Cu NMR study of the superconducting thiospinel Cu1.5Rh1.5S4

The ⁶³Cu NMR Knight shift K and spin-lattice relaxation rate 1/T₁ have been measured to study the thiospinel superconductor Cu_1.5Rh_1.5S₄ from a microscopic viewpoint. K is negative and has a weak dependence on temperature, and the hyperfine coupling constant H_hf^d is estimated to be −52.4 kOe/μ_B. 1/T₁ is proportional to the temperature in the normal state. In the superconducting state, 1/T₁ takes a coherence peak just below T_c, and decreases exponentially well below T_c, from whose temperature dependence the superconducting energy gap has been proved to be close to 2Δ = 3.52k_BT_c given by the BCS theory.     

Modulation-doping in 3–5 μm GaAs/AlAs/AlGaAs double barrier quantum well infrared photodetectors: an alternative to achieve high photovoltaic performance and high temperature detection

In this work we propose new detector designs, which allow achieving mid-infrared photovoltaic (PV) detection at temperatures as high as 180 K. The devices, which are grown by molecular beam epitaxy, are modulation-doped (MD) double barrier quantum well infrared photodetectors (QWIPs) based on AlGaAs/AlAs/GaAs. As the photocurrent spectra and I–V characteristics (in the dark and under infrared illumination) show that the dopant location is a relevant design parameter regarding the performance of PV QWIPs, we begin our work with a comparison of the performance of a set of MD samples (where we have varied the dopant location in the AlGaAs barriers) with respect to a well-doped sample of nominally the same structure. We find that the responsivity and detectivity of the MD devices seem to be higher than those of the well-doped detector, specially when the dopant is located in the substrate-sided barrier. Then, in order to improve the dark current-limited performance, we designed a new set of substrated-sided MD detectors that exhibit an extremely low dark current, even at high temperatures, otherwise no drop in the zero bias peak responsivity. Therefore, the association of the notable PV signal detection in the 3–5 μm range of these MD detectors together with the dark current reduction of the new structures has allowed us to achieve a 140 K zero bias peak responsivity of 0.015 A/W and a 180 K zero bias peak responsivity of 0.01 A/W at 4.4 μm.     

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Highly dense sintered YBa₂Cu₄O₈ has been produced by hot isostatic pressing (HIP). The electrical resistivity of this material has been measured as a function of temperature T and pressure in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of are found to be well described by a model based on the standard Bloch-Grüneisen theory. It is pointed out that is liner in T only under isobaric conditions, while is strongly nonlinear in all high-T_c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm² at 4 K, while the resistivity is 630 μΩ cm at 294 K.     

Magnetic moments and Fe hyperfine field interactions in Y(Fe1−xRux)2 ternaries

Measurements of magnetization and ⁵⁷Fe Mössbauer spectra have been made for Y(Fe_1−xRu_x)₂. The C15 type cubic structure is stabilized for xx 0.7. The C15 compounds is ferromagnetic with T_c200 K and its saturation moment decreases monotonically with increasing x, while the ⁵⁷Fe hyperfine field decreases only slightly with x. From these results, it is deduced that the Ru atoms have an induced moment of ≈1μ_B in the range x 0.2. In the C14 type phase, no magnetic ordering develops even at 4.2 K.     

Stress dependence of the magnetic permeability of MnZn ferrous ferrites

In monocrystalline MnZn ferrous ferrites the magnetic permeability μ as a function of temperature T has been observed to show a striking dependence on applied stress. Stresses cause the μ(T) curve to cant around a point near its maximum arising from compensation of the magnetocrystalline anisotropy due to the ferrous ions. By grinding the surface of a monocrystalline toroid, a tensile stress was exerted on the interior of the specimen. The canting of the μ(T) curve observed when the damaged surface layer was removed step by step was employed to study the underlying physical mechanism. In the case of polycrystalline materials the maximum of μ(T) is usually strongly suppressed. Nevertheless, it proved possible to investigate the dependence of μ(T) on applied stress in a polycrystalline MnZn ferrous ferrite pot core.     

