Showing 1–17 of 17 results for author: Tyagi, S

CVD Growth of Tin Selenide Thin Films for Optoelectronic Applications

Authors: Shivam Tyagi, Shubham Yadav, Vaibhav Wani, Shivam Singh, Soumya Biswas, Krishna Nand Prajapati, Vinayak B Kamble

Abstract: Tin Selenide (SnSe) thin films were grown onto glass and alumina substrates by Chemical Vapor Deposition (CVD) method. The structural, micro-structural and morphological characterizations of the as grown thin films were investigated using XRD, SEM and Raman spectroscopy which reveals that the films on glass are phase pure oriented SnSe while those on Alumina are polycrystalline SnSe with SnSe2 imp… ▽ More Tin Selenide (SnSe) thin films were grown onto glass and alumina substrates by Chemical Vapor Deposition (CVD) method. The structural, micro-structural and morphological characterizations of the as grown thin films were investigated using XRD, SEM and Raman spectroscopy which reveals that the films on glass are phase pure oriented SnSe while those on Alumina are polycrystalline SnSe with SnSe2 impurity phase. The optical properties of the films grown onto glass substrate were studied by UV-vis spectroscopy. The optical band gap calculated is 1 to 1.3 eV for indirect and direct transition in film deposited on glass substrates. The Arrhenius plots of the two films show very different thermal activations i.e. 0.088 eV for tin vacancy acceptor level close to valance band maxima in pure SnSe and 0.44 eV for mid gap selenium vacancies of SnSe2. Photoresponse was observed by illuminating the sample (Glass and alumina Substrate) using white and UV light (400 nm) for a fixed time pulses. The deposited onto Alumina substrate were found to show better photoresponse due to SnSe/SnSe2 p-n heterojunctions. △ Less

Submitted 30 August, 2022; originally announced August 2022.

Experimental detection of surface spin-polarized electron accumulation in topological insulators using scanning tunneling microscopy

Authors: S. Tyagi, M. Dreyer, D. Bowen, D. Hinkel, P. J. Taylor, A. L. Friedman, R. E. Butera, C. Krafft, I. Mayergoyz

Abstract: Spin-momentum locking in the surface mode of topological insulators (TI) leads to the surface accumulation of spin-polarized electrons caused by bias current flows through TI samples. Here, we demonstrate that scanning tunneling microscopy can be used to sense this surface spin-polarized electron accumulation. We present experimental results of this sensing for Sn-doped Bi$_2$Se$_3$ samples by emp… ▽ More Spin-momentum locking in the surface mode of topological insulators (TI) leads to the surface accumulation of spin-polarized electrons caused by bias current flows through TI samples. Here, we demonstrate that scanning tunneling microscopy can be used to sense this surface spin-polarized electron accumulation. We present experimental results of this sensing for Sn-doped Bi$_2$Se$_3$ samples by employing Fe-coated W tips as well as non-magnetic W tips. We observe a linear increase in the spin-accumulation as a function of bias current through TI samples. △ Less

Submitted 7 April, 2020; originally announced April 2020.

Imaging of Strain Driven Magnetic Domains and Strong Spin-Phonon Coupling in Epitaxial Thin Films of SrRuO3

Authors: Shekhar Tyagi, V. G. Sathe, Gaurav Sharma, Rajeev Rawat, Mael Guennou, Jens Kreisel

Abstract: Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the… ▽ More Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the two transitions is different. It is observed that the perpendicular magnetic anisotropy is dominating in films which results in domain spin orientation preferably in out of plane direction. The Raman studies showed that the lattice is highly influenced by the magnetic order. The analysis of the phonon spectra around magnetic transition reveals the existence of strong spin-phonon coupling and the calculations resulted in spin-phonon coupling strength (λ) values of λ ~ 5 cm-1 and λ ~ 8.5 cm-1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively. △ Less

Submitted 12 January, 2020; originally announced January 2020.

Comments: 6 pages, 4 figures

Study Of Surface Spin-Polarized Electron Accumulation In Topological Insulators Using Scanning Tunneling Microscopy

Authors: S. Tyagi, M. Dreyer, D. Bowen, D. Hinkel, P. J. Taylor, A. L. Friedman, R. E. Butera, C. Krafft, I. Mayergoyz

Abstract: The results of scanning tunneling microscopy experiments using iron-coated tungsten tips and current-carrying bismuth selenide ($Bi_2Se_3$) samples are reported. Asymmetry in tunneling currents with respect to the change in the direction of bias currents through $Bi_2Se_3$ samples has been observed. It is argued that this asymmetry is the manifestation of surface spin-polarized electron accumulati… ▽ More The results of scanning tunneling microscopy experiments using iron-coated tungsten tips and current-carrying bismuth selenide ($Bi_2Se_3$) samples are reported. Asymmetry in tunneling currents with respect to the change in the direction of bias currents through $Bi_2Se_3$ samples has been observed. It is argued that this asymmetry is the manifestation of surface spin-polarized electron accumulation caused by the ninety-degree electron spin-momentum locking in the topologically protected surface current mode. It is demonstrated that the manifestation of surface spin-polarized electron accumulation is enhanced by tin doping of $Bi_2Se_3$ samples. Furthermore, the appearance of spin-dependent density of states in current carrying $Bi_2Se_3$ samples has also been observed. △ Less

Submitted 31 October, 2019; originally announced November 2019.

