Enrico Giannini - Academia.edu (original) (raw)

Papers by Enrico Giannini

Research paper thumbnail of First Scanning Tunnelling Spectroscopy on Bi2Sr2Ca2Cu3O10 (Bi2223) single crystals

We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high... more We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high quality Bi2223 crystals. We present atomic resolution and show spectroscopic data acquired on two different samples. In one case, for Tc= 109K and a transition width of only 1K, we obtained an extremely homogeneous sample with a gap value of 60 meV over at least 50 nm. In the other case, the respective parameters were Tc= 111K with a transition width of 1.7K and yielded a slightly less homogeneous sample with a gap of 45 meV. We evidence strong similarities with Bi2Sr2CaCu2O8 (Bi2212) and discuss the doping level of our samples.

Research paper thumbnail of Temperature and time scaling of the peak-effect vortex configuration in FeTe_{0.7}Se_{0.3}

Physical Review B, 2012

An extensive study of the magnetic properties of FeTe 0.7 Se 0.3 crystals in the superconducting ... more An extensive study of the magnetic properties of FeTe 0.7 Se 0.3 crystals in the superconducting state is presented. We show that weak collective pinning, originating from spatial variations of the charge carrier mean free path (δl pinning), rules in this superconductor. Our results are compatible with the nanoscale phase separation observed on this compound and indicate that in spite of the chemical inhomogeneity spatial fluctuations of the critical temperature are not important for pinning. A power law dependence of the magnetization vs time, generally interpreted as signature of single vortex creep regime, is observed in magnetic fields up to 8 T. For magnetic fields applied along the c axis of the crystal the magnetization curves exhibit a clear peak effect whose position shifts when varying the temperature, following the same dependence as observed in YBa 2 Cu 3 O 7−δ .

Research paper thumbnail of Tuning magnetotransport in a compensated semimetal at the atomic scale

Nature communications, 2015

Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides con... more Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and i...

Research paper thumbnail of Crystal-clear – The '2014 Most Superlative Crystal Growth Contest' for School Classes

CHIMIA International Journal for Chemistry, 2014

To celebrate the International Year of Crystallography among the general public, a consortium of ... more To celebrate the International Year of Crystallography among the general public, a consortium of chemists, physicists and crystallographers of the University of Geneva organised in Spring 2014 an incentive crystal growth contest for Geneva scholars aged 4 to 19. Starting from a kit containing a salt and user instructions, classes had to prepare a crystal that met specific criteria according to their category of age. The composition of the salt - potassium dihydrogen phosphate (KDP) - was only disclosed to the participants during the final Awards Ceremony. This contest positively exceeded our expectations with almost 100 participating classes (ca. 1800 participants) and 54 specimens received over all categories.

Research paper thumbnail of Fluorine Substitution in High Temperature Superconductors

International Journal of Modern Physics B, 1999

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. ... more Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. We obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200-400 °C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and

Research paper thumbnail of Growth and characterization of Bi 2 Sr 2 Ca 2 Cu 3 O 10 and (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10   single crystals

Superconductor Science and Technology, 2004

Page 1. Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10− δ single crystal... more Page 1. Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10− δ single crystals This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2004 Supercond. Sci. Technol. 17 220 ...

Research paper thumbnail of Dissipation behaviour of YBa 2 Cu 3 O x single crystals studied through magnetization and magnetization decay measurements

Superconductor Science and Technology, 1993

Page 1. Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magn... more Page 1. Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magnetization decay measurements This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1993 Supercond. Sci. Technol. 6 771 ...

