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One hundred years after ferroelectricity was first reported [Physical Review 17, 475 (1921)], a qualitatively different phenomenon, asymmetric ferroelectricity, is reported in atomic-scale engineered crystals. Unlike the high-temperature centrosymmetric structure that prevail in ferroelectric crystals, the crystals here break inversion symmetry by design. This is achieved by sequenced stacking of three molecular components. When these tailored crystals become hysteretic below an ordering temperature, two unequal polarization states emerge. This unusual bistability points to an asymmetric energy landscape, controlled by sample architecture.

Topological surface states (TSSs) coexist with a rapidly formed two-dimensional electron gas (2DEG) at the surface of Bi2Se3. While this complex band structure has been widely studied for its interactions between the two states in terms of electrical conductivity and carrier density, the resulting thermopower has not been investigated as thoroughly. Here, we report measurements of the temperature dependent Seebeck coefficient ( S) and electrical conductivity ( σ) on an undoped 10 nm thin Bi2Se3 film over the temperature range of 100–300 K to find an overall metal-like behavior. The measured S is consistent with the theory when assuming that both the TSS and the 2DEG contribute to thermoelectric transport. Our analysis further shows that the coefficient corresponds to a Fermi level situated well above the conduction band minima of the 2DEG, resulting in comparable contributions from the TSS and the 2DEG. The thermoelectric power factor ( S2 σ) at 300 K increases by 10%–30% over the bulk. This work provides insights into understanding and enhancing thermoelectric phenomena in topological insulators.

Interfacing bulk conducting topological Bi_{2}Se_{3} films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. However, bulk insulating topological (Bi_{1-x}Sb_{x})_{2}Te_{3} films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two layers thick. This massive suppression of proximity pairing is evidenced by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi_{2}Se_{3}/Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.

Interfacing bulk conducting topological Bi_{2}Se_{3} films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. However, bulk insulating topological (Bi_{1-x}Sb_{x})_{2}Te_{3} films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two layers thick. This massive suppression of proximity pairing is evidenced by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi_{2}Se_{3}/Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.

Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudo-magnetic-field effects, helical flat bands and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here, we show that the Dirac surface states of the topological insulator Bi2Se3 can be reversibly tuned by an externally applied elastic strain. Performing in-situ x-ray diffraction and in-situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi2Se3 films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting reversible changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of Bi2Se3 and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes.

Rapid materials discovery in inorganic chemistry should combine predictive computational tools with fast experimental syntheses. We apply such a tandem approach to explore the Ba–Ru–S phase space, where no ternary compounds are yet known to exist. Related ternary oxide ruthenates and ternary iron sulfides exhibit interesting electronic properties due to d-electron correlations, such as superconductivity, metamagnetism, and quantum phase transitions. We use a combination of evolutionary algorithms and density functional theory to inform traditional and in situ diffraction methods. In the course of our investigation, we find that convex hull constructions of the binary constituents inform interpretation of the ternary hull, which in this case has two compounds near thermodynamic stability. Our experimental study does not reveal formation of the candidates BaRu2S2 or BaRuS3, but it does provide the structure of a high-temperature polymorph of BaS2. This methodology can be exploited to study other ternary sys...

Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudomagnetic field effects, helical flat bands, and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here we show that the Dirac surface states of the topological insulator Bi

Bismuth selenide becomes superconducting upon coupling to metallic niobium, and its topological states pair up to form a gap., A topological insulator film coupled to a simple isotropic s-wave superconductor substrate can foster helical pairing of the Dirac fermions associated with the topological surface states. Experimental realization of such a system is exceedingly difficult, however using a novel “flip-chip” technique, we have prepared single-crystalline Bi2Se3 films with predetermined thicknesses in terms of quintuple layers (QLs) on top of Nb substrates fresh from in situ cleavage. Our angle-resolved photoemission spectroscopy (ARPES) measurements of the film surface disclose superconducting gaps and coherence peaks of similar magnitude for both the topological surface states and bulk states. The ARPES spectral map as a function of temperature and film thickness up to 10 QLs reveals key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length is found to be much larger than the decay length of the topological surface states.