Pressure–temperature studies on conductivity of TlX(X=Cl,Br,I) compounds:: I: phase diagram. II: ionic mobility

We have measured the conductivity σ of TlX(X=Cl, Br, I) compounds up to 5.3 GPa and between 300–823 K. The σ–T dependence for all compounds can be divided into three distinct regions: (i) low temperature (LT), <400 K, with unusual negative σ–T dependence, (ii) intermediate temperature (IT), 400<650 K, with positive σ–T dependence and (iii) high temperature (HT), T>650 K, with positive σ–T dependence. The σ–T isobars were used to construct the T–P solid phase diagram for each compound. The LT region data indicate a new meta-stable phase in the 1.0–3.5 GPa range. The LT→IT transition is characterized by an inverse σ–T dependence followed by normal Arrhenius behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and HT regions above 3 GPa for each compound in the P–T plot yields a value close to its respective normal (1 atm) T_melt suggesting a solid order–disorder transition type paralleling -AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5–4.1 GPa of all compounds is at variance with the Arrhenius behavior observed for unperturbed ion migration implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius σ–T dependence prevails in both IT and HT regions with Q_c values of 85–100 kJ mol⁻¹ and 50–75 kJ mol⁻¹, respectively. The higher conductivities at 0.4 GPa for TlBr and TlI relative to their 1 atm data and the increasing σ with P are in strong contrast to the normal σ-P behavior of TlCl. The dependence of activation volume ΔV^‡ on T for TlCl, i.e. ΔV^‡>0, shows abnormally high values with a maximum at 500 K for P<3.0 GPa but reasonable ΔV^‡ values appear above 3.0 GPa. The ΔV^‡–T dependence for both TlBr and TlI with ΔV^‡<0 is incompatible with an ion transport mechanism suggesting an electronic conduction process and implying an ionic–metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in terms of electronegativity differences and bonding character rather than structure.     

Magnetic properties of the Bi1.95Sr2.05−xLaxCuOy (Bi-2201) superconducting phase

We have investigated the reversible mixed-state magnetization M of three lanthanum substituted Bi_1.95Sr_2.05−xLa_xCuO_y (Bi-2201) ceramic samples having different critical temperatures T_c ranging from 20.0 to 35.5 K. As for the Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) phase, we found that anisotropy of Bi-2201 is large. A manifestation of this anisotropy is the field independent magnetization M^* observed at a temperature T^*. In the framework of the London model, and including thermal fluctuations of vortices, we found for the temperature dependence of the penetration depth λ_ab(T) = λ_ab(0)[1 − (T/T_c0)ⁿ]^−1/2, with n 1.7 and λ_ab (T = 0) 4000 Å. The estimated upper critical fields μ₀H_c2,c are of the order of 10 T. We observe a peculiar negative slope M/T at low temperature and sufficiently high external magnetic field. This feature seems to be a characteristic of the Bi-2201 phase. However, we do not know whether it is associated with the superconducting mixed-state. A small amount of magnetic impurities could also be responsible for this behavior. Finally, the behavior of the reversible magnetization of the Bi-2201 samples investigated, which are situated at the optimal and in the overdoped region, did not indicate any unusual temperature dependence for the upper critical field H_c2,c.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.     

Out-of-plane conductivity in Bi2Sr2CuOy crystals

We present extensive measurements of anisotropic resistivity on Bi₂Sr₂CuO_y crystals grown from melts with different Bi/Sr ratios and doped with Pb. We find that the temperature variation of the c-axis resistivity _c(T) is correlated with the in-plane resistivity _ab. Depending on the starting compositions, the normal-state in-plane resistivity _ab can either show localized conduction at low temperature or be metallic (d_ab/dT < 0) in the whole temperature range. Correspondingly, a change of the T dependence of _c from nonmetallic (d_c/dT < 0) in the whole measured temperature range (4.2–300 K) to a mixed (d_c/dT < 0 at low T but d_c/dT> 0 at high T) conduction is observed. In accompany, the magnitude of _c at low T decreases by about two orders. We have quantified the trend of the _c(T) and examined some current proposals concerning the out-of-plane transport.     

