Here we report about the interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating CaCuO2 and SrTiO3 blocks. Hard x-ray photoelectron spectroscopy shows that the valence bands alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline earth interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.
The electronic and magnetic properties of epitaxial LaNiO3/LaAlO3 superlattices can be tuned by layer thickness and substrate-induced strain. Here, we report on direct measurements of the k-space-resolved electronic structure of buried nickelate layers in superlattices under compressive strain by soft x-ray photoemission. After disentangling strong extrinsic contributions to the angle-dependent signal caused by photoelectron diffraction, we are able to extract Fermi surface information from our data. We find that with decreasing LaNiO3 thickness down to two unit cells (2 uc) quasiparticle cohe…
We present a study of interlayer coupling and proximity effects in a La$_{0.66}$Ca$_{0.33}$MnO$_3$(10 nm)/YBa$_2$Cu$_3$O$_7$(10 nm) superlattice. Using element-sensitive x-ray probes, the magnetic state of Mn can be probed without seeing the strong diamagnetism of the superconductor, which makes this approach ideal to study changes in the magnetic properties across the superconducting transition. By a combined experiment using {\it in situ} transport measurements during polarized soft x-ray measurements, we were able to see no noticeable influence of the superconducting state on the magnetic p…
We use resonant soft x-ray scattering (RSXS) to quantify the hole distribution in a superlattice of insulating La2CuO4 (LCO) and overdoped La_{2-x}Sr_{x}CuO4 (LSCO). Despite its non-superconducting constituents, this structure is superconducting with T_{c}=38 K. We found that the conducting holes redistribute electronically from LSCO to the LCO layers. The LCO layers were found to be optimally doped, suggesting they are the main drivers of superconductivity. Our results demonstrate the utility of RSXS for separating electronic from structural effects at oxide interfaces.
We investigate the effect of the superlattice structure on the single particle transport along the c-axis of high temperature superconductors. In particular, superlattice systems that consists of metals/insulators and d-wave superconductors (NS/IS superlattice) are considered. We find that for the NS superlattice in the large mass anisotropy limit of the metal, the density of state in the low energy section is bulk d-wave like except that the position of the quasi-particle peak can be considerably smaller than the gap value, while for the IS superlattice, the quasi-particle peak remains at the…
We report a highly unusual angular variation of the upper critical field (H_c2) in epitaxial superlattices CeCoIn_5(n)/YbCoIn_5(5), formed by alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor CeCoIn_5 with a strong Pauli effect and normal metal YbCoIn_5, respectively. For the n=3 superlattice, H_{c2}(θ) changes smoothly as a function of the field angle θ. However, close to the superconducting transition temperature, H_{c2}(θ) exhibits a cusp near the parallel field (θ=0 deg). This cusp behavior disappears for n=4 and 5 superlattices. This sudden disappearance suggest…
Ultra-thin tri-color (tri-layer) titanate superlattices ([3u.c. LaTiO3/2u.c. SrTiO3/3u.c. YTiO3], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO3 (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3u.c. LaTiO3/2u.c. SrTiO3] bi-layers and all the tri-color structures, whereas a [3u.c. YTiO3/2u.c. SrTiO3] bi-layer shows …
The competition of superconductivity and magnetism in superlattices composed of alternating YBa$_2$Cu$_3$O$_{7-d}$ and La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 nm to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of superlattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symme…
We present an Angle-Resolved PhotoElectron Spectroscopy study of the changes in the electronic structure of electron doped Ba(Fe(1-x)Co(x))2As2 across the superconducting phase transition. By changing the polarization of the incoming light, we were able to observe the opening of the gap for the inner hole pocket alpha, and to compare its behavior with the outer hole-like band beta. Measurements along high symmetry directions show that the behavior of beta is consistent with an isotropic gap opening, while slight anisotropies are detected for the inner band alpha. The implications of these resu…
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials, including high-$T_c$ cuprates, iron pnictides, and heavy-fermion compounds. A new approach to this most fundamental and hotly debated subject focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. A recent state-of-the-art molecular-beam-epitaxy technique has enabled us to fabricate superlattices consisting of different heavy-fermion compounds with atomic thickness. These Kondo s…
