Colossal transverse magnetoresistance due to nematic superconducting phase fluctuations in a copper oxide.

Electronic anisotropy ("nematicity") has been detected in cuprate superconductors by various experimental techniques. Using angle-resolved transverse resistance (ARTR) measurements, a very sensitive and background-free technique that can detect 0.5% anisotropy in transport, we have observed it also in La2-xSrxCuO4 (LSCO) for 0.02 ≤ x ≤ 0.25. A central enigma in LSCO is the rotation of the nematic director (orientation of the largest longitudinal resistance) with temperature; this has not been seen before in any material. Here, we address this puzzle by measuring the angle-resolved transverse magnetoresistance (ARTMR) in LSCO. We report the discovery of colossal transverse magnetoresistance (CTMR)-an order-of-magnitude drop in the transverse resistivity in the magnetic field of 6 T. We show that the apparent rotation of the nematic director is caused by anisotropic superconducting fluctuations, which are not aligned with the normal electron fluid, consistent with coexisting bond-aligned and diagonal nematic orders. We quantify this by modeling the (magneto-)conductivity as a sum of normal (Drude) and paraconducting (Aslamazov-Larkin) channels but extended to contain anisotropic Drude and Cooper-pair effective mass tensors. Strikingly, the anisotropy of Cooper-pair stiffness is much larger than that of the normal electrons. It grows dramatically on the underdoped side, where the fluctuations become quasi-one-dimensional. Our analysis is general rather than model dependent. Still, we discuss some candidate microscopic models, including coupled strongly-correlated ladders where the transverse (interladder) phase stiffness is low compared with the longitudinal intraladder stiffness, as well as the anisotropic superconducting fluctuations expected close to the transition to a pair-density wave state.

Atomic-Layer Engineering of La2-xSrxCuO4-La2-xSrxZnO4 Heterostructures.

The fabrication of trilayer superconductor-insulator-superconductor (SIS) Josephson junctions with high-temperature superconductor (HTS) electrodes requires atomically perfect interfaces. Therefore, despite great interest and efforts, this remained a challenge for over three decades. Here, we report the discovery of a new family of metastable materials, La2-xSrxZnO4 (LSZO), synthesized by atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). We show that LSZO is insulating and epitaxially compatible with an HTS compound, La2-xSrxCuO4 (LSCO). Since the "parent" compound La2ZnO4 (LZO) is easier to grow, here we focus on this material as our insulating layer. Growing LZO at very low temperatures to reduce cation interdiffusion makes LSCO/LZO interfaces atomically sharp. We show that in LSCO/LZO/LSCO trilayers, the superconducting properties of the LSCO electrodes remain undiminished, unlike in previous attempts with insulator barriers made of other materials. This opens prospects to produce high-quality HTS tunnel junctions.

Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface.

Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported. Micrometre-scale single-crystal domains formed as isolated faceted islands or merged together to achieve full monolayer coverage. The crystal structure of this phase has ten boron atoms and two hexagonal vacancies in its unit cell. First-principles calculations indicate that charge transfer, rather than covalent bonding, binds this two-dimensional boron to the Cu(100) surface. The electronic band structure of this material features multiple anisotropic tilted Dirac cones.

Visualizing the unusual spectral weight transfer in DyBa2Cu3O7-δ thin film.

We report a Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) study of a DyBa2Cu3O7-δ (DBCO) thin film (Tc ~ 79 K) synthesized by the molecular beam epitaxy (MBE). We observed an unusual transfer of spectral weight in the local density of states (LDOS) spectra occurring only within the superconducting gap. By a systematic control of the tip-sample distance and the junction resistance, we demonstrate that the spectral weight transfer can be switched at a nano-meter length scale. These results suggest that an interaction between the STM tip and the sample alters the electronic configurations in the film. This probably originates from a combination of an intrinsic band bending at the interface between the surface and the bulk, and a tip-induced band bending. These results may open a new avenue for band engineering and applications of thin films of high-Tc cuprates.