Research paper thumbnail of Reversible melting and equilibrium phase formation of (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10  

Superconductor Science and Technology, 2002

Research paper thumbnail of Growth and superconducting properties of Bi 2 Sr 2 Ca 2 Cu 3 O 10 single crystals

Superconductor Science and Technology, 2004

Single crystals of Bi2Sr2Ca2Cu3O10 (Bi-2223) have been grown using the travelling solvent floatin... more Single crystals of Bi2Sr2Ca2Cu3O10 (Bi-2223) have been grown using the travelling solvent floating zone technique in an image furnace. Annealing the crystals under high pressures of O2 increased their critical temperature to 109 K, and resulted in sharp superconducting transitions of DgrTc = 1 K. The superconducting anisotropy of Bi-2223 was found to be {\sim } 50 , from measurements

Research paper thumbnail of SEM investigation of outgrowths formation mechanism in Ag-sheathed Bi,Pb(2223) tapes

Superconductor Science and Technology, 2002

Research paper thumbnail of Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

Proceedings of the National Academy of Sciences, 2008

The mechanism of electron pairing in high-temperature superconductors is still the subject of int... more The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper-oxygen planes), in the anisotropic electronlattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c-axis value of 30 Å when the light polarization is in the direction of the copper-oxygen bond, but its decay slows down at 45º. These findings suggest a selective dynamical lattice involvement with the anisotropic electron-phonon coupling being on a time scale (1 to 3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase.

Research paper thumbnail of Lower Critical Field and Mixed State Magnetisation of Single Crystal Bi_2Sr_2Ca_2Cu_3O_10

The magnetisation of Bi_2Sr_2Ca_2Cu_3O_10 (Bi-2223) single crystals (Tc = 109 K, DeltaTc = 1 K) h... more The magnetisation of Bi_2Sr_2Ca_2Cu_3O_10 (Bi-2223) single crystals (Tc = 109 K, DeltaTc = 1 K) has been investigated using a SQUID magnetometer. The anisotropy of Bi-2223 is found to be gamma ˜ 50 from measurements of the lower critical field, H_c1, with phHallelphc and phHallelphab. The effects of surface barriers are overcome by sweeping the magnetic field very slowly, allowing an accurate determination of H_c1 with phHallelphc from ˜100 K down to 10 K. We estimate the in-plane penetration depth to be lambda_ab(0) = 1200 ÅThe irreversibility line has been obtained from field-cooled and zero-field-cooled magnetisation data for applied magnetic fields above the second magnetisation peak. A fit to the irreversibility line is achieved using the Lindemann melting criterion for pancake vortices in a Josephson-coupled layered superconductor.

Research paper thumbnail of Charge Carrier Interaction with a Purely Electronic Collective Mode: Plasmarons and the Infrared Response of Elemental Bismuth

Physical Review Letters, 2007

We present a detailed optical study of single crystal bismuth using infrared reflectivity and ell... more We present a detailed optical study of single crystal bismuth using infrared reflectivity and ellipsometry. Colossal changes in the plasmon frequency are observed as a function of temperature due to charge transfer between hole and electron Fermi pockets. In the optical conductivity, an anomalous temperature dependent mid-infrared absorption feature is observed. An extended Drude model analysis reveals that it can be connected to a sharp upturn in the scattering rate, the frequency of which exactly tracks the temperature dependent plasmon frequency. We interpret this absorption and increased scattering as the first direct optical evidence for a charge carrier interaction with a collective mode of purely electronic origin; here electron-plasmon scattering. The observation of a plasmaron as such is made possible only by the unique coincidence of various energy scales and exceptional properties of semi-metal bismuth.

Research paper thumbnail of Preeminent Role of the Van Hove Singularity in the Strong-Coupling Analysis of Scanning Tunneling Spectroscopy for Two-Dimensional Cuprate Superconductors

