arXiv subjects

Ivan Bozovic

Publications and source records attributed to Ivan Bozovic.

12 recordsLinked to original sources

arXiv2025

Anisotropic electron damping and energy gap in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

The many body electron-electron interaction in cuprates causes the broadening of the electronic bands in \textit{\textbf{k}}-space, leading to a deviation from the standard Fermi liquid. While a \textit{\textbf{k}}-dependent anisotropic electronic scattering (\textit{\textbf{k}}-DAES) has been assessed by photoemission, its fingerprint in \textit{\textbf{Q}}-space has been scarcely considered. Here, we explore the \textit{\textbf{Q}}-dependent electron dynamics in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ through the evolution of low-energy charge excitations as measured by resonant inelastic x ray scattering (RIXS). In the normal state, the RIXS spectra display a continuum of excitations down to 0~meV, while the superconducting state features a spectral weight suppression below 80 meV without any enhancement at higher energies. To interpret the energy and \textit{\textbf{Q}}-evolution of our data, we introduce a phenomenological expression of the charge susceptibility by including the \textit{\textbf{k}}-DAES. We show that only the charge susceptibility with \textit{\textbf{k}}-DAES captures the RIXS data, highlighting the importance of \textit{\textbf{k}}-DAES when describing the \textit{\textbf{Q}}-dependence of charge excitations from 0 to few eV scale. Furthermore, we also find that the inclusion of \textit{\textbf{k}}-DAES is essential when quantitative parameters such as the electronic energy gap are extracted from RIXS data.

cond-mat.str-el↗

arXiv2025

Decoupling momentum and energy relaxation rates in cuprate strange metals via giant THz nonlinearities

Understanding the $T$-linear normal-state resistivity of cuprates remains a central physics challenge. The associated momentum relaxation rate, $\Gamma_M$, saturates near the conjectured ``Planckian" bound $\Gamma_M\sim kT/\hbar$, but the mechanism underlying the anomalous scattering remains unresolved. Here we employ nonlinear terahertz spectroscopy to systematically study La$_{2-x}$Sr$_x$CuO$_4$ across a broad temperature and doping range. We measure the normal-state third-order susceptibility, $|\chi^{(3)}|\approx 6\times10^{-9}$ m$^2$/V$^2$, among the largest in the THz regime, enabling direct access to the rarely measured electronic energy relaxation rate, $\Gamma_E$. Strikingly, $\Gamma_E$ is 10-40 times smaller than $\Gamma_M$, revealing that the scatterings responsible for momentum loss and $T$-linear resistivity do not remove appreciable energy from the electrons. While $\Gamma_M (T)$ is consistent with quasi-elastic scattering from bosonic modes above their characteristic energy scale, this is incompatible with the increasing temperature dependence of $\Gamma_E(T)$. Our results exclude phonons as the source of $T$-linear resistivity and impose strong constraints on possible mechanisms.

cond-mat.supr-con↗

arXiv2022

Evolution of the cyclotron mass with doping in La$_{2-x}$Sr$_x$CuO$_4$

The recent observation of cyclotron resonance in optimally-doped La$_{2-x}$Sr$_x$CuO$_4$ using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. One can measure the cyclotron mass in more disordered cuprates possesing short scattering times therefore expanding the study to materials and dopings in which quantum oscillations have not been observed. Here we present the measurement of the carrier mass of the hole doped cuprate La$_{2-x}$Sr$_x$CuO$_4$ across a a range of dopings spanning the slightly underdoped ($x=0.13$) to highly overdoped ($x=0.26$), near to the termination of the superconducting dome. These results reveal a systematic increase of $m_c$ with doping, up to values greater than thirteen times the bare electron mass. This is in contrast with those masses extracted from the heat capacity, which show a peak near the pseudogap critical point $p^*$ and/or Lifshitz transition. The cyclotron frequency is linear in field up to 31 T for all dopings giving no evidence for field induced Fermi surface reconstructions. The cyclotron mass is found to be positive for all dopings, but with a magnitude systematically below the heat capacity mass for under and optimally doped samples, while exceeding it for overdoped samples. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite frequency resonance is -- in conventional theories -- a {\it topological quantity} only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near $p^*$ nor near the Lifshitz transition, showing that any singularity -- if it exists -- is not strong enough to affect the cyclotron mass.

cond-mat.supr-con↗

arXiv2021

Effect of radiation-induced defects on the superfluid density and optical conductivity of overdoped La$_{2-x}$Sr$_x$CuO$_4$

Using a combination of time-domain THz spectroscopy (TDTS) and mutual inductance measurements, we have investigated the low-energy electrodynamic response of overdoped La$_{2-x}$Sr$_x$CuO$_4$ films that have been exposed to ion irradiation. Films went through three rounds of irradiation (2, 4, and 6 $\times 10^{13}$ ions/cm$^2$) and mutual inductance and TDTS experiments were performed between each step. Together with the as-grown film, this gives four different levels of disorder. The transport scattering rate that is measured directly in the THz experiments is an approximately linear function of the radiation dose at all temperatures. This is consistent with a proportionate increase in elastic scattering. In the superconducting state we find that the relation between $T_c$, the superfluid density, and the scattering rates are quantitatively at odds with the predictions based on the extant theory of Abrikosov-Gorkov-like pair breaking in a dirty $d$-wave superconductor. Increasing disorder causes only a small change in the superconducting transition temperature for the overdoped films, but the changes to the $\omega \sim 0$ superfluid density are much larger.

cond-mat.supr-con↗

arXiv2021

Homogeneous superconducting gap in DBCO synthesized by oxide molecular beam epitaxy