Range formulation for a staring electro-optical and imaging system incorporating the effects of atmospheric transmission, focal plane 1/f noise and fixed pattern noise

To increase the detection range in staring FPAs, commonly the integration time T_i is increased, as the range is assumed to increase as one-fourth power of T_i, (T_i^1/4). It is shown here that the range dependence on T_i is weaker than T_i^1/4, because of the effect of atmosphere. Since the atmospheric transmission coefficient decreases with increasing range, the effect of increasing T_i on the range is considerably reduced. It is also shown that when detector 1/f noise dominates over other noise sources, the dependence of range on T_i is much weaker than T_i^1/4, having a logarithmic dependence Calculations have been done by integrating equations involving spectral functions—photon flux, atmospheric transmission coefficient, optics transmission coefficient and responsivity—over wavelength, in the spectral range from 8 μm to 12 μm. LOWTRAN2 has been used for spectral atmospheric transmission coefficients, for different conditions of ambient temperature (−30°C to 55°C) and relative humidity (50–85%). The range formulation also distinguishes between distant targets (point sources) and nearby targets (extended sources). The analysis is applicable for terrestrial imaging, where the temperature difference of the target and the background is small. The effect of fixed pattern noise (FPN) in mercury cadmium telluride (MCT) FPAs has also been considered by modeling FPN in terms of a composition variation in the MCT. It is seen that range, both in the point source and the extended source cases, is not a sensitive function of FPN.     

Antiferromagnetism and the Kondo behaviour in Ce(Pt1−xPdx)Ga

We present the results of lattice parameters at room temperature, the static magnetic susceptibility and the magnetic resistivity between 1.8 and 300 K, and the low-temperature specific-heat measurements for the series Ce(Pt_1−xPd_x)Ga, (x=0.0, 0.2, 0.5, 0.8 and 1.0). Two maxima in the temperature dependence of the magnetic resistive curve for each sample are observed, one above 100 K, and another at around 4 K, which due to an interplay between crystal-field effect and the Kondo effect. As determined from the peak values of the temperature dependence of the specific heat data C(T), all samples exhibit antiferromagnetic ordering from 1.3 K for CePdGa to 3.4 K for CePtGa. The large reduction of entropy for each sample below T_N is associated with the Kondo effect.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.     

Aspects of “low field” magnetotransport in epitaxial thin films of the ferromagnetic metallic oxide SrRuO3

Epitaxial thin films of the conductive ferromagnetic oxide SrRuO₃ were grown on an (0 0 1) SrTiO₃ (STO) substrate by using DC sputtering technique. The magnetic and magnetoresistive properties of the films were measured by applying the magnetic field both perpendicular (out-of-plane) and parallel (in-plane) to the film plane and ever maintaining the direction of the applied field perpendicular to that of the transport current. The films grown on an (0 0 1) STO substrate showed identical magnetization properties in two orthogonal crystallographic directions of the substrate, [1 0 0]_S and [0 0 1]_S (in-plane and out-of-plane geometry), which suggests the presence of a multi domain structure within the plane of the film. For such samples, no anisotropic field (hard axis) along de [0 0 1]s direction, i.e., perpendicular to the film-plane could be detected. Nevertheless, a distinguishable temperature dependent out-of-plane anisotropic magnetoresistance (MR) along with strong temperature dependent low field hysteretic MR(H) behavior was detected for the studied films. A negative MR ratio MR(T)=[ρ(μ₀H=9 T; T)−ρ( μ₀H=0 T; T)]/ρ( μ₀H=0 T; T) on the order of a few percent, with maximums of 6% and 4% (right at the Curie temperature, T_C 160 K) was calculated for an in-plane and out-of plane measuring geometry, respectively. In addition there is an equally strong MR effect at low temperatures, which might be related to the temperature dependence of the magnetocrystalline anisotropy together with a magnetization rotation. Both the MR(T) behavior and the achieved values (except for T<30 K) are similar to those obtained on SrRuO₃ films grown on 2° miscut (0 0 1) STO substrates with the current parallel to the field and parallel to the direction, which was identified as the easier axis for magnetization.     