Physical Review Letters, 2008

In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) whi... more In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-Tc superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with great accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analyses of strong-coupling effects can lead to severe errors. PACS numbers: 68.37.Ef, 74.72.Hs, 74.50.+r Scanning tunneling spectroscopy of Bi-based cuprate high-T c superconductors (HTS) shows a d-wave gap and a strong dip-hump feature which is nearly always stronger for occupied than for empty states. 1 It has been proposed that the dip-hump structure results from the interaction of electrons with a collective mode, 2 but the dip asymmetry has not received an explanation so far. Indeed such a coupling leads to electron-hole symmetric spectra in classical superconductors. 3,4,5 The dip-hump was also observed by photoemission, but in those experiments it is not possible to probe the electron-hole asymmetry. Sometimes photoemission spectra are even symmetrized, 6 thus ignoring the relevance of the asymmetry seen in tunneling. The fact that in two dimensions the density of states (DOS) has a prominent van-Hove singularity (VHS), unlike in 3D, introduces naturally an asymmetry and thereby modifies the strongcoupling analysis and the corresponding determination of the collective mode frequency in an essential way.

Research paper thumbnail of Imaging the Essential Role of Spin Fluctuations in High-T_{c} Superconductivity

Physical Review Letters, 2009

We have used scanning tunneling spectroscopy to investigate short-length electronic correlations ... more We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi2Sr2Ca2Cu3O 10+δ (Bi-2223). We show that the superconducting gap and the energy Ω dip , defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure, and are locally anti-correlated. Based on fits of our data to a microscopic strong-coupling model we show that Ω dip is an accurate measure of the collective mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy, and is most likely the (π, π) spin resonance. PACS numbers: 74.50.+r, 74.20.Mn, 74.72.Hs The presence of phonon signatures in the electron tunneling spectra of classical superconductors [1], and their quantitative explanation by the Eliashberg equations [2, 3], stand among the most convincing validations of the BCS phonon-mediated pairing theory . For high-T c superconductors, the pairing mechanism still remains an intriguing mystery. Several cuprate superconductors present a spectroscopic feature, known as the dip-hump , that resembles the phonon signatures of classical superconductors. There is still no consensus on the origin of the dip-hump nor its connection to high-T c superconductivity. In this work we report on a scanning tunneling microscopy (STM) study of the three-layer compound Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223). We observe that the gap magnitude, a direct measure of the pairing strength, is periodically modulated on a lengthscale of about 5 crystal unit-cells. This variation follows the superstructure, a periodic modulation of atomic positions naturally present in Bi-based cuprates. By fitting the STM data with a strong-coupling model we demonstrate that the dip feature originates from a collective excitation. This allows us to image the collective mode energy (CME) at the atomic scale and reveal a modulation that also follows the superstructure. The CME and the gap are locally anti-correlated. These findings support that the collective mode probed in our study is related to superconductivity, and is most likely the anti-ferromagnetic spin resonance detected by neutron scattering [10]. Our results, in particular the CME value of 30-40 meV, are in agreement with the spin-fluctuation-mediated pairing scenario , in which the spin resonance in high-T c 's is a consequence of pairing.

Research paper thumbnail of Vortex phase diagram and temperature-dependent second-peak effect in overdoped Bi_{2}Sr_{2}CuO_{6+δ} crystals

Physical Review B, 2010

We study the vortex phase diagram of the single-layer Bi2Sr2CuO 6+δ (Bi2201) superconductor by me... more We study the vortex phase diagram of the single-layer Bi2Sr2CuO 6+δ (Bi2201) superconductor by means of bulk magnetization measurements on high-quality oxygen-overdoped crystals. In striking contrast with the results found in the moderately-doped two and three-layer Bi-based cuprates, Bi2201 exhibits a strong temperature-dependent second-peak effect. By means of measurements of the in and out-of-plane first-penetration field we provide direct evidence that this phenomenon is mainly associated to an increase of the electromagnetic anisotropy on warming. The effect of oxygen-doping δ on the vortex phase diagram results in both the irreversibility and second-peak lines shifting to higher temperatures and fields. This enhanced stability of the Bragg glass phase suggests that the interlayer coupling between Cu-O layers increases with δ. In addition, we found that the critical temperature follows the parabolic relation with the number of holes per Cu-O plane that holds for most single and two-layer cuprates.

