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Author(s): L. Alff (corresponding author) [1]; Y. Krockenberger [1]; B. Welter [1]; M. Schonecke [1]; R. Gross [1]; D. Manske [2]; M. Naito [3] The ground state of superconductors is [...]

The layer-by-layer growth of epitaxial yttrium-iron-garnet ([Y.sub.3][Fe.sub.5][O.sub.12]) thin films on (111) and (110) [Gd.sub.3][Ga.sub.5][O.sub.12] substrates are described that are verified by the intensity oscillations of reflections high energy electron diffraction during pulsed laser deposition. Magnetic anisotropy similar to bulk crystals is seen for as-grown films that are not subjected to a postannealing process.

We report on the layer-by-layer growth of epitaxial yttrium-iron-garnet (Y3Fe5O12) thin films on (111) and (110) Gd3Ga5O12 substrates as verified by the intensity oscillations of reflection high energy electron diffraction during pulsed laser deposition. X-ray diffraction indicates pseudomorphic lattice structure with slight expansion along out of plane from cubic one. The saturation magnetization well reproduces the full moment observed for bulk crystals. The crystallographic magnetic anisotropy was also preserved regardless of shape anisotropy in different orientations. Magnetic anisotropy similar to bulk crystals was observed for as-grown films which have not been subject to a postannealing process. [ABSTRACT FROM AUTHOR]

We report on the layer-by-layer growth of epitaxial yttrium-iron-garnet (Y3Fe5O12) thin films on (111) and (110) Gd3Ga5O12 substrates as verified by the intensity oscillations of reflection high energy electron diffraction during pulsed laser deposition. X-ray diffraction indicates pseudomorphic lattice structure with slight expansion along out of plane from cubic one. The saturation magnetization well reproduces the full moment observed for bulk crystals. The crystallographic magnetic anisotropy was also preserved regardless of shape anisotropy in different orientations. Magnetic anisotropy similar to bulk crystals was observed for as-grown films which have not been subject to a postannealing process. [ABSTRACT FROM AUTHOR]

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La1.9Y0.1CuO4, to confirm bulk superconductivity (Tc ≃ 21 K) in its undoped state. The gradual expelling of an external magnetic field is observed over a depth range of ∼ 100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the n-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities. [ABSTRACT FROM AUTHOR]

We report a study of transport and magnetic properties of high-quality c axis-oriented superconducting 'T'-RE2-xCexCuO4 (RE = La, Pr, Nd) thin films grown by molecular beam epitaxy at optimal as well as J: = 0.00 doping levels. While the superconducting coherence length &xgr;GL decreases from RE = Nd to La, it remains nearly constant as a function of doping. For optimally doped T'-cuprates, the electronic conductivity is higher than the dopant-free sample, while Tc is higher for the dopant-free samples for RE = Nd and Pr. We estimated the superconducting penetration depth &lgr;ab by using a relationship obtained from optical measurements and compared it to the diamagnetic response, which corresponds to the superfluid density ns. Our data suggest that electron doping increases the superfluid density ns significantly, but the pairing interaction is not affected. In the normal state, the temperature dependence of the resistivity was used to deduce the Fermi temperature Tp, from which we estimated the charge-carrier concentration n. A systematic relationship between n, ns, and the superconducting transition temperature Tc was not observed. We finally conclude that the superconducting ground state is merely influenced by Ce doping with respect to Tc and &xgr;GL but solely by the ÄE element. [ABSTRACT FROM AUTHOR]

We have investigated the insulator-metal transition (IMT) of the 5d system Sr2IrO4 by optical spectroscopy. Change of the band filling as well as tuning of the spin-orbit interaction (SOI) strength has been realized in single crystalline thin films with continuous cation substitutions. Upon the IMT, the spectral weight of the Mott-gap region is transferred into the in-gap states, and its transfer rate was found to be enhanced similarly to those for the correlated electron system close to the IMT boundary. In particular, the SOI-controlled IMT leads to a characteristic spectral evolution as observed for bandwidth-controlled IMT. The observed feature unambiguously demonstrates the cooperative role of SOI and electron-correlation in realizing the Mott insulator state of Sr2IrO4. [ABSTRACT FROM AUTHOR]