The new field of quantum computing uses qubits (quantum bits) in place of classical bits to carry out certain types of computation. Physical systems that act as qubits encompass a wide range of technologies, from ions, to local defect states in crystals, and on to microelectronic devices addressable with wire interconnects. Materials issues arise in all of these, and this issue of MRS Bulletin describes how materials challenges and opportunities arise and have been used to make qubit-based quantum circuits using very different materials systems. In this overview article, we first review the universal ideas of how information is introduced and processed in a quantum computer. Comparing quantum to classical computers, for a given number of bits, the information content in a quantum computer is exponentially larger. But quantum computers face a daunting challenge: How do we keep the information from degrading and eventually disappearing? Maintaining the coherence of a quantum computer comes down to specific materials issues for all the approaches studied so far. Advances in materials design and processing have enabled enormous increases in performance, and we review the work described in each of the articles in this issue.

We present a new type of transmon split-junction qubit which can be tuned by Meissner screening currents in the adjacent superconducting film electrodes. The best detected relaxation time ($T_1$) was of the order of 50 $\mu$s and the dephasing time ($T_2$) about 40 $\mu$s. The achieved period of oscillation with magnetic field was much smaller than in usual SQUID-based transmon qubits, thus a strong effective field amplification has been realized. This Meissner qubit allows an efficient coupling to superconducting vortices. We present a quantitative analysis of the radiation-free energy relaxation in qubits coupled to Abrikosov vortices. The observation of coherent quantum oscillations provides strong evidence that vortices can exist in coherent quantum superpositions of different position states. According to our suggested model, the wave function collapse is defined by Caldeira-Leggett dissipation associated with viscous motion of the vortex cores.

Expanding the library of known inorganic materials with functional electronic or magnetic behavior is a long-standing goal in condensed matter physics and materials science. Recently, the transition metal chalcogenides including selenium and sulfur have been of interest because of their correlated-electron properties, as seen in the iron-based superconductors and the transition metal dichalcogenides. However, the chalcogenide chemical space is less explored than that of oxides, and there is an open question of whether there may be new materials heretofore undiscovered. We perform a systematic combined theoretical and experimental search over ternary phase diagrams that are empty in the Inorganic Crystal Structure Database containing cations, transition metals, and one of selenium or sulfur. In these 27 ternary systems, we use a probabilistic model to reduce the likelihood of false negative predictions, which results in a list of 24 candidate materials. We then conduct a variety of synthesis experiments to check the candidate materials for stability. While the prediction method did obtain previously unknown compositions that are predicted stable within density functional theory, none of the candidate materials formed in our experiments. We come to the conclusion that these phase diagrams are ``empty'' in the case of bulk synthesis, but it remains a possibility that alternate synthesis routes may produce some of these phases.

In March 2012, a group of researchers met to discuss emerging topics in ceramic science and to identify grand challenges in the field. By the end of the workshop, the group reached a consensus on eight challenges for the future:—understanding rare events in ceramic microstructures, understanding the phase-like behavior of interfaces, predicting and controlling heterogeneous microstructures with unprecedented functionalities, controlling the properties of oxide electronics, understanding defects in the vicinity of interfaces, controlling ceramics far from equilibrium, accelerating the development of new ceramic materials, and harnessing order within disorder in glasses. This paper reports the outcomes of the workshop and provides descriptions of these challenges.

More than one hundred years ago Paul Drude derived the frequency-dependent conductivity σ(o) of a gas of point charges, showing that the width of the spectrum was related to the scattering rate of carriers. Seventy five years later, in the late 1970's and early 1980's, it was recognized that σ(o) of a two-dimensional superconductor above it's transition temperature Tc could be described by the Drude response of interpenetrating gases of electrons and vortices. Here we describe measurements of σ(o) in the high-Tc superconductor system Bi2Sr2Ca1-xDyxCu2O8+δ (BSCCO) using time-domain THz spectroscopy that provide evidence for vortices above Tc. We compare this evidence with results obtained by other probes of fluctuations in cuprate superconductors.