Negative refraction, which reverses many fundamental aspects of classical optics, can be obtained in systems with negative magnetic permeability and negative dielectric permittivity. This Letter documents an experimental realization of negative refraction at millimeter waves, finite magnetic fields and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and superconducting thin films. In the present case the superconducting YBa_2Cu_3O_7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO_3 layers for frequencies and magneti…
Significant progress has been achieved in fabricating high quality bulk and thinfilm iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high field applications, large critical current densities (Jc) and irreversibility fields (Hirr) are indispensable along all crystal directions. On the other hand, the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. Here we show that artif…
The bulk polycrystalline sample FeSe1/2Te1/2 is synthesized by solid state reaction route in an evacuated sealed quartz tube at 750 oC. The presence of superconductivity is confirmed through magnetization/thermoelectric/resistivity studies. It is found that the superconducting transition temperature (Tc) is around 12 K. Heat capacity (Cp) of superconducting FeSe1-xTex exhibited a hump near Tc, instead of well defined Lambda transition. X-ray Photo electron spectroscopy (XPS) studies revealed well defined positions for divalent Fe, Se and Te but with sufficient hybridization of Fe (2p) and Se/T…
Superlattices of the repeated structure La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4) (LSCO-LCO), where none of the constituents is superconducting, show a superconducting transition of T_c \simeq 25 K. In order to elucidate the nature of the superconducting state we have performed a low-energy muSR study. By applying a magnetic field parallel (Meissner state) and perpendicular (vortex state) to the film planes, we could show that superconductivity is sheet like, resulting in a very anisotropic superconducting state. This result is consistent with a simple charge-transfer model, which takes into account …
Twisted Van der Waals systems offer the unprecedented possibility to tune different states of correlated quantum matter with an external non-invasive electrostatic doping. The nature of the superconducting order presents a recurring open question in this context. In this work, we quantitatively assess the case of spin-fluctuation-mediated pairing for $Γ$-valley twisted transition metal dichalcogenide homobilayers. We self-consistently and dynamically calculate the doping dependent superconducting transition temperature $T_{\mathrm{c}}$ revealing a superconducting dome with a maximal $T_{\mathr…
We have used spin-polarized neutron reflectometry to investigate the magnetization profile of superlattices composed of ferromagnetic Gd and superconducting Nb layers. We have observed a partial suppression of ferromagnetic (F) order of Gd layers in [Gd($d_F$)/Nb(25nm)]$_{12}$ superlattices below the superconducting (S) transition of the Nb layers. The amplitude of the suppression decreases with increasing $d_F$. By analyzing the neutron spin asymmetry we conclude that the observed effect has an electromagnetic origin - the proximity-coupled S layers screen out the external magnetic field and …
By using a molecular beam epitaxy technique, we fabricate a new type of superconducting superlattices with controlled atomic layer thicknesses of alternating blocks between heavy fermion superconductor CeCoIn_5, which exhibits a strong Pauli pair-breaking effect, and nonmagnetic metal YbCoIn_5. The introduction of the thickness modulation of YbCoIn_5 block layers breaks the inversion symmetry centered at the superconducting block of CeCoIn_5. This configuration leads to dramatic changes in the temperature and angular dependence of the upper critical field, which can be understood by considerin…
Electronic Structure and Superconductivity in La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ Artificial High-$T_c$ Superlattices Probed by Hard and Soft X-ray Spectroscopy
Temple University（天普大学）；Diamond Light Source Ltd.（钻石光源有限公司）；Advanced Light Source, Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室先进光源中心）；Max Planck Institute for Solid State Research（马克斯·普朗克固态研究所）；Institute of Crystallography, Italian National Research Council, IC-CNR（意大利国家研究委员会晶体学研究所）；Rome International Center for Materials Science Superstripes, RICMASS（罗马国际材料科学超条带研究中心）
The conducting interface of LaAlO$_3$/SrTiO$_3$ heterostructures has been studied by hard X-ray photoelectron spectroscopy. From the Ti~2$p$ signal and its angle-dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO$_3$ overlayers. Our results point to an electronic reconstruction in the LaAlO$_3$ overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the Berezinskii-Kosterlitz…
We show, by means of low-energy muon spin rotation measurements, that few-unit-cells thick La(2)CuO(4) layers synthesized digitally by molecular beam epitaxy synthesis are antiferromagnetically ordered. Below a thickness of about 5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few Angstrom) to high-temperature superconducting layers, without transmitting supercurrents.