The interface magnetic state in the garnet superlattices has been investigated by measuring the magnetization-induced second harmonic generation. Among the superlattice films being composed of sequentially stacked two or three different compositions of garnet layers including a magnetic yttrium iron garnet, while the centrosymmetric bicolor superlattices show little magnetic field effect, the tricolor superlattice exhibits a large nonlinear Kerr rotation manifesting a polar magnetic state formed at the interface. Furthermore, we found that the interface magnetic state is switchable by a tiny magnetic field (∼5 mT) over a wide temperature range up to about 550 K, and the material itself is visibly transparent, which will promise a potential magnetoelectric optical element. [ABSTRACT FROM AUTHOR]

Abstract: Na x CoO2 [x =0.51, 0.54, and 0.59] thin films have been grown on SrTiO3 (100)-oriented single crystals with a 5° vicinal cut towards [010] by pulsed laser deposition. We analysed the films by X-ray diffractometry, atomic force microscopy (AFM), and dc-transport measurements. X-ray diffraction patterns of the films show single phase and c-axis textured growth with the film plane closely aligned to the [001]-direction of 5° miscut SrTiO3 (001) substrates. In addition to the structural analysis of these films we performed transport measurements along and perpendicular to the substrate tilt direction and determined the resistivity anisotropy as a function of temperature. The results enable the development of a strategy for the fabrication of Na x CoO2 based thermoelectric thin film devices. [Copyright &y& Elsevier]

Abstract: Single phase thin films of Na x CoO2 have been grown by pulsed laser deposition. The growth conditions were studied based on the log p O 2 –1/T phase diagram using different types of substrate materials. For Na0.58CoO2 metallic behavior is found down to 4.2 K. [Copyright &y& Elsevier]

Abstract: Single phase, c-axis oriented infinite layer (IL) Sr1− x Nd x CuO2 thin films were epitaxially grown on (110) DyScO3 substrates by molecular beam epitaxy (MBE). Electron impact emission spectroscopy (EIES) is the tool of choice for a stringent stoichiometry control which is essential for Sr1− x Nd x CuO2 thin film preparation. As-grown films contain excess oxygen and therefore a reduction process is necessary to induce superconductivity. In the present study special attention was paid to the underdoped region since the optimization of the reduction process becomes more difficult for lower doping levels x. Sr0.95Nd0.05CuO2 films (underdoped) became superconducting with =30K and =11.4K after an annealing procedure. [Copyright &y& Elsevier]

Sr2RuO4 is a superconductor with a low transition temperature (Tc∼1 K), but has a unique character of p-wave symmetry of the order parameter, demanding epitaxial thin films for Josephson junction studies. We have grown c-axis oriented epitaxial films by a pulsed laser deposition technique on (001) surfaces of (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 substrates. Careful tuning of growth conditions yielded in a layer-by-layer growth of high crystallinity films. The films show a residual resistivity ratio (ρ300 K/ρ2 K) as large as 82 and a superconducting transition with a zero resistivity at 0.6 K. [ABSTRACT FROM AUTHOR]

Epitaxial Y3Fe5O12 thin films were grown on (111) Gd3Ga5O12 substrates by pulsed laser deposition and subsequent annealing above 1270 K. The as-grown films deposited at temperatures below 1070 K were amorphous and the absence of trivalent iron ions has been verified by electron spin resonance spectroscopy. The ex situ annealing above 1270 K in oxygen atmosphere converted the films into epitaxial ones with ferrimagnetic properties. The higher temperature annealing up to 1670 K enabled the higher TC and the shorter lattice constant approaching those of bulk, while the saturation magnetization stayed at 2μB/f.u. at 300 K. Slightly expanded lattice constants and reduced saturation magnetization values may originate from oxygen deficient crystal structures. Nonetheless, the epitaxial films preserved a fairly smooth surface with a root mean square roughness below 2 nm. [ABSTRACT FROM AUTHOR]

Abstract: Layered transition metal oxides AMO2 (A: alkaline, M: transition metal) may show novel electronic properties: the cobaltate Na x CoO2 · yH2O for example is a possible spin-triplet superconductor. Here, we report on the fabrication of isostructural polycrystalline Na x RhO2 (with x =0.66 and x =1.0) and its magnetic and structural properties measured by neutron powder diffraction and SQUID-magnetometry. [Copyright &y& Elsevier]

The observation of superconductivity in the layered transition metal oxide NaxCoO2·yH2O [K. Takada et al., Nature (London) 422, 53 (2003)] has caused a tremendous upsurge of scientific interest due to its similarities and its differences to the copper based high-temperature superconductors. Two years after the discovery, we report the fabrication of single-phase superconducting epitaxial thin films of Na0.3CoO2·1.3D2O grown by pulsed laser deposition technique. This opens additional roads for experimental research exploring the superconducting state and the phase diagram of this unconventional material. [ABSTRACT FROM AUTHOR]

In order to realize superconductivity in cuprates with the T'-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T'-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd2-xCexCuO4 (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO2 plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd2CuO4 film is doped with electrons despite the absence of the Ce substitution. [ABSTRACT FROM AUTHOR]

A correction to the article "Epitaxial Growth of NaxCoO2 Thin Films By Pulsed-Laser Deposition" that was previously published in the journal "Applied Physics Letters" is presented.