We have grown infinite-layer (Ca1−xSrxCuO2) epitaxial films using ozone assisted molecular beam epitaxy. In situ reflective high energy electron diffraction analysis shows that two-dimensional defect-free films can be obtained for limited composition (0.15

We report continuous frequency shifting of a train of mode-locked pulses of light by up to /spl plusmn/400 GHz using electrooptic frequency shifting (EOFS). Dynamic, continuous, and accurate shifting of the optical pulses is achieved by controlling the phase and power of a single frequency microwave signal in a traveling wave phase modulator structure. Operation of the device under different microwave power and optical pulse length settings was investigated in order to study spectral distortion predicted to occur at high levels of frequency shifting and when using long pulses. Possible techniques for future device improvement were proposed and preliminary tests were performed. The use of a second-harmonic component of the microwave drive signal leads to a longer region of constant amplitude gradient and results in reduced distortion.

Although the pairing symmetry of the high-Tc superconductors is widely accepted to be dx2 − y2, measurements in apparent contradiction with this picture have been reported in Bi2Sr2Ca1Cu2O8 + x and attributed to s-wave symmetry or complex mixtures of s-wave and d-wave. To clarify this issue, we have measured the transport properties of asymmetric 45° [100]/[110] Bi2Sr2Ca1Cu2O8 + x bicrystal Josephson junctions, a geometry sensitive to the pairing symmetry. All samples investigated show complicated but polarity-symmetric modulation of their critical currents with magnetic field, providing unambiguous, clear evidence for a predominant dx2 − y2 pairing symmetry at Bi2Sr2Ca1Cu2O8 + x interfaces.

In this paper, we present a semiclassical wave equation analysis using a coupled-mode expansion formalism to study the effect of a microwave field on a train of short optical pulses co-propagating in a traveling-wave modulator structure. This device can continuously shift the central frequency of light pulses. We demonstrate the effect of the applied field on the light spectrum, the dependence on the relation between microwave wavelength and pulse length, and the effect of changing the interaction length or the power of the applied microwave field. The results of the calculation of these effects are presented.

Interstitial oxygen critical to the emergence of superconductivity in ${\mathrm{Fe}}_{1+y}\mathrm{Te}{\mathrm{O}}_{x}$ thin films has been detected. Its location and concentration are measured by atomic-resolution electron-energy-loss spectroscopy with $x\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}0.09$. The density functional theory calculations show that oxygen incorporation leads to local disorder in the magnetic moments of Fe, hole doping by oxygen forming ionic bonds with Fe, and a large magnetic- and position-dependent increase or reduction in the Te-Fe-Te bond angles. An examination of bonding based on charge density further reveals covalent charge between Fe and Te, and its reduction with O doping.

We have measured the complex conductivity $\ensuremath{\sigma}$ of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ thin film between 0.2 and 0.8 THz. We find $\ensuremath{\sigma}$ in the superconducting state to be well described as the sum of contributions from quasiparticles, condensate, and order parameter fluctuations which draw $30%$ of the spectral weight from the condensate. An analysis based on this decomposition yields a quasiparticle scattering rate on the order of ${k}_{B}T/\ensuremath{\Elzxh}$ for temperatures below ${T}_{c}$.

Disorder causes both superconducting cuprate and colossal magnetoresistive manganite based tunnel junctions to depart from properties based solely on the superconductivity or magnetism inherent in the materials. Amorphous barrier layers cause diffuse reflection of quasiparticle states at the insulator–superconductor interface, resulting in a local quenching of the d-wave order parameter. A subdominant order parameter evolves at low temperatures in these layers. In manganites, interface disorder reduces spin polarization and causes the tunneling magnetoresistance in spin valve devices to be smaller than in junctions with less disorder.

Equilibrium conductivity fluctuations of mesoscopic domains are found in film and bulk single-crystal manganite colossal magnetoresistive material. Temperature and field dependences of the Boltzmann factors for a collection of two-state fluctuators give measures of the magnetic moment and entropy differences between the states, and of the fluctuator volumes. The large resistance step size implies dramatic current inhomogeneities. Occasional anomalous temperature dependences indicate that the film inhomogeneous phase is stabilized by a repulsive interaction between conducting regions.