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film. It is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one corresponding to the superconductivity and the other one to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

cond-mat.supr-con↗

arXiv2020

Electron pairing in the pseudogap state revealed by shot noise in copper-oxide junctions

In the quest to understand high-temperature superconductivity in copper oxides, a vigorous debate has been focused on the pseudogap - a partial gap that opens over portions of the Fermi surface in the 'normal' state above the bulk critical temperature ($T_{c}$). The pseudogap has been attributed to precursor superconductivity, to the existence of preformed pairs, or to competing orders such as charge-density waves. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among these alternatives. Here, we report measurements of the shot noise of tunneling current in high-quality La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$/La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO/LCO/LSCO) heterostructures fabricated using atomic-layer-by-layer molecular beam epitaxy, for several doping levels. The data delineate three distinct regions in the bias voltage-temperature ($V-T$) space. Well outside the superconducting gap region, the shot noise agrees quantitatively with independent tunneling of charge-e carriers. Deep within the gap, shot noise is greatly enhanced, reminiscent of multiple Andreev reflections. Starting above $T_{c}$ and extending to biases much larger than the gap, there is a broad region in which the noise substantially exceeds the expectations of single-charge tunneling, indicating pairing of carriers. Pairs are detectable deep into the pseudogap region of temperature and bias.

cond-mat.supr-con↗

arXiv2019

Locating the missing superconducting electrons in overdoped cuprates

Overdoped high-temperature cuprate superconductors have been widely believed to be described by the physics of d-wave BCS-like superconductivity. However, recent measurements indicate that as the doping is increased, the superfluid density decreases smoothly to zero rather than increasing as expected by BCS theory in the absence of disorder. 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 both the superfluid and the uncondensed carrier density as a function of doping. A significant fraction of the carriers remains uncondensed in a wide Drude-like peak even as $T\rightarrow0$, which, when taken with the linear-in-temperature superfluid density, is inconsistent with existing theories for the role of disorder in suppressing the superfluid density in a d-wave superconductor. Our almost eight orders of magnitude in 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.

cond-mat.supr-con↗

arXiv2017

Optical Nanoscopy of High-Tc Cuprate Nano-Constriction Devices Patterned by Helium Ion Beams

Helium-ion beams (HIB) focused to sub-nanometer scales have emerged as powerful tools for high-resolution imaging as well as nano-scale lithography, ion milling or deposition. Quantifying irradiation effects is essential for reliable device fabrication but most of the depth profiling information is provided by computer simulations rather than experiment. Here, we use atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB. By imaging the infrared dielectric response we find that amorphization caused by the nominally 0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film and by about 500 nm laterally. This unexpectedly widespread structural and electronic damage can be attributed to a Helium depth distribution substantially modified by internal device interfaces. Our study introduces AFM-SNOM as a quantitative nano-scale tomographic technique for non-invasive 3D characterization of irradiation damage in a wide variety of devices.

cond-mat.mes-hall↗

arXiv2016

The role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors

We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor.

cond-mat.str-el↗

arXiv2015

Frequency dispersion of nonlinear response of thin superconducting films in Berezinskii-Kosterlitz-Thouless state

The effects of microwave radiation on the transport properties of atomically thin $La_{2-x}Sr_xCuO_4$ films were studied in the 0.1-13 GHz frequency range. Resistance changes induced by microwaves were investigated at different temperatures near the superconducting transition. The nonlinear response decreases by several orders of magnitude within a few GHz of a cutoff frequency $ν_{cut} \approx$ 2 GHz. Numerical simulations that assume an ac response to follow the dc V-I characteristics of the films reproduce well the low frequency behavior, but fail above $ν_{cut}$. The results indicate that two-dimensional superconductivity is resilient against high-frequency microwave radiation, because vortex-antivortex dissociation is dramatically suppressed in two-dimensional superconducting condensates oscillating at high frequencies.

cond-mat.supr-con↗

arXiv2009

Anomalous Expansion of the Copper-Apical Oxygen Distance in Superconducting La$_{2}$CuO$_{4}$ - La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ 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-Ångstrom resolution the complete atomic structure of an ultrathin superconducting bilayer film, composed of La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ and La$_{2}$CuO$_{4}$ neither of which is superconducting by itself. The results show that phase-retrieval diffraction techniques enable accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that close to the sample surface the unit cell size remains constant while the copper-apical oxygen distance shows a dramatic increase, by as much as 0.45 Å. The apical oxygen displacement is known to have a profound effect on the superconducting transition temperature.

cond-mat.mtrl-sci↗

arXiv2006

Microstructure of strained La2CuO4+delta thin films on varied sub-strates

Layered perovskite La2CuO4+delta(LCO) thin films were epitaxially grown on SrTiO3 (STO) and LaSrAlO4 (LSAO) substrates by atomic-layer-by-layer molecular beam epitaxy. The lattice de-fects and residual strain in these films were investigated by means of transmission electron mi-croscopy and electron energy loss spectroscopy. The LCO films showed a high epitaxial quality with flat interfaces and top surfaces. Misfit dislocations with Burgers vector a<010> and shear defects were frequently observed at or near the film/substrate interfaces and in the films, respec-tively, for all the LCO films. In one LCO film, grown on STO at the highest temperature, 700 oC, was a two layered structure. The top layer was identified as rhombohedral LaCuO2 by electron energy loss spectroscopy combined with electron diffraction. In addition, stacking faults were observed in plane-view micrographs of one LCO film grown on the STO substrate. The residual strains were evaluated from the spacing of misfit dislocations at the film/substrate interface and from the split electron diffraction patterns. Possible mechanisms of strain relaxations are dis-cussed based on the observed defects.

cond-mat.mtrl-sci↗