Electronic nematicity in Sr2RuO4.

We have measured the angle-resolved transverse resistivity (ARTR), a sensitive indicator of electronic anisotropy, in high-quality thin films of the unconventional superconductor Sr2RuO4 grown on various substrates. The ARTR signal, heralding the electronic nematicity or a large nematic susceptibility, is present and substantial already at room temperature and grows by an order of magnitude upon cooling down to 4 K. In Sr2RuO4 films deposited on tetragonal substrates the highest-conductivity direction does not coincide with any crystallographic axis. In films deposited on orthorhombic substrates it tends to align with the shorter axis; however, the magnitude of the anisotropy stays the same despite the large lattice distortion. These are strong indications of actual or incipient electronic nematicity in Sr2RuO4.

Locating the Missing Superconducting Electrons in the Overdoped Cuprates La_{2-x}Sr_{x}CuO_{4}.

Overdoped high-temperature cuprate superconductors have often been understood within the standard BCS framework of superconductivity. However, measurements in a variety of overdoped cuprates indicate that the superfluid density is much smaller than expected from BCS theory and decreases smoothly to zero as the doping is increased. Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La_{2-x}Sr_{x}CuO_{4} films to determine the total, superfluid, and uncondensed spectral weight as a function of doping. A significant fraction of the carriers remains uncondensed in a wide Drude-like peak as T→0, while the superfluid density remains linear in temperature. These observations are seemingly inconsistent with existing, realistic theories of impurity scattering suppressing the superfluid density in a BCS-like d-wave superconductor. Our large measurement frequency range gives us a unique look at the low frequency spectral weight distribution, which may suggest the presence of quantum phase fluctuations as the critical doping is approached.

Large-area single-crystal sheets of borophene on Cu(111) surfaces.

Borophene, a theoretically proposed two-dimensional (2D) boron allotrope1-3, has attracted much attention4,5 as a candidate material platform for high-speed, transparent and flexible electronics6-9. It was recently synthesized, on Ag(111) substrates10,11, and studied by tunnelling and electron spectroscopy12. However, the exact crystal structure is still controversial, the nanometre-size single-crystal domains produced so far are too small for device fabrication and the structural tunability via substrate-dependent epitaxy is yet to be proven. We report on the synthesis of borophene monitored in situ by low-energy electron microscopy, diffraction and scanning tunnelling microscopy (STM) and modelled by ab initio theory. We resolved the crystal structure and phase diagram of borophene on Ag(111), but found that the domains remain nanoscale for all growth conditions. However, by growing borophene on Cu(111) surfaces, we obtained large single-crystal domains, up to 100 µm2 in size. The crystal structure is a novel triangular network with a concentration of hexagonal vacancies of η = 1/5. Our experimental data, together with first principles calculations, indicate charge-transfer coupling to the substrate without significant covalent bonding. Our work sets the stage for fabricating borophene-based devices and substantiates the idea of borophene as a model for development of artificial 2D materials.

Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating.

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.

Hall effect in quantum critical charge-cluster glass.

Upon doping, cuprates undergo a quantum phase transition from an insulator to a d-wave superconductor. The nature of this transition and of the insulating state is vividly debated. Here, we study the Hall effect in La2-xSrxCuO4(LSCO) samples doped near the quantum critical point atx∼ 0.06. Dramatic fluctuations in the Hall resistance appear belowTCG∼ 1.5 K and increase as the sample is cooled down further, signaling quantum critical behavior. We explore the doping dependence of this effect in detail, by studying a combinatorial LSCO library in which the Sr content is varied in extremely fine steps,Δx∼ 0.00008. We observe that quantum charge fluctuations wash out when superconductivity emerges but can be restored when the latter is suppressed by applying a magnetic field, showing that the two instabilities compete for the ground state.

Sub-millikelvin stabilization of a closed cycle cryocooler.