Coherent terahertz spectroscopy is used to measure the complex conductivity of thin films of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ in the vortex state as a function of frequency, temperature, and applied magnetic field. We report an unusual sublinear power law dependence of the high-frequency magnetoconductivity $\ensuremath{\Delta}\ensuremath{\sigma}(H)\ensuremath{\equiv}\ensuremath{\sigma}(H)\ensuremath{-}\ensuremath{\sigma}(0)$ on magnetic field. Over a broad range of field and temperature, $\ensuremath{\Delta}{\ensuremath{\sigma}}_{2}$ varies as ${H}^{\ensuremath{\alpha}}$, with $\ensuremath{\alpha}\ensuremath{\approx}\frac{1}{2}$. We present a model based on the nonlinear London electrodynamics predicted for a $d$-wave superconductor which quantitatively explains these results using only parameters obtained from zero-field measurements.

We present two types of experiments which probe the high-frequency conductivity of BSCCO films in the presence of large superfluid velocities. In the first the superflow is the static velocity field associated with the vortex lattice. In the second the velocity is dynamic, induced by high-intensity single-cycle electromagnetic pulses. We show that intrinsic pair-breaking in a d -wave superconductor can provide a unified explanation for both effects.

Using ALL-MBE, we have engineered a novel, metastable cuprate superconductor, BiSr 2 Ca 7− x Dy x Cu 8 O 19+ y , in which only the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer (only few A thick), and the top superconducting electrode, thus constituting an artificial intra-cell Josephson junction. In this way, we have fabricated the first high- T c tunnel (SIS) junctions. They exhibit very sharp quasiparticle tunneling I–V characteristics, consistent with tunneling between 2D superconductors with d-wave pairing.

Using ALL-MBE, we have engineered a novel, artificial (metastable) cuprate superconductor, BiSr 2 Ca 7-x Dy x CU 8 O, 19+y' in which only the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer (only few A thick), and the top superconducting electrode, thus constituting an artificial intra-cell Josephson junction. In this way, we have fabricated the first high-T C tunnel (SIS) junctions. They exhibit significant Josephson supercurrents and very sharp quasiparticle tunneling I-V characteristics. Next, single-crystal thin films of manganites grown by ALL-MBE have shown remarkable anisotropy of colossal magnetoresistance (CMR). We have also fabricated lateral superlattices with alternating stripes (nano-wires) of La 1-x Ca x MnO 3 and La 1-x Sr x MnO 3 , few tens of nanometers wide. They show substantial in-plane anisotropy in transport properties, and a fivefold increase in low-field sensitivity. Our most recent results include successful growth of La 1-x Sr x MnO 3 on top of Bi 2 Sr 2 CaCu 2 O 8 , and vice versa.

We have measured the low-field magnetoresistance of molecular-beam-epitaxy-grown tetragonal ${\mathrm{La}}_{0.7}$ ${\mathrm{Ca}}_{0.3}$ ${\mathrm{MnO}}_{3}$ films as a function of temperature, and both magnitude and direction of the applied magnetic field. We observed low-field anisotropic hysteresis that depends on the direction of the applied field in the plane of the film. The hysteretic effect can result in a sharp drop in resistance during magnetization reversal which is more than 10 times steeper than the already 'colossal' magnetoresistance. We also present evidence of biaxial magnetocrystalline anisotropy with easy axes along the Mn-O bond ([100],[010]) directions. We show that the low-field anisotropic hysteresis arises from the combined effects of magnetocrystalline anisotropy, anisotropic magnetoresistance, and 'colossal' magnetoresistance. Based on a comparison of the data with a simple phenomenological model for the magnetoresistance, we argue that magnetization reversal must proceed by a domain process.

Lattice distortion due to oxygen octahedral rotations have a significant role in mediating the magnetism in oxides, and recently attracts a lot of interests in the study of complex oxides interface. However, the direct experimental evidence for the interrelation between octahedral rotation and magnetism at interface is scarce. Here we demonstrate that interfacial octahedral rotation are closely linked to the strongly modified ferromagnetism in (LaMnO3+delta)(N)/(SrTiO3)(N) superlattices. The maximized ferromagnetic moment in the N = 6 superlattice is accompanied by a metastable structure (space group Imcm) featuring minimal octahedral rotations (a(-) a(-) c(-), alpha similar to 4.2 degrees, gamma similar to 0.5 degrees). Quenched ferromagnetism for N

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. As ${T}_{c}$ is reduced by a factor of 2 by underdoping, the superconducting state gap $\ensuremath{\Delta}$ does not fall proportionally, but instead stays constant or increases slightly, in violation of the BCS mean-field theory result. The different doping dependences of $\ensuremath{\Delta}$ and $k{T}_{c}$ indicate that they represent different energy scales. The measurements also show that $\ensuremath{\Delta}$ is highly anisotropic and consistent with a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above ${T}_{c}$. The existence of a normal state gap is related to the failure of $\ensuremath{\Delta}$ to scale with ${T}_{c}$ in theoretical models that predict pairing without phase coherence above ${T}_{c}$.