Research paper thumbnail of Effect of oxygen-doping on Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ} vortex matter: Crossover from electromagnetic to Josephson interlayer coupling

Physical Review B, 2008

We study the effect of oxygen-doping on the critical temperature, T c , the vortex matter phase d... more We study the effect of oxygen-doping on the critical temperature, T c , the vortex matter phase diagram and the nature of the coupling mechanism between the Cu-O layers in the three-layer Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223) compound. Contrary to previous reports, in the overdoped (OD) regime we do find a variation of T c with increasing the oxygen partial-pressure of the postannealing treatment. This variation is less significant than in the bi-layer parental compound Bi 2 Sr 2 CaCu 2 O 10+δ (Bi-2212) and does not follow the universal T c vs. δ relation. Magnetic measurements reveal that increasing δ enlarges the field and temperature stability of the Bragg glass phase. These findings imply that the interlayer coupling between Cu-O layers enhances with δ. The anisotropy parameter estimated from directional first-penetration field measurements monotonously decreases from 50 in the underdoped (UD) to 15 in the OD regimes. However, the in-plane penetration depth presents a boomerang-like behaviour with δ, reaching its minimum value close to optimal doping. These two facts lead to a crossover from a Josephson(OD) to electromagnetic(UD)-dominated coupling of adjacent Cu-O layers in the vicinity of optimal doping.

Research paper thumbnail of In-plane optical spectral weight transfer in optimally doped Bi2Sr2Ca2Cu3O10

Physical Review B, 2006

We examine the redistribution of the in-plane optical spectral weight in the normal and supercond... more We examine the redistribution of the in-plane optical spectral weight in the normal and superconducting state in tri-layer Bi2Sr2Ca2Cu3O10 (Bi2223) near optimal doping (Tc = 110 K) on a single crystal via infrared reflectivity and spectroscopic ellipsometry. We report the temperature dependence of the low-frequency integrated spectral weight W (Ωc) for different values of the cutoff energy Ωc. Two different model-independent analyses consistently show that for Ωc = 1 eV, which is below the charge transfer gap, W (Ωc) increases below Tc, implying the lowering of the kinetic energy of the holes. This is opposite to the BCS scenario, but it follows the same trend observed in the bi-layer compound Bi2Sr2CaCu2O8 (Bi2212). The size of this effect is larger in Bi2223 than in Bi2212, approximately scaling with the critical temperature. In the normal state, the temperature dependence of W (Ωc) is close to T 2 up to 300 K.

Research paper thumbnail of Strong coupling to magnetic fluctuations in the charge dynamics of iron-based superconductors

Physical Review B, 2010

We present a comprehensive comparison of the infrared charge response of two systems, characteris... more We present a comprehensive comparison of the infrared charge response of two systems, characteristic of classes of the 122 pnictide (SrFe2As2) and 11 chalcogenide (Fe1.087Te) Fe compounds with magnetically-ordered ground states. In the 122 system, the magnetic phase shows a decreased plasma frequency and scattering, and associated appearance of strong mid-infrared features. The 11 system, with a different magnetic ordering pattern, also shows decreased scattering, but an increase in the plasma frequency, while no clear mid-infrared features appear below the ordering temperature. We suggest how this marked contrast can be understood in terms of the diverse magnetic ordering patterns of the ground state, and conclude that while the high temperature phases of these systems are similar, the magnetic ordering strongly affects the charge dynamical response. In addition, we propose an optical absorption mechanism which appears to be consistent with information gained from several different experiments.

