Ivan Božović - Academia.edu (original) (raw)

Papers by Ivan Božović

$Research paper thumbnail of Ultrafast Bragg Coherent Diffraction Imaging of Epitaxial Thin Films using Deep Complex-valued Neural Networks$

Research Square (Research Square), Feb 21, 2023

Frontiers in Physics, 2022

We summarize the basic ideas of our topological model of the pseudogap state of high temperature ... more We summarize the basic ideas of our topological model of the pseudogap state of high temperature superconductors (HTS) as a condensate of charged magnetic monopoles, with a focus on new experimental signatures. These include the surface quantum Hall effect, the generation of electric fields when applying magnetic fields by the oblique Meissner effect, and the generation of circular electric fields surrounding electric currents by the oblique Ampère law.

Journal of Superconductivity and Novel Magnetism, 2016

Journal of Superconductivity and Novel Magnetism, 2015

Scientific Reports, 2017

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (Tc... more In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (Tc) has a dome-shaped doping dependence. The cause of demise of bothTcand superfluid densitynson the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La2−xSrxCuO4(LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanation — competition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing single-crystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static mag...

Physica C: Superconductivity and its Applications, 2014

La 2-x Sr x CuO 4 nanowire devices have been fabricated and characterized using electrical transp... more La 2-x Sr x CuO 4 nanowire devices have been fabricated and characterized using electrical transport measurements. Nanowires with widths down to 80 nm are patterned using high-resolution electron beam lithography. However, the narrowest nanowires show incomplete superconducting transitions with some residual resistance at T = 4 K. Here, we report on the refinement of the fabrication process to achieve narrower nanowire devices with complete superconducting transitions, opening the path to the study of novel physics arising from dimension-limited superconductivity on the nanoscale.

Molecular beam epitaxy enables one to synthesize HTS thin films with rms surface roughness in the... more Molecular beam epitaxy enables one to synthesize HTS thin films with rms surface roughness in the range 0.2-0.5 nm, much less than the unit cell height (1-2 nm).^1 One can also make atomically smooth multilayers and superlattices in which HTS or spacer layers can be just one unit cell thick if so desired. A detailed study of transport properties of

Journal of Physics A: Mathematical and General, 1982

For pt.III see ibid., vol.14, p.1825-34, (1981). The irreducible representations of the line grou... more For pt.III see ibid., vol.14, p.1825-34, (1981). The irreducible representations of the line groups (constructed in the preceding papers of this series) provide a useful labelling scheme for electron energy bands of stereo-regular polymers and quasi one-dimensional solids. To implant automatic symmetry assignation into a band-structure computing routine one needs a method of dealing with representations of line groups, which are of infinite order. The author demonstrates that the set K of coset representatives of the translational subgroup T of the line group L-which has a finite (and usually quite small) number of elements-contains all the relevant information. Dealing only with K, one can complete the symmetry assignation, decompose a reducible representation and derive selection rules.

Journal of Physics A: Mathematical and General, 1981

For pt.II see ibid., vol.14, no.4, p.777 (1981). Systematic degeneracies of electron and phonon e... more For pt.II see ibid., vol.14, no.4, p.777 (1981). Systematic degeneracies of electron and phonon energy spectra of stereoregular polymers are determined by the dimensions of unitary irreducible representations (reps) of line groups (L), as far as the spatial symmetries are concerned. However, time reversal ( theta ) invariance of the Hamiltonian can bring in some extra degeneracies. These are determined for the reps of all the line groups. A physical interpretation of the results is given in terms of the action of theta onto the quantum numbers arising from line-group symmetry. Finally, corepresentations of the groups LA=L+ theta L and the corresponding symmetry-adapted bases are also obtained.

Zeitschrift für Physik B Condensed Matter, 1993

$Research paper thumbnail of In situ monitoring during pulsed laser deposition of complex oxides using reflection high energy electron diffraction under high oxygen pressure$

Applied Physics Letters, 1997

A suitable in situ monitoring technique for growth of thin films is reflection high energy electr... more A suitable in situ monitoring technique for growth of thin films is reflection high energy electron diffraction (RHEED). Deposition techniques, like pulsed laser deposition (PLD) and sputter deposition, used for fabrication of complex oxide thin films use relatively high oxygen pressures (up to 100 Pa) and are, therefore, not compatible with ultrahigh vacuum RHEED equipment. We have developed a RHEED system which can be used for growth monitoring during the deposition of complex oxides at standard PLD conditions. We are able to increase the deposition pressure up to 50 Pa using a two-stage differential pumping system. Clear RHEED patterns are observable at these high pressures. The applicability of this system is demonstrated with the study of homoepitaxial growth of SrTiO3 as well as the heteroepitaxial growth of YBa2Cu3O7−δ on SrTiO3. Intensity oscillations of the RHEED reflections, indicating two-dimensional growth, are observed up to several tens of nanometers film thickness in ...

Applied Physics Letters, 1990

We report the growth of untwinned epitaxial thin films of Bi-Sr-Ca-Cu-O by atomically layered het... more We report the growth of untwinned epitaxial thin films of Bi-Sr-Ca-Cu-O by atomically layered heteroepitaxy on SrTiO3 substrates. These films are c-axis oriented as-layered and do not exhibit 90° in-plane defects, i.e., a-b ‘‘twinning.’’ By misorienting the surface normal from {100} by approximately 4° towards 〈111〉, the cubic symmetry of the {100} surface is adequately broken to completely align the b axis of the superconducting film with respect to the substrate. Reflection high-energy electron diffraction patterns observed during growth and post-growth x-ray diffraction analysis indicate that the incommensurate structural modulation occurs along the same direction as the step edges.

Applied Physics Letters, 1998

We report the results of magnetization measurements on pseudomorphic (fully strained) c-axis orie... more We report the results of magnetization measurements on pseudomorphic (fully strained) c-axis oriented colossal magnetoresistance manganite thin films grown by molecular beam epitaxy. We observe uniaxial magnetic anisotropy (hard axis/easy plane) with the easy plane being the film plane. Within the plane a weaker biaxial anisotropy is observed with [100] (Mn–O bond direction) easy axes. The magnetization dependence of the uniaxial anisotropy constant follows the predicted magnetization dependence of the magnetostriction constants within single-ion models indicating that the anisotropy energy is dominated by strain-induced anisotropy from the lattice constant mismatch with the SrTiO3 substrate. These results indicate a magnetostriction constant λ100≈+7×10−5, and an induced orbital moment of at least 0.02μb/Mn ion. We predict that by appropriate substrate selection an equilibrium out-of-plane magnetization can be produced.

Applied Physics Letters, 1996

We have fabricated thin films of La1−xCaxMnOδ with tetragonal symmetry. For low temperatures and ... more We have fabricated thin films of La1−xCaxMnOδ with tetragonal symmetry. For low temperatures and magnetic fields the measured magnetoresistance is anisotropic: initially positive for applied magnetic field perpendicular to the film plane and negative for field applied parallel to the film plane. At high temperatures the magnetoresistance is negative for all fields and field orientations. We also observe an in-plane magnetoresistance anisotropy with an angular dependence corresponding to that observed in transition metal ferromagnets. We suggest an interpretation requiring a substantial spin-orbit interaction in the material.

PNAS Nexus

Electronic anisotropy (“nematicity”) has been detected in cuprate superconductors by various expe... more 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-alig...

SPIE Proceedings, 2012

Molecular beam epitaxy technique has enabled synthesis of atomically smooth thin films, multilaye... more 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 Ångström 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, highresolution 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 CuO 2 plane, evidence for local pairs, and establishing tight limits on the temperature range of superconducting fluctuations.