We present a detailed analysis of magnetoresistance in tetragonal thin films of ferromagnetic La{sub 0.7}Ca{sub 0.3}MnO{sub {delta}} grown by atomic layer-by-layer molecular-beam epitaxy. The field and temperature dependence of the resistivity {rho} show an explicit dependence only on the fractional magnetization {ital M}/{ital M}{sub sat} and are consistent with a single approximate form {rho}{approx_equal}{rho}(0)exp[{minus}{ital C}({ital M}/{ital M}{sub sat}){sup 2}] for small and intermediate magnetization both above and below the Curie temperature ({ital T}{sub {ital C}}). The microscopic magnetization and susceptibility ({chi}) have been inferred from the low-field magnetoresistance above {ital T}{sub {ital C}} with {chi} following a Curie-Weiss form. We observe a sharp transition in the low-field ({ital H}) dependence of {rho} from quadratic ({proportional_to}{ital H}{sup 2}) above {ital T}{sub {ital C}} to linear ({proportional_to}{ital H}) below, consistent with a temperature-independent {rho}({ital M}). Finally we show that within a simple model this temperature-independent form for {rho} reproduces the behavior of the measured low-field sensitivity below {ital T}{sub {ital C}}. {copyright} {ital 1996 The American Physical Society.}

We report angle-resolved photoemission results on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and oxygen depleted ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ investigating the electronic structure changes above ${T}_{c}$ in materials with hole doping levels ranging from insulating to slightly overdoped. Near optimal hole doping, the Fermi surface is large and consistent with band calculations. In underdoped samples with ${T}_{c}$ of 60--70 K, portions of this Fermi surface are not seen. This change is related to the opening of an energy gap near $(\ensuremath{\pi},0)$ above ${T}_{c}$.

We have used pulsed organometallic beam epitaxy (POMBE) to simultaneously deposit â- and ĉ-axis oriented Y1Ba2Cu3O7−x (YBCO) thin films at arbitrary locations on LaAlO3(100) substrates. Using photolithography and ion milling, several types of â-ĉ weak links have been fabricated at the boundary between the two films. The current-voltage (I-V) characteristics show a flux flow type behavior. The resistive transitions are broad and the critical current density is low, indicating weak coupling across these boundaries. With magnetic field applied parallel to the grain boundary plane, nonhysteretic I-V curves are obtained and the critical current goes to zero at an applied magnetic field of ∼7500 G.

We review the progress towards an understanding of the electrodynamics in the vortex-state which arises in response to a magnetic field. B. Experimentally, the goal is to characterize the conductivity of cuprate superconductors in the presence of an applied magnetic field over a broad frequency range from microwave to infrared wavelengths. The theoretical challenge is to understand these measurements with microscopic, as well as phenomenological, models. In describing the experiments to date we will focus on a relatively new technique, coherent terahertz spectroscopy, which is uniquely capable of measuring the complete complex conductivity tensor.

Atomic Layer-by-Layer Molecular Beam Epitaxy can be used to assemble complex oxides, an atomic oxide layer at a time, and to artificially layer a wide range of novel materials and artificial structures combining such layers at atomically flat interfaces. This requires accurate control of the flux of each atomic beam. Additionally, activated oxygen like ozone provides substantially higher reactivity than is obtained with oxygen and allows virtually all oxide phases to be grown at temperatures high enough to get good crystal growth at background pressure less than 10-5 Torr. Materials with novel composite properties can be synthesized, and films to study fundamental aspects of correlated phases like high temperature superconductors and magnetically ordered compounds can be grown and processed into devices.

