not been observed previously in any element; unusually, its calculated electronic density of states exhibits a pronounced semimetal-like minimum near the Fermi energy. We present total-energy calculations that provide theoretical support for the observed phase transition sequence. Our calculations indicate a large stability range of the 16-atom cubic phase relative to various other crystal structures tested here.

Radiofrequency tissue ablation with a cooled needle in vitro: ultrasonography, dose response, and lesion temperature.

RATIONALE AND OBJECTIVES: Radiofrequency (RF) tissue ablation with a cooled needle electrode (probe) can produce large lesions. By using this technique on ex vivo calf livers, the authors evaluated the role of ultrasound (US), dose response, and temperature course with time. METHODS: RF ablation was produced with a 14-gauge probe with a 2-cm exposed tip. The lesions were examined with US and macroscopically after various treatment durations. Tissue temperature was measured with thermosensors inserted 1, 2, and 3 cm from the probe. RESULTS: Before treatment the tip of the probe was easy to visualize with US, but treatment microbubbles obscured the lesion and probe. After treatment, the lesions appeared hypoechoic. Lesion size was underestimated based on US findings. Lesion size was logarithmically correlated to treatment duration. Lesion temperature increased at an increased rate with higher wattage applied and with decreased distance from the probe. CONCLUSION: US is useful for probe placement before treatment and might be of value after treatment. Lesion size increases reproducibility with treatment duration.

Prostatectomy and impotence: can temperature variations around the prostate during TURP explain postprostatectomy impotence?

A considerable number of patients (4-35%) experience sexual dysfunction after treatment for BPH with a transurethral resection of the prostate (TURP). The aim of this study was to elucidate whether heat, generated by the resectoscope during TUR-P, can be transmitted to the surrounding tissue to an extent that may damage the penile innervation.

Rectal compliance determined by rectal endosonography. A new application of endosonography.

PURPOSE: The aim of this study was to develop a method for determination of rectal compliance that allows direct measurement of corresponding changes in the rectal cross-sectional area or perimeter and rectal pressure. METHODS: We developed an anal probe for transrectal endosonography. The probe was tested in vitro, and rectal compliance of six healthy patients was determined. RESULTS: In vitro measurements proved the method to be well reproducible. The method allowed calculation of an endosonographic rectal compliance, which correlated well with rectal compliance measured by the standard method. CONCLUSION: Endosonographic determination of rectal compliance is possible, and the endosonographic method may give a more precise and reproducible estimation of rectal compliance.

[Use of bladder thermistor catheters in an intensive care unit. Comparative study of core temperature measurements with bladder thermometers and rectal thermometers in an intensive care unit]. / Brug af blaeretermistorkateter på intensiv afdeling. Sammenlignende undersøgelse af kernetemperaturmåling med bloere- og rektaltermometer på en intensiv afdeling.

Bladder temperature measured by a thermistor tipped urinary catheter was compared to rectal temperature in 14 ICU patients. During a period of five days 2 x 232 measurements were recorded, and good correlation was found between rectal and bladder temperatures (R = 0.97). There did not seem to be any drift in the bladder thermistor. Measurement was accurate during periods of oliguria. We find that bladder temperature measurement is reliable, safe, convenient and accurate for routine use in the ICU.

Interstitial hyperthermia of colorectal liver metastases with a US-guided Nd-YAG laser with a diffuser tip: a pilot clinical study.

Ultrasound (US)-guided interstitial hyperthermia performed with the neodymium yttrium aluminum garnet laser with a diffuser tip involves placement of a laser fiber in solid tissue followed by irradiation from within the center of the tumor to be treated. The authors previously described the development of a US-guided technique that permits simultaneous interstitial laser irradiation and temperature measurements, with real-time US monitoring. With the use of a dedicated diffuser tip modification of the bare laser fiber, it has proved possible to produce spherical coagulations with a diameter comparable to that of liver metastases of clinical relevance. This technique was used in 11 patients with 16 colorectal liver metastases, 12 of which were radically ablated. Real-time US and interstitial temperature monitoring during treatment had a positive predictive value of 86% and negative predictive value of 100% in judging the final result. The technique is feasible, effective, and safe, but the effect on patient survival must be tested in future randomized clinical studies.

Laryngeal mask and anaesthetic waste gas exposure.

In recent years there has been a growing awareness of the possible hazards caused by anaesthetic gases in operating theatres. The laryngeal mask airway provides an alternative both to tracheal intubation and the face mask although the implications for operating theatre contamination have not been quantified. This paper describes the incidence and magnitude of exposure of theatre personnel to waste anaesthetic gases during laryngeal mask airway anaesthesia. The leakage of anaesthetic gases to the anaesthetist's breathing zone was monitored using a Bruel & Kjaer Multi Gas Monitor, Type 1302 during 50 general anaesthetics employing either spontaneous (n = 24) or controlled (n = 26) ventilation. All patients were anaesthetised with propofol, alfentanil and nitrous oxide. There was no statistically significant association between the amount of anaesthetic gas leakage and ventilation method. The laryngeal mask airway meets occupational safety requirements on nitrous oxide concentrations in the operating theatre environment.

3-dimensional ultrasound based demonstration of the posterior urethra during voiding combined with urodynamics.

A new method for continuous 3-dimensional demonstration of the posterior urethra during micturition is described. The method is based on a series of transverse ultrasonic scans through the urethra. It seems to provide on optimal model of the urethra and used in combination with urodynamics this technique also has a potential in diagnostics and urodynamic research.

Technique, errors and safeguards in modern Küntscher nailing.

Intramedullary nailing after reaming of the medullary cavity according to Küntscher, provides a rigid fixation and makes early mobilization possible, and is even applicable to infected non-unions. Experience with 150 cases of fresh femoral fractures and more than 80 cases of non-union of the femur, the tibia, the humerus and the forearm, demonstrates that thorough familiarity with the instrumentation and the pitfalls of the technique, as well as the correct clinical indications of the method are critical to the achievement of good results.