Research paper thumbnail of On the magnetic behavior of BSCCO(2223) Ag tapes

Physica C: Superconductivity, 1995

Pressed and rolled BSCCO(2223) Ag tapes have been widely characterized by SQUID magnetometry with... more Pressed and rolled BSCCO(2223) Ag tapes have been widely characterized by SQUID magnetometry with the applied field both parallel and perpendicular to the c-axis. The main results can be summarized as follows: (1) at low temperature (T = 5 K), the anisotropy of the indashplane and out-of-plane current densities Jab/Jc is estimated to be 50; (2) the magnetic response of

Research paper thumbnail of First Scanning Tunnelling Spectroscopy on Bi2Sr2Ca2Cu3O10 (Bi2223) single crystals

We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high... more We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high quality Bi2223 crystals. We present atomic resolution and show spectroscopic data acquired on two different samples. In one case, for Tc= 109K and a transition width of only 1K, we obtained an extremely homogeneous sample with a gap value of 60 meV over at least 50 nm. In the other case, the respective parameters were Tc= 111K with a transition width of 1.7K and yielded a slightly less homogeneous sample with a gap of 45 meV. We evidence strong similarities with Bi2Sr2CaCu2O8 (Bi2212) and discuss the doping level of our samples.

Research paper thumbnail of Temperature and time scaling of the peak-effect vortex configuration in FeTe_{0.7}Se_{0.3}

Physical Review B, 2012

An extensive study of the magnetic properties of FeTe 0.7 Se 0.3 crystals in the superconducting ... more An extensive study of the magnetic properties of FeTe 0.7 Se 0.3 crystals in the superconducting state is presented. We show that weak collective pinning, originating from spatial variations of the charge carrier mean free path (δl pinning), rules in this superconductor. Our results are compatible with the nanoscale phase separation observed on this compound and indicate that in spite of the chemical inhomogeneity spatial fluctuations of the critical temperature are not important for pinning. A power law dependence of the magnetization vs time, generally interpreted as signature of single vortex creep regime, is observed in magnetic fields up to 8 T. For magnetic fields applied along the c axis of the crystal the magnetization curves exhibit a clear peak effect whose position shifts when varying the temperature, following the same dependence as observed in YBa 2 Cu 3 O 7−δ .

Research paper thumbnail of Tuning magnetotransport in a compensated semimetal at the atomic scale

Nature communications, 2015

Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides con... more Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and i...

Research paper thumbnail of Crystal-clear – The '2014 Most Superlative Crystal Growth Contest' for School Classes

CHIMIA International Journal for Chemistry, 2014

To celebrate the International Year of Crystallography among the general public, a consortium of ... more To celebrate the International Year of Crystallography among the general public, a consortium of chemists, physicists and crystallographers of the University of Geneva organised in Spring 2014 an incentive crystal growth contest for Geneva scholars aged 4 to 19. Starting from a kit containing a salt and user instructions, classes had to prepare a crystal that met specific criteria according to their category of age. The composition of the salt - potassium dihydrogen phosphate (KDP) - was only disclosed to the participants during the final Awards Ceremony. This contest positively exceeded our expectations with almost 100 participating classes (ca. 1800 participants) and 54 specimens received over all categories.

Research paper thumbnail of Fluorine Substitution in High Temperature Superconductors

International Journal of Modern Physics B, 1999

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. ... more Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. We obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200-400 °C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and

Research paper thumbnail of Growth and characterization of Bi 2 Sr 2 Ca 2 Cu 3 O 10 and (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10   single crystals

Superconductor Science and Technology, 2004

Page 1. Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10− δ single crystal... more Page 1. Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10− δ single crystals This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2004 Supercond. Sci. Technol. 17 220 ...

Research paper thumbnail of Dissipation behaviour of YBa 2 Cu 3 O x single crystals studied through magnetization and magnetization decay measurements

Superconductor Science and Technology, 1993

Page 1. Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magn... more Page 1. Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magnetization decay measurements This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1993 Supercond. Sci. Technol. 6 771 ...

