arXiv subjects

G. Logvenov

Publications and source records attributed to G. Logvenov.

52 records · Page 3Linked to original sources

arXiv2013

Magnetic phase diagram of low-doped La{2-x}SrxCuO4 thin films studied by low-energy muon-spin rotation

The magnetic phase diagram of La2-xSrxCuO4 thin-films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Neel temperature TN is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase (x>0.02) the transition temperature to the cluster spin-glass state Tg is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed.

cond-mat.mtrl-sci↗

arXiv2013

Orbital control of noncollinear magnetic order in nickelate heterostructures

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and quantum confinement of the valence electrons. The data are discussed in the light of recent theoretical predictions.

cond-mat.str-el↗

arXiv2012

Emergence of superconductivity from the dynamically heterogeneous insulating state in La_{2-x}Sr_{x}CuO_{4}

A central issue for copper oxides is the nature of the insulating ground state at low carrier densities and the emergence of high-temperature superconductivity from that state with doping. Even though this superconductor-insulator transition (SIT) is a zero-temperature transition, measurements are not usually carried out at low temperatures. Here we use magnetoresistance to probe both the insulating state at very low temperatures and the presence of superconducting fluctuations in La_{2-x}Sr_{x}CuO_{4}(LSCO) films, for doping levels that range from the insulator to the superconductor (x=0.03-0.08). We observe that the charge glass behavior, characteristic of the insulating state, is suppressed with doping, but it coexists with superconducting fluctuations that emerge already on the insulating side of the SIT. The unexpected quenching of the superconducting fluctuations by the competing charge order at low temperatures provides a new perspective on the mechanism for the SIT.

cond-mat.supr-con↗

arXiv2011

History dependent magnetoresistance in lightly doped La_{2-x}Sr_{x}CuO_{4} thin films

The in-plane magnetoresistance (MR) in atomically smooth La_{2-x}Sr_{x}CuO_{4} thin films grown by molecular-beam-epitaxy was measured in magnetic fields B up to 9 T over a wide range of temperatures T. The films, with x=0.03 and x=0.05, are insulating, and the positive MR emerges at T<4 K. The positive MR exhibits glassy features, including history dependence and memory, for all orientations of B. The results show that this behavior, which reflects the onset of glassiness in the dynamics of doped holes, is a robust feature of the insulating state.

cond-mat.str-el↗

arXiv2011

Graded Orbital Occupation near Interfaces in a La2NiO4 - La2CuO4 Superlattice

X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6 octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni L edge, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.

cond-mat.str-el↗

arXiv2011

Distinct oxygen hole doping in different layers of $\rm Sr_{2}CuO_{4-δ}-La_{2}CuO_4$ superlattices

X-ray absorption in $\rm Sr_{2}CuO_{4-δ}-La_{2}CuO_4$ (SCO-LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for $x \lesssim x_{optimal}$ in bulk $\rm La_{2-x}Sr_{x}CuO_4$ and suggests that this hole state is on apical oxygen atoms and polarized in the $a-b$ plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft X-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice.

cond-mat.supr-con↗

arXiv2011

Superconductivity in La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4) superlattices

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 the layered structure and the difference in the chemical potential between LCO and LSCO, as well as Sr interdiffusion. Using a pancake-vortex model we could estimate a strict upper limit of the London penetration depth to 380 nm in these superlattices. The temperature dependence of the muon depolarization rate in field cooling experiments is very similar to what is observed in intercalated BSCCO and suggests that vortex-vortex interaction is dominated by electromagnetic coupling but negligible Josephson interaction.

cond-mat.supr-con↗

arXiv2011

Magnetism in the 2D Limit and Interface Superconductivity in Metal-Insulator La(2-x)Sr(x)CuO(4) Superlattices

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.

cond-mat.str-el↗

arXiv2011

Temporal correlations of superconductivity above the transition temperature in La$_{2-x}$Sr$_x$CuO$_4$ probed by terahertz spectroscopy

The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity. In this work we use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above the critical temperature (T$_c$) in La$_{2-x}$Sr$_x$CuO$_4$ thin films over a doping range that spans almost the entire superconducting dome (x = 0.09 to 0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above Tc. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge transport anomalies of the pseudogap in LSCO at temperatures well above T$_c$.

cond-mat.supr-con↗

arXiv2011

On the possibility of fast vortices in the cuprates: A vortex plasma model analysis of THz conductivity and diamagnetism in La$_{2-x}$Sr$_{x}$CuO$_4$

We present measurements of the fluctuation superconductivity in an underdoped thin film of La$_{1.905}$Sr$_{0.095}$CuO$_4$ using time-domain THz spectroscopy. We compare our results with measurements of diamagnetism in a similarly doped crystal of La$_{2-x}$Sr$_x$CuO$_4$. We show through a vortex-plasma model that if the fluctuation diamagnetism solely originates in vortices, then they must necessarily exhibit an anomalously large vortex diffusion constant, which is more than two orders of magnitude larger than the Bardeen-Stephen estimate. This points to either the extremely unusual properties of vortices in the under-doped d-wave cuprates or a contribution to the diamagnetic response that is not superconducting in origin.