We consider the effect of gap nodes on the electrodynamic response of superconductors in the vortex state. We show that for a gap with line nodes it is not possible to ignore the effect of field-induced pairbreaking on the electrodynamic response. We describe measurements of both the diagonal and the Hall components of the resistivity which support this conclusion.

We report angle-resolved photoemission spectroscopy (ARPES) results from Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (2212) single-crystal thin films grown with atomic layer-by-layer molecular beam epitaxy (ALL-MBE). This was made possible by adapting the top-post cleaving technique to thin films of 2212. We have observed clear photoemission features from these films that disperse through the Fermi level. We have observed the superconducting energy gap with ARPES in thin films. Both the magnitude of the gap as well as its anisotropy are consistent with the results from bulk crystals. Furthermore, we observe the pile-up and dip in spectral weight associated with the gap. These results open possibilities for future research using thin-film samples in this materials system that take advantage of the ability to grow metastable materials with ALL-MBE.

The magnetic flux noise generated by films and crystals of B${\mathrm{i}}_{2}$S${\mathrm{r}}_{2}$CaC${\mathrm{u}}_{2}$ ${\mathrm{O}}_{8+y}$ and YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$ ${\mathrm{O}}_{7\ensuremath{-}x}$, up to 30 \ensuremath{\mu}m thick and cooled in nominally zero magnetic field, has been measured at opposing surfaces by two dc superconducting quantum interference devices. For both materials, the noise sources at the two surfaces were highly correlated at specific temperatures in a given cooldown. This result suggests that the observed vortices moved as rigid rods. At other temperatures, the noise was mostly uncorrelated, suggesting that the relevant vortices were pinned at more than one point along their length.

Molecular beam epitaxy technique has enabled synthesis of atomically smooth thin films, multilayers, and superlattices of cuprates and other complex oxides. Such heterostructures show high temperature superconductivity and enable novel experiments that probe the basic physics of this phenomenon. For example, it was established that high temperature superconductivity and anti-ferromagnetic phases separate on Angstrom scale, while the pseudo-gap state apparently mixes with high temperature superconductivity over an anomalously large length scale (the “Giant Proximity Effect”). We review some recent experiments on such films and superlattices, including X-ray diffraction, atomic force microscopy, angle-resolved time of flight ion scattering and recoil spectroscopy, transport measurements, high- resolution transmission electron microscopy, resonant X-ray scattering, low-energy muon spin resonance, and ultrafast photo-induced reflection high energy electron diffraction. The results include an unambiguous demonstration of strong coupling of in-plane charge excitations to out-of-plane lattice vibrations, a discovery of interface high temperature superconductivity that occurs in a single CuO2 plane, evidence for local pairs, and establishing tight limits on the temperature range of superconducting fluctuations.

The as-grown single crystal Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ with the tetragonal PbO-type structure is nonsuperconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy and accompanied by improved homogeneity in the remaining PbO-type phase, as well as an increase in the L${}_{3}$/L${}_{2}$ intensity ratio of the Fe-L${}_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.

A technique has been developed for synthesis of heterostructures containing various Bi- and Dy-based cuprates and other complex oxides, with a high level of control of deposition of individual atomic monolayers as well as capability for atomic-layer engineering. It produces multilayers and superlattices which display striking long-range crystalline order and atomically abrupt interfaces; this enables deposition of one-unit-cell thick 2212 layers which display HTSC, and of barriers less than 1 nm thick with no pinholes over macroscopic areas. Employing Dy-doped 1278 barriers, trilayer Josephson junctions with I c R n =5−7 mV have been fabricated. Phase locking is seen in stacked junctions. Tunneling transport (but no supercurrent) is observed in junctions with insulating titanate barriers. Modulation doping is attained by omitting or inserting entire monolayers, and artificial metastable high-T c compounds have been synthesized in this way. Finally, long-range proximity effects are seen in 2212-2201-2212 trilayers.

Oxides exhibit most of the interesting phenomena known to occur in solid-state systems. As a class of materials they may be richer in phenomenology than any other comparable class. Oxides can be insulators, semiconductors, or metals. The copperoxide-based compounds we have studied are superconductors with the highest critical temperatures. In some oxides, electrons manifest simple single-particle transport properties, with a high mobility; in others, they show strongly correlated behavior resulting in a Mott-Hubbard transition, localization, and charge- or spin-density waves. In some oxides, electron-phonon coupling leads to polaronic transport. Others show collective states such as magnetism; in some there are large local magnetic moments that can couple to form ferromagnetic or antiferromagnetic phases that exist up to high temperatures. Yet others have large nonlinear dielectric and optical properties. In fact, it would seem there is very little that some such oxide couldn't do for or to the experimenter.