Research paper thumbnail of Reversible melting and equilibrium phase formation of (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10  

Superconductor Science and Technology, 2002

Research paper thumbnail of Growth and superconducting properties of Bi 2 Sr 2 Ca 2 Cu 3 O 10 single crystals

Superconductor Science and Technology, 2004

Single crystals of Bi2Sr2Ca2Cu3O10 (Bi-2223) have been grown using the travelling solvent floatin... more Single crystals of Bi2Sr2Ca2Cu3O10 (Bi-2223) have been grown using the travelling solvent floating zone technique in an image furnace. Annealing the crystals under high pressures of O2 increased their critical temperature to 109 K, and resulted in sharp superconducting transitions of DgrTc = 1 K. The superconducting anisotropy of Bi-2223 was found to be {\sim } 50 , from measurements

Research paper thumbnail of SEM investigation of outgrowths formation mechanism in Ag-sheathed Bi,Pb(2223) tapes

Superconductor Science and Technology, 2002

Research paper thumbnail of Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

Proceedings of the National Academy of Sciences, 2008

The mechanism of electron pairing in high-temperature superconductors is still the subject of int... more The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper-oxygen planes), in the anisotropic electronlattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c-axis value of 30 Å when the light polarization is in the direction of the copper-oxygen bond, but its decay slows down at 45º. These findings suggest a selective dynamical lattice involvement with the anisotropic electron-phonon coupling being on a time scale (1 to 3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase.

Research paper thumbnail of Lower Critical Field and Mixed State Magnetisation of Single Crystal Bi_2Sr_2Ca_2Cu_3O_10

The magnetisation of Bi_2Sr_2Ca_2Cu_3O_10 (Bi-2223) single crystals (Tc = 109 K, DeltaTc = 1 K) h... more The magnetisation of Bi_2Sr_2Ca_2Cu_3O_10 (Bi-2223) single crystals (Tc = 109 K, DeltaTc = 1 K) has been investigated using a SQUID magnetometer. The anisotropy of Bi-2223 is found to be gamma ˜ 50 from measurements of the lower critical field, H_c1, with phHallelphc and phHallelphab. The effects of surface barriers are overcome by sweeping the magnetic field very slowly, allowing an accurate determination of H_c1 with phHallelphc from ˜100 K down to 10 K. We estimate the in-plane penetration depth to be lambda_ab(0) = 1200 ÅThe irreversibility line has been obtained from field-cooled and zero-field-cooled magnetisation data for applied magnetic fields above the second magnetisation peak. A fit to the irreversibility line is achieved using the Lindemann melting criterion for pancake vortices in a Josephson-coupled layered superconductor.

Research paper thumbnail of Charge Carrier Interaction with a Purely Electronic Collective Mode: Plasmarons and the Infrared Response of Elemental Bismuth

Physical Review Letters, 2007

We present a detailed optical study of single crystal bismuth using infrared reflectivity and ell... more We present a detailed optical study of single crystal bismuth using infrared reflectivity and ellipsometry. Colossal changes in the plasmon frequency are observed as a function of temperature due to charge transfer between hole and electron Fermi pockets. In the optical conductivity, an anomalous temperature dependent mid-infrared absorption feature is observed. An extended Drude model analysis reveals that it can be connected to a sharp upturn in the scattering rate, the frequency of which exactly tracks the temperature dependent plasmon frequency. We interpret this absorption and increased scattering as the first direct optical evidence for a charge carrier interaction with a collective mode of purely electronic origin; here electron-plasmon scattering. The observation of a plasmaron as such is made possible only by the unique coincidence of various energy scales and exceptional properties of semi-metal bismuth.

Research paper thumbnail of Preeminent Role of the Van Hove Singularity in the Strong-Coupling Analysis of Scanning Tunneling Spectroscopy for Two-Dimensional Cuprate Superconductors

Physical Review Letters, 2008

In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) whi... more In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-Tc superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with great accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analyses of strong-coupling effects can lead to severe errors. PACS numbers: 68.37.Ef, 74.72.Hs, 74.50.+r Scanning tunneling spectroscopy of Bi-based cuprate high-T c superconductors (HTS) shows a d-wave gap and a strong dip-hump feature which is nearly always stronger for occupied than for empty states. 1 It has been proposed that the dip-hump structure results from the interaction of electrons with a collective mode, 2 but the dip asymmetry has not received an explanation so far. Indeed such a coupling leads to electron-hole symmetric spectra in classical superconductors. 3,4,5 The dip-hump was also observed by photoemission, but in those experiments it is not possible to probe the electron-hole asymmetry. Sometimes photoemission spectra are even symmetrized, 6 thus ignoring the relevance of the asymmetry seen in tunneling. The fact that in two dimensions the density of states (DOS) has a prominent van-Hove singularity (VHS), unlike in 3D, introduces naturally an asymmetry and thereby modifies the strongcoupling analysis and the corresponding determination of the collective mode frequency in an essential way.

