General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small.
We describe a long wavelength infrared detector where an InAs/GaSb superlattice absorber is surrounded by a pair of electron-blocking and hole-blocking unipolar barriers. A 9.9 μm cutoff device without antireflection coating based on this complementary barrier infrared detector design exhibits a responsivity of 1.5 A/W and a dark current density of 0.99×10−5 A/cm2 at 77 K under 0.2 V bias. The detector reaches 300 K background limited infrared photodetection (BLIP) operation at 87 K, with a black-body BLIP D∗ value of 1.1×1011 cm Hz1/2/W for f/2 optics under 0.2 V bias.
Bitter decoration and magneto-optical studies reveal that in heavy-ion irradiated superconductors, a 'porous' vortex matter is formed when vortices outnumber columnar defects (CDs). In this state ordered vortex crystallites are embedded in the 'pores' of a rigid matrix of vortices pinned on CDs. The crystallites melt through a first-order transition while the matrix remains solid. The melting temperature increases with density of CDs and eventually turns into a continuous transition. At high temperatures a sharp kink in the melting line is found, signaling an abrupt change from crystallite melting to melting of the rigid matrix.PACS numbers: 74.60. Ec, 74.60.Ge, 74.72.Hs Melting of heterogeneous systems, and in particular of nanocrystals embedded in porous rigid matrices, is a complex process with many uncontrolled parameters. Metal and semiconductor nanocrystals with free surfaces, for example, usually show a decrease in their melting temperature with decreasing size [1], whereas nanocrystals encapsulated in a porous matrix often display an increase in melting temperature [2]. Although the contribution of the different factors is still a matter of debate, the melting process is known to depend on the size, dimensionality, material properties of the nanocrystals and the matrix, as well as the interface energies between the materials [1,2]. In this work we investigate an analogous, but a more controllable composite system, which is a 'porous' vortex matter consisting of vortex nanocrystals encapsulated in a matrix of strongly pinned vortices. As shown below, this system is present in the commonly heavy-ion irradiated superconductors when the vortices outnumber the columnar defects (CDs). The rigid matrix is created by vortices localized on the network of random CDs, while the softer nanocrystals are formed within the 'pores' of this matrix by the interstitial vortices. The size of the nanocrystals can be readily varied from several hundred down to a few vortices by changing the applied field or the density of CDs. We find that this composite vortex matter reveals a number of intriguing mechanisms: Similarly to the metallic nanocrystals in a matrix, we observe for the first time a pronounced upward shift in the vortex melting temperature T m , while preserving the first-order nature of the transition (FOT). With increasing density of CDs, the size of the pores decreases, resulting in a larger shift in T m . We also find a critical point at which the FOT changes into a continuous melting. Moreover, the crystallites can melt while the matrix remains rigid. As a result, at high temperatures we find an abrupt breakdown in the upward shift of T m and a sharp kink in the FOT line, which apparently result from the collapse of the matrix due to vortex depinning from the CDs.The reported findings were obtained using Bitter decoration and differential magneto-optical (MO) [3] techniques. High quality Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals (T c ≈ 89 K) were covered by various patterned masks and irradiated at GANIL by 1 Ge...
We analyze and compare different aspects of InAs/InAsSb and InAs/GaSb type-II superlattices for infrared detector applications and argue that the former is the most effective when implemented for mid-wavelength infrared detectors. We then report results on an InAs/InAsSb superlattice based mid-wavelength high operating temperature barrier infrared detector. At 150 K, the 50% cutoff wavelength is 5.37 μm, the quantum efficiency at 4.5 μm is ∼52% without anti-reflection coating, the dark current density under −0.2 V bias is 4.5 × 10−5 A/cm2, and the dark-current-limited and the f/2 black-body (300 K background in 3–5 μm band) specific detectivities are 4.6 × 1011 and 3.0 × 1011 cm-Hz1/2/W, respectively. A focal plane array made from the same material exhibits a mean noise equivalent differential temperature of 18.7 mK at 160 K operating temperature with an f/2 optics and a 300 K background, demonstrating significantly higher operating temperature than InSb.
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