Abstract: Recent reports on superconductivity in parent compounds [RE = Pr, Nd, Sm, Eu, Gd] prepared by metal-organic decomposition (MOD) shed new light on the electronic and magnetic phase diagram of electron-doped cuprates. A thorough reduction process is the key to inducing superconductivity in square-planar coordinated cuprates. In this work, Pr2CuO4 films were grown epitaxially by molecular beam epitaxy (MBE) on (001)SrTiO3, (110)GdScO3 and (110)DyScO3 substrates. A modified two-step reduction process is required in order to obtain single-phase superconducting samples. The optimized superconducting properties are , while the room-temperature resistivity is about . Hence, we show that even amorphous Pr2CuO y films deposited on (110)DyScO3 become superconducting after the application of the two-step annealing process. These results indicate that superconductivity is induced to Pr2CuO4 by systematic optimization of the reduction conditions, irrespective of the synthesis route. [Copyright &y& Elsevier]

We have observed a ferromagnetic-induced piezoresistance component in a micromechanical cantilever inte-grated with a GaMnAs piezoresistor. The temperature and the magnetic-field dependence of the piezoresistance around the ferromagnetic transition reveal that the piezoresistance is modulated by ferromagnetic ordering. The sign of the ferromagnetic-induced component of the piezoresistance is opposite to that of the conventional piezoresistance, though of comparable magnitude. This ferromagnetic-induced piezoresistance has a delayed response to mechanically induced strain, with an experimentally determined response time of 230 ± 35 ns. [ABSTRACT FROM AUTHOR]

We have investigated the 3d orbital excitations in CaCuO_{2} (CCO), Nd_{2}CuO_{4} (NCO), and La_{2}CuO_{4} (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d_{xy} orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.

High-temperature superconductivity appears in cuprate materials that have been tuned in a way where the copper-oxygen bond configuration and coordination is in a state of minimal energy. In competition with the Jahn-Teller effect, which impedes the formation of infinitely connected CuO 2 planes, the state of minimal energy persists for planar copper-oxygen bond length variations of up to 10%. We have synthesized the infinite-layer phases of CaCuO 2 and SrCuO 2 as single-crystalline films using molecular beam epitaxy and performed in-plane scanning transmission electron microscopy mapping. For the infinite-layer phase of CaCuO 2 with a short Cu-O bond length, the CuO 2 planes maintain their minimal energy by forming distinguished atomic stripes. In contrast, atomic stripe formation does not occur in the infinite-layer phase of SrCuO 2 , which has a larger Cu-O bond length. The polar field provided by the charge reservoir layer in cuprates with infinitely connected CuO 2 planes holds the key over the emergence of superconductivity and is vital to maintain infinitely connected CuO 2 planes themselves., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO 3 . We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO 3 films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions-the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm 2 V -1 s -1 -combined with first-principles electronic structure calculations establishes SrRuO 3 as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

Magnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (T C ), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr 3 OsO 6 , has the highest T C (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with J eff  = 3/2 driven by the large spin-orbit coupling of Os 6+ 5d 2 orbitals. Moreover, the Sr 3 OsO 6 films are epitaxially grown on SrTiO 3 substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications.

In many of today's most interesting materials, strong interactions prevail upon the magnetic moments, the electrons, and the crystal lattice, forming strong links between these different aspects of the system. Particularly, in two-dimensional cuprates, where copper is either five- or six-fold coordinated, superconductivity is commonly induced by chemical doping which is deemed to be mandatory by destruction of long-range antiferromagnetic order of 3d(9) Cu(2+) moments. Here we show that superconductivity can be induced in Pr2CuO4, where copper is four-fold coordinated. We induced this novel quantum state of Pr2CuO4 by realizing pristine square-planar coordinated copper in the copper-oxygen planes, thus, resulting in critical superconducting temperatures even higher than by chemical doping. Our results demonstrate new degrees of freedom, i.e., coordination of copper, for the manipulation of magnetic and superconducting order parameters in quantum materials.