Research paper thumbnail of Imaging the Essential Role of Spin Fluctuations in High-T_{c} Superconductivity

Physical Review Letters, 2009

We have used scanning tunneling spectroscopy to investigate short-length electronic correlations ... more We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi2Sr2Ca2Cu3O 10+δ (Bi-2223). We show that the superconducting gap and the energy Ω dip , defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure, and are locally anti-correlated. Based on fits of our data to a microscopic strong-coupling model we show that Ω dip is an accurate measure of the collective mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy, and is most likely the (π, π) spin resonance. PACS numbers: 74.50.+r, 74.20.Mn, 74.72.Hs The presence of phonon signatures in the electron tunneling spectra of classical superconductors [1], and their quantitative explanation by the Eliashberg equations [2, 3], stand among the most convincing validations of the BCS phonon-mediated pairing theory . For high-T c superconductors, the pairing mechanism still remains an intriguing mystery. Several cuprate superconductors present a spectroscopic feature, known as the dip-hump , that resembles the phonon signatures of classical superconductors. There is still no consensus on the origin of the dip-hump nor its connection to high-T c superconductivity. In this work we report on a scanning tunneling microscopy (STM) study of the three-layer compound Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223). We observe that the gap magnitude, a direct measure of the pairing strength, is periodically modulated on a lengthscale of about 5 crystal unit-cells. This variation follows the superstructure, a periodic modulation of atomic positions naturally present in Bi-based cuprates. By fitting the STM data with a strong-coupling model we demonstrate that the dip feature originates from a collective excitation. This allows us to image the collective mode energy (CME) at the atomic scale and reveal a modulation that also follows the superstructure. The CME and the gap are locally anti-correlated. These findings support that the collective mode probed in our study is related to superconductivity, and is most likely the anti-ferromagnetic spin resonance detected by neutron scattering [10]. Our results, in particular the CME value of 30-40 meV, are in agreement with the spin-fluctuation-mediated pairing scenario , in which the spin resonance in high-T c 's is a consequence of pairing.

Research paper thumbnail of Vortex phase diagram and temperature-dependent second-peak effect in overdoped Bi_{2}Sr_{2}CuO_{6+δ} crystals

Physical Review B, 2010

We study the vortex phase diagram of the single-layer Bi2Sr2CuO 6+δ (Bi2201) superconductor by me... more We study the vortex phase diagram of the single-layer Bi2Sr2CuO 6+δ (Bi2201) superconductor by means of bulk magnetization measurements on high-quality oxygen-overdoped crystals. In striking contrast with the results found in the moderately-doped two and three-layer Bi-based cuprates, Bi2201 exhibits a strong temperature-dependent second-peak effect. By means of measurements of the in and out-of-plane first-penetration field we provide direct evidence that this phenomenon is mainly associated to an increase of the electromagnetic anisotropy on warming. The effect of oxygen-doping δ on the vortex phase diagram results in both the irreversibility and second-peak lines shifting to higher temperatures and fields. This enhanced stability of the Bragg glass phase suggests that the interlayer coupling between Cu-O layers increases with δ. In addition, we found that the critical temperature follows the parabolic relation with the number of holes per Cu-O plane that holds for most single and two-layer cuprates.