Evolution of zero-dispersion peaks in fluctuation spectra with temperature

Zero-dispersion peaks (ZDPs) can arise in fluctuation spectra of underdamped systems for which the dependence of the eigenfrequency on energy has an extremum. It is shown that ZDPs exist only for temperatures T higher than the critical one, T_c. The shape of the ZDP is described analytically as well as the process of the onset of this peak at T ≈ T_c. In the low-friction limit the shape of the ZDP turns out to be universal for T > T_c.     

Zero field μ spin relaxation in Cd1−xMnxTe diluted magnetic semiconductors

Zero field μSR measurements were carried out on samples of the typical diluted magnetic semiconductor Cd_1−xMn_xTe as a function of composition in the range 0.27x0.65, at temperatures in both the “spin glass’ regions of the magnetic phase diagram. The results show the onset of complex diffusion-trapping behaviour at temperatures T60 K for all concentrations. Below 50 K the exponential relaxation found for the main signal is consistent with the interactions of the muon spin with rapidly fluctuating and rather large local hyperfine fields in these concentrated random diluted magnetic systems. In spite of the loss of signal near and below the transition temperature, the present results show that rapid spin fluctuations persist below T_g.     

Dynamical critical slowing down in CsNiF3

Dynamical critical slowing down in CsNiF₃ is studied using the a.c. susceptibility measurements at 9.5 GHz in zero external static magnetic field. The dynamical critical exponent for the relaxation time of the in-plane spin fluctuations is obtained for the temperature interval 6 K < T < 20 K. For this temperature interval where one-dimensional spin fluctuations are dominant, very good qualitative and quantitative agreement with the spinwave calculation of the dynamical response is obtained at long wavelength and low frequency. The dynamical critical exponent for the relaxation time is measured to be 0.96 ± 0.6. At higher temperature, a gradual crossover to an isotropic Heisenberg chain behaviour is observed. For temperatures close to the 3-d antiferromagnetic ordering temperature T_N, a crossover to 3-d fluctuation regime gives rise to a speeding-up of the spin relaxation rate.     

Demonstration of 640 × 512 pixels long-wavelength infrared (LWIR) quantum dot infrared photodetector (QDIP) imaging focal plane array

We have exploited the artificial atom-like properties of epitaxially grown self-assembled quantum dots (QDs) for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays (FPAs). QD infrared photodetectors (QDIPs) are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR dot-in-a-well (DWELL) structures based on the InAs/InGaAs/GaAs material system. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. DWELL QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. The most recent devices exhibit peak responsivity out to 8.1 μm. Peak detectivity of the 8.1 μm devices has reached 1 × 10¹⁰ Jones at 77 K. Furthermore, we have fabricated the first long-wavelength 640 × 512 pixels QDIP imaging FPA. This QDIP FPA has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60 K operating temperature.     

Anomalous magnetic behavior of the Co0.53Ga0.47 spin glass above the freezing temperature

We present here the detailed analysis of the magnetic behavior of the Co_0.53Ga_0.47 alloy, especially at temperatures above the freezing temperature T_f = 10 K. Low field static magnetization measurements were performed by using the SQUID magnetometer in the temperature range 5–65 K and magnetic fields up to 100 Oe. The temperature dependence of the field cooled susceptibility π_FC(T) at T > T_f has an anomaly, which is displayed in the double change of the curvature near T_s = 24 K. The data of magnetization M_FC in an external field H lie on a universal curve M_FC(H/T) at temperatures T_f < T < T_s. The plots of π^-1_FC(T) and non-linear magnetic susceptibility π^nl_FC(T^-3) are linear lines in the temperature range T_f−T_s. The strong deviation of π^-1_FC(T) and π^nl_FC(T^-3) from straight line, taking place at T T_s, indicates that T_s is an upper temperature limit of the classical superparamagnetic behavior with the constant cluster moment. The results suggest that such phenomena may be fairly universal for spin glasses.