The electronic structure of interfaces between YBa$_2$Cu$_3$O$_6$ and SrTiO$_3$ is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA+U). We find a metallic state in cuprate/titanate heterostructures with the hole carriers concentrated substantially in the CuO$_2$-layers and in the first interface TiO$_2$ and SrO planes. This effective interface doping appears due to the polarity of interfaces, caused by the first incomplete copper oxide unit cell. Interface-induced high pre-doping of CuO$_2$-layers is a key mechanism controlling the superconducting…
We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^\circ$C on Al$_2$O$_3$(1$\bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this we…
We propose an explanation of the superconducting transitions discovered in the heavy fermion superlattices by Mizukami et al. (Nature Physics 7, 849 (2011)) in terms of Berezinskii-Kosterlitz-Thouless transition. We observe that the effective mass mismatch between the heavy fermion superconductor and the normal metal regions provides an effective barrier that enables quasi 2D superconductivity in such systems. We show that the resistivity data, both with and without magnetic field, are consistent with BKT transition. Furthermore, we study the influence of a nearby magnetic quantum critical poi…
The electronic structures of the ferromagnetic superconductors $\mathrm{UGe}_2$ and $\mathrm{UCoGe}$ in the paramagnetic phase were studied by angle-resolved photoelectron spectroscopy using soft X-rays ($hν=400-500$). The quasi-particle bands with large contributions from $\mathrm{U}~5f$ states were observed in the vicinity of $E_\mathrm{F}$, suggesting that the $\mathrm{U}~5f$ electrons of these compounds have an itinerant character. Their overall band structures were explained by the band-structure calculations treating all the $\mathrm{U}~5f$ electrons as being itinerant. Meanwhile, the st…
We report the structural and electron transport properties of 3R-Nb_{1+x}S_2 (x >= .07) and 3R-Ga_xNbS_2 (.1 <= x <= .33) prepared as polycrystalline pellets as well as single crystals grown by vapour transport. We observe a resistance minimum in these compounds between 20-60 K, with the T_min proportional to x. The resistance scales as rho/rho_min(T/T_min) between .2 < T/T_min < 2 for different phases with x <= .25 whose resistivity differs by an order of magnitude. Powder X-ray diffraction (XRD) also shows progressively increasing intensity of superlattice lines with cation concentration. Th…
After an extended introduction, the thesis considers the electronic properties of BSCCO and the recent progress in understanding the electronic structure of this material. The main result of this part of the work is a model of the Green's function that is later used for calculating the two-particle excitation spectrum. Then, the matrix element effects in the photoemission spectra of cuprates are discussed with a focus on the recently discovered anomalous behavior of the ARPES spectra that partially originates from the momentum-dependent photoemission matrix element. The momentum- and excitatio…
Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not give rise to complex and uncontrolled strain effects due to structural transitions at low temperature. The growth dynamics and the structure have been studied in-situ with reflection high energy electron diffraction …
Charge order has recently been identified as a leading competitor of high-temperature superconductivity in moderately doped cuprates. We provide a survey of universal and materials-specific aspects of this phenomenon, with emphasis on results obtained by scattering methods. In particular, we discuss the structure, periodicity, and stability range of the charge-ordered state, its response to various external perturbations, the influence of disorder, the coexistence and competition with superconductivity, as well as collective charge dynamics. In the context of this journal issue which honors Ro…
Graphene moire superlattices have emerged as a platform hosting and abundance of correlated insulating, topological, and superconducting phases. While the origins of strong correlations and non-trivial topology are shown to be directly linked to flat moire bands, the nature and mechanism of superconductivity remain enigmatic. In particular, only alternating twisted stacking geometries of bilayer and trilayer graphene are found to exhibit robust superconductivity manifesting as zero resistance and Fraunhofer interference patterns. Here we demonstrate that magic-angle twisted tri-, quadri-, and …
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. Here we fabricate hybrid superlattices consisting of alternating atomic layers of heavy-fermion superconductor CeCoIn$_5$ and antiferromagnetic (AFM) metal CeRhIn$_5$, in which the AFM order can be suppressed by applying pressure. We find that the superconducting…
In this article, we report magneto-heat capacity analysis of superconducting SnAs, which is characterized through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and magneto-transport measurements. The studied SnAs superconductor evidenced the presence of superconductivity at around 4K, and the same is seen to persist up to an applied field of 250Oe. The bulk nature of superconductivity is determined through AC susceptibility along with heat capacity measurements. Magneto-heat capacity measurements show SnAs to be a fully gapped s wave superconductor. This finding is well supp…