Research paper thumbnail of Effect of oxygen-doping on Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ} vortex matter: Crossover from electromagnetic to Josephson interlayer coupling

Physical Review B, 2008

We study the effect of oxygen-doping on the critical temperature, T c , the vortex matter phase d... more We study the effect of oxygen-doping on the critical temperature, T c , the vortex matter phase diagram and the nature of the coupling mechanism between the Cu-O layers in the three-layer Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223) compound. Contrary to previous reports, in the overdoped (OD) regime we do find a variation of T c with increasing the oxygen partial-pressure of the postannealing treatment. This variation is less significant than in the bi-layer parental compound Bi 2 Sr 2 CaCu 2 O 10+δ (Bi-2212) and does not follow the universal T c vs. δ relation. Magnetic measurements reveal that increasing δ enlarges the field and temperature stability of the Bragg glass phase. These findings imply that the interlayer coupling between Cu-O layers enhances with δ. The anisotropy parameter estimated from directional first-penetration field measurements monotonously decreases from 50 in the underdoped (UD) to 15 in the OD regimes. However, the in-plane penetration depth presents a boomerang-like behaviour with δ, reaching its minimum value close to optimal doping. These two facts lead to a crossover from a Josephson(OD) to electromagnetic(UD)-dominated coupling of adjacent Cu-O layers in the vicinity of optimal doping.

Research paper thumbnail of In-plane optical spectral weight transfer in optimally doped Bi2Sr2Ca2Cu3O10

Physical Review B, 2006

We examine the redistribution of the in-plane optical spectral weight in the normal and supercond... more We examine the redistribution of the in-plane optical spectral weight in the normal and superconducting state in tri-layer Bi2Sr2Ca2Cu3O10 (Bi2223) near optimal doping (Tc = 110 K) on a single crystal via infrared reflectivity and spectroscopic ellipsometry. We report the temperature dependence of the low-frequency integrated spectral weight W (Ωc) for different values of the cutoff energy Ωc. Two different model-independent analyses consistently show that for Ωc = 1 eV, which is below the charge transfer gap, W (Ωc) increases below Tc, implying the lowering of the kinetic energy of the holes. This is opposite to the BCS scenario, but it follows the same trend observed in the bi-layer compound Bi2Sr2CaCu2O8 (Bi2212). The size of this effect is larger in Bi2223 than in Bi2212, approximately scaling with the critical temperature. In the normal state, the temperature dependence of W (Ωc) is close to T 2 up to 300 K.

Research paper thumbnail of Strong coupling to magnetic fluctuations in the charge dynamics of iron-based superconductors

Physical Review B, 2010

We present a comprehensive comparison of the infrared charge response of two systems, characteris... more We present a comprehensive comparison of the infrared charge response of two systems, characteristic of classes of the 122 pnictide (SrFe2As2) and 11 chalcogenide (Fe1.087Te) Fe compounds with magnetically-ordered ground states. In the 122 system, the magnetic phase shows a decreased plasma frequency and scattering, and associated appearance of strong mid-infrared features. The 11 system, with a different magnetic ordering pattern, also shows decreased scattering, but an increase in the plasma frequency, while no clear mid-infrared features appear below the ordering temperature. We suggest how this marked contrast can be understood in terms of the diverse magnetic ordering patterns of the ground state, and conclude that while the high temperature phases of these systems are similar, the magnetic ordering strongly affects the charge dynamical response. In addition, we propose an optical absorption mechanism which appears to be consistent with information gained from several different experiments.

Research paper thumbnail of On the magnetic behavior of BSCCO(2223) Ag tapes

Physica C: Superconductivity, 1995

Pressed and rolled BSCCO(2223) Ag tapes have been widely characterized by SQUID magnetometry with... more Pressed and rolled BSCCO(2223) Ag tapes have been widely characterized by SQUID magnetometry with the applied field both parallel and perpendicular to the c-axis. The main results can be summarized as follows: (1) at low temperature (T = 5 K), the anisotropy of the indashplane and out-of-plane current densities Jab/Jc is estimated to be 50; (2) the magnetic response of