Intrinsic temperature oscillations (with the amplitude up to 1 K) of a closed cycle cryocooler are stabilized by a simple thermal damping system. It employs three different materials with different thermal conductivity and specific heat at various temperatures. The amplitude of oscillations of the sample temperature is reduced to less than 1 mK, in the temperature range from 4 K to 300 K, while the cooling power is virtually undiminished. The damping system is small, inexpensive, can be retrofitted to most existing closed cycle cryocoolers, and may improve measurements of any temperature-sensitive physics properties.

Fluctuating charge-density waves in a cuprate superconductor.

Cuprate materials hosting high-temperature superconductivity (HTS) also exhibit various forms of charge and spin ordering whose significance is not fully understood. So far, static charge-density waves (CDWs) have been detected by diffraction probes only at particular doping levels or in an applied external field . However, dynamic CDWs may also be present more broadly and their detection, characterization and relationship with HTS remain open problems. Here we present a method based on ultrafast spectroscopy to detect the presence and measure the lifetimes of CDW fluctuations in cuprates. In an underdoped La(1.9)Sr(0.1)CuO4 film (T(c) = 26 K), we observe collective excitations of CDW that persist up to 100 K. This dynamic CDW fluctuates with a characteristic lifetime of 2 ps at T = 5 K that decreases to 0.5 ps at T = 100 K. In contrast, in an optimally doped La(1.84)Sr(0.16)CuO4 film (T(c) = 38.5 K), we detect no signatures of fluctuating CDWs at any temperature, favouring the competition scenario. This work forges a path for studying fluctuating order parameters in various superconductors and other materials.

The Meissner effect in a strongly underdoped cuprate above its critical temperature.

The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed. Here we probe the local diamagnetic response in the normal state of an underdoped La(1.94)Sr(0.06)CuO(4) layer (T(c)'≤5 K), which is brought into close contact with two nearly optimally doped La(1.84)Sr(0.16)CuO(4) layers (T(c)≈32 K). We show that the entire 'barrier' layer of thickness, much larger than the typical c axis coherence lengths of cuprates, exhibits a Meissner effect at temperatures above T(c)' but below T(c). The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.

Large oscillations of the magnetoresistance in nanopatterned high-temperature superconducting films.

Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometres(8-11). Here, we report the results of measurements on loops of La(1.84)Sr(0.16)CuO(4), a high-temperature superconductor that loses its resistance to electric currents when cooled below approximately 38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little-Parks effect. Moreover, unlike Little-Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry, or with a period of h/4e, as predicted for superconductors that exhibit stripes.

Anomalous expansion of the copper-apical-oxygen distance in superconducting cuprate bilayers.

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Angstrom resolution the complete atomic structure of epitaxial La(2-x)Sr(x)CuO(4) ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 A. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

Electron-phonon interactions in superconducting La1.84Sr0.16CuO4 films.

We have measured quasiparticle tunneling across a junction perpendicular to the superconducting copper oxide planes. The tunneling spectra show peaks in the density of states. There are 11 minima in the second derivative d2I/dV2, where I is the current and V the voltage, suggesting multiple boson-quasiparticle interactions. These minima match precisely with the published Raman scattering data, leading us to conclude that the relevant bosons in superconducting La1.84Sr0.16CuO4 films are phonons.

X-ray nanocrystallography of individual carbon nanotubes.

By numerical simulations, we show that with the next-generation synchrotron sources one should be able to record within minutes an X-ray diffraction pattern of a single nanotube and from it decipher its detailed atomic structure. In calculated diffractograms we can even discern signatures of presence of a single adatom and locate its position. New synchrotrons will allow the existing method of electron nanocrystallography to be undertaken using X-ray beams, thus facilitating in situ environmental studies.

Nonequilibrium phase transitions in cuprates observed by ultrafast electron crystallography.

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable "structural isosbestic" point and a threshold effect for the dependence of c-axis expansion (Deltac) on fluence (F), with Deltac/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at approximately 0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.