cond-mat.supr-con↗

arXiv2011

Photoinduced melting of superconductivity in the high-Tc superconductor La2-xSrxCuO4 probed by time-resolved optical and THz techniques

Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4 thin films is investigated utilizing optical pump-probe and optical pump - THz probe techniques as a function of temperature and excitation fluence. The absorbed energy density required to suppress superconductivity is found to be about 8 times higher than the thermodynamically determined condensation energy density and nearly temperature independent between 4 and 25 K. These findings indicate that during the time when superconducting state suppression takes place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the phonons with energy lower than twice the maximum value of of the SC gap and only 10% is spent on Cooper pair breaking.

cond-mat.supr-con↗

arXiv2009

Superconducting transition at 38 K in insulating-overdoped La2CuO4-La1.64Sr0.36CuO4 superlattices: Evidence for interface electronic redistribution from resonant soft x-ray scattering

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.

cond-mat.str-el↗

arXiv2008

High-temperature interface superconductivity between metallic and insulating cuprates

High-temperature superconductivity confined to nanometer-size interfaces has been a long standing goal because of potential applications^{1,2} and the opportunity to study quantum phenomena in reduced dimensions^{3,4}. However, this is a challenging target: in conventional metals the high electron density restricts interface effects such as carrier depletion/accumulation to a region much narrower than the coherence length, the scale necessary for superconductivity to occur. In contrast, in copper oxides the carrier density is low while the critical temperature (T_c) is high and the coherence length very short; so, this provides a breakthrough opportunity but at a price: the interface must be atomically perfect. Here we report on superconductivity in bilayers consisting of an insulator (La_2CuO_4) and a metal (La_{1.55}Sr_{0.45}CuO_{4}), neither of which is superconducting in isolation. However, in bilayers T_c is either ~15 K or ~30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm from the interface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and this enhanced superconductivity is also shown to originate from the interface layer about 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observed previously^5 but the essential role of the interface was not recognized at the time. Our results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi two-dimensional high T_c phases and to significantly enhance superconducting properties in known or new superconductors.

cond-mat.supr-con↗

arXiv2007

Surface Structure Analysis of Atomically Smooth BaBiO$_3$ Films

Using low energy Time-of-Flight Scattering and Recoil Spectroscopy (TOF-SARS) and Mass Spectroscopy of Recoiled Ions (MSRI) we analyze the surface structure of an atomically smooth BaBiO$_3$ film grown by molecular beam epitaxy. We demonstrate high sensitivity of the TOF-SARS and MSRI spectra to slight changes in the orientation of the ion scattering plane with respect to the crystallographic axes. The observed angle dependence allows us to clearly identify the termination layer as BiO$_2$. Our data also indicate that angle-resolved MSRI data can be used for high resolution studies of surface structure of complex oxide thin films.

cond-mat.other↗

arXiv2004

Half - h/2e - Oscillations of SQUIDs

The current-voltage characteristics of Superconducting Quantum Interference Devices (SQUIDs) are known to modulate as a function of applied magnetic field with a period of one flux quantum $Φ_0=h$/2e. Here we report on the fabrication and properties of SQUIDs modulating with a period of $1/2\timesΦ_0$. The characteristics of these bicrystal SQUIDs are consistent with either a strong sin(2$ϕ$) component of the current-phase relation of the Josephson current, or with an interaction between the Cooper-pairs, causing an admixture of quartets to the condensate.

cond-mat.supr-con↗

arXiv2002

A Structural Probe of the Doped Holes in Cuprate Superconductors

An unresolved issue concerning cuprate superconductors is whether the distribution of carriers in the CuO(2) plane is uniform or inhomogeneous. Because the carriers comprise a small fraction of the total charge density and may be rapidly fluctuating, modulations are difficult to detect directly. Here we demonstrate that in anomalous x-ray scattering at the oxygen K edge of the cuprates, the contribution of carriers to the scattering amplitude is selectively magnified 82 times. This enhances diffraction from the doped holes by more than 10^3, permitting direct structural analysis of the superconducting ground state. Scattering from thin films of La(2)CuO(4+x) (Tc = 39 K) at T=(50 +/- 5) K on the reciprocal space intervals (0,0,0.21) -> (0,0,1.21) and (0,0,0.6) -> (0.3,0,0.6) show a rounding of the carrier density near the substrate suggestive of a depletion zone or similar effect. The structure factor for off-specular scattering was less than 3x10-7 e, suggesting an absence of in-plane hole ordering in this material.

cond-mat.supr-con↗