Hc2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.Hc2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.

A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified atomic monolayers to fabricate, for the first time, intra-cell barriers. Apart from manufacturing trilayer Josephson junctions withI cRn>5 mV, this technique enables one to customize both the materials and the devices according to the needs of a specific experiment. A number of fundamental issues, such as the dimensionality of the HTSC state, existence of long-range proximity effects, occurrence of resonant tunneling with a specified number of hops, etc., have been addressed in this way. Synthesis of the first “artificial” metastable HTSC compounds is also reported.

We have very recently discovered a new hydrogen-producing photocatalyst is BiNbO4. BiNbO4 powders prepared by solid state reaction were tested for photocatalytic activity in methanol solutions under UV irradiation. When the material is tested without the presence of a Pt co-catalyst, photocatalytic activity for H2 evolution is superior to that of TiO2. It was also found that BiNbO4 photodegrades into metallic Bi and reduced Nb oxides after use; materials were characterized by SEM, XRD, and XPS. Adding Pt to the surface of the photocatalyst increases photocatalytic activity and importantly, helps to prevent photodegradation of the oxide material. With 1 wt. % Pt loading, photodegradation is essentially absent. BiNbO4 photodegrades into metallic Bi and reduced Nb oxides after use; materials were characterized by SEM, XRD, and XPS. Adding Pt to the surface of the photocatalyst increases photocatalytic activity and importantly, helps to prevent photodegradation of the oxide material. With 1 wt. % Pt loading, photodegradation is essentially absent.

Microwave Fabry-Perot resonators containing nonlinear mesoscopic elements (such as superconducting nanowires) can be used to explore many-body circuit QED. Here, we report on observations of a superconductor-normal pulsing regime in microwave (GHz) coplanar waveguide resonators consisting of superconducting MoGe films interrupted by a gap that is bridged by one or more suspended superconducting nanowires. This regime, which involve MHz-frequency oscillations in the amplitude of the supercurrent in the resonator, are achieved when the steady-state amplitude of the current in the driven resonator exceeds the critical current of the nanowires. Thus we are able to determine the temperature dependence of the critical current, which agrees well with the corresponding Bardeen formula. The pulsing regime manifests itself as an apparent "crater" on top of the fundamental Lorentzian peak of the resonator. Once the pulsing regime is achieved at a fixed drive power, however, it remains stable for a range of drive frequencies corresponding to subcritical steady state currents in the resonator. We develop a phenomenological model of resonator-nanowire systems, from which we are able to obtain a quantitative description of the amplitude oscillations and also, inter alia, to investigate thermal relaxation processes in superconducting nanowires. For the case of resonators comprising two parallel nanowires and subject to an external magnetic field, we find field-driven oscillations of the onset power for the amplitude oscillations, as well as the occurrence (for values of the magnetic field that strongly frustrate the nanowires) of a distinct steady state in which the pulsing is replaced by stochastic amplitude-fluctuations. We conclude by giving a brief discussion of how circuit-QED-based systems have the potential to facilitate understanding of quantum phase-slips in superconducting nanowires., Comment: 40 pages, 20 figures

${\mathit{H}}_{\mathrm{c}2}$(T) has been measured for thin BSCO films over a larger combined range of magnetic field and reduced temperature than for any other superconductor. ${\mathit{H}}_{\mathit{c}2}$(T) diverged anomalously as the temperature decreased: At the lowest temperature, it was 5 times that expected for a conventional superconductor. Such a strong divergence cannot be explained by any conventional model.

Optical wavelength shifting of 1.054-nm laser pulses in excess of +or- 10 wavenumbers (+or- 300 GHz) was demonstrated. The wavelength shifter consists of synchronous microwave and optical waveguides fabricated monolithically on LiNbO/sub 3/. An optical pulse experiences a constant refractive index gradient that travels with the pulse and causes the wavelength shift. >