Phonon Invisibility Driven by Robust Magneto-Elastic Coupling in AlFeO3 Thin Film

Authors: Shekhar Tyagi, Gaurav Sharma, R. J. Choudhary, Vasant G. Sathe

Abstract: The thin films of lead free magneto-electric compound AlFeO3 have been deposited using pulsed laser deposition technique. X-ray diffraction, X-ray absorption spectroscopy and reflectivity measurements established the orthorhombic structure and material density of 4.5g/cc which is comparable with bulk AlFeO3. The Raman mode corresponding to AlFeO3 was found to vanish when magnetic field of 800 Oe w… ▽ More The thin films of lead free magneto-electric compound AlFeO3 have been deposited using pulsed laser deposition technique. X-ray diffraction, X-ray absorption spectroscopy and reflectivity measurements established the orthorhombic structure and material density of 4.5g/cc which is comparable with bulk AlFeO3. The Raman mode corresponding to AlFeO3 was found to vanish when magnetic field of 800 Oe was applied at room temperature. Additionally, it was observed that the Raman phonon mode present at the room temperature becomes invisible in the temperature window of 280K-236K and reappears below it. The detailed analysis of magnetization showed a change in magnetic order in this temperature interval. The invisibility of Raman phonon mode corresponding to AlFeO3 have been attributed to the lattice deformation caused by the magnetoelastic effect. The presence of strong spin-lattice coupling is also validated by the renormalization of phonon frequencies below 200 K. △ Less

Submitted 13 January, 2019; originally announced January 2019.

Strain Healing of Spin-Orbit Coupling: A Cause for Enhanced Magnetic Moment in Epitaxial SrRuO3 Thin Films

Authors: Shekhar Tyagi, Gaurav Sharma, D. M. Phase, V. R. Reddy, V. G. Sathe

Abstract: Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin… ▽ More Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 Bohr-magnetron/Ru ion observed in bulk compound. Contrary to the expectation the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment. Strain disorder is shown to be an efficient mean to control the spin-orbit coupling. △ Less

Submitted 3 January, 2019; originally announced January 2019.

Comments: 10 pages, 6 figures

Spin-lattice coupling mediated giant magnetodielectricity across the spin reorientation in Ca2FeCoO5

Authors: Gaurav Sharma, Shekhar Tyagi, V. R. Reddy, A. M. Awasthi, R. J. Choudhary, A. K. Sinha, Vasant Sathe

Abstract: The structural, phonon, magnetic, dielectric, and magneto dielectric responses of the pure bulk Brownmillerite compound Ca2FeCoO5 are reported. This compound showed giant magneto dielectric response (10%-24%) induced by strong spin-lattice coupling across its spin reorientation transition (150-250 K). The role of two Debye temperatures pertaining to differently coordinated sites in the dielectric… ▽ More The structural, phonon, magnetic, dielectric, and magneto dielectric responses of the pure bulk Brownmillerite compound Ca2FeCoO5 are reported. This compound showed giant magneto dielectric response (10%-24%) induced by strong spin-lattice coupling across its spin reorientation transition (150-250 K). The role of two Debye temperatures pertaining to differently coordinated sites in the dielectric relaxations is established. The positive giant magneto-dielectricity is shown to be a direct consequence of the modulations in the lattice degrees of freedom through applied external field across the spin reorientation transition. Our study illustrates novel control of magneto-dielectricity by tuning the spin reorientation transition in a material that possess strong spin lattice coupling. △ Less

Submitted 1 November, 2018; originally announced November 2018.

Comments: 7 pages, 12 figures

Journal ref: Phys. Rev. B 99, 024436 (2019)

Raman spectroscopic investigation of relaxor behavior in Pr doped SrTiO3 and Origin of Fano resonance

Authors: Vivek Dwij, Binoy Krishna De, Shekhar Tyagi, Gaurav Sharma, V. G. Sathe

Abstract: Detailed Raman spectroscopy studies on polycrystalline Sr1-xPrxTiO3 (x=0.01, 0.025, 0.05, 0.075, 0.09, 0.13, 0.15, 0.17) samples are reported elucidating the microscopic mechanism of relaxor ferroelectrics. The polar mode was observed upto very high temperature ~1000 K suggesting that the dipoles exists at temperatures well above the characteristic relaxor temperatures and they develop a short ran… ▽ More Detailed Raman spectroscopy studies on polycrystalline Sr1-xPrxTiO3 (x=0.01, 0.025, 0.05, 0.075, 0.09, 0.13, 0.15, 0.17) samples are reported elucidating the microscopic mechanism of relaxor ferroelectrics. The polar mode was observed upto very high temperature ~1000 K suggesting that the dipoles exists at temperatures well above the characteristic relaxor temperatures and they develop a short range correlation at 505 K leading to formation of PNRs. The TO2 polar mode showed anomalous softening in cooling below T~505 K supporting the growth of PNRs. Fano resonance is reported in the Pr doped compounds which decreases with increasing doping. Our work on Pr doped SrTiO3 reveals that the local TiO6 octahedral tilt scales with the intensity of the polar mode and hence can be used as an order parameter for relaxor transition. The study showed that the paraelectric to relaxor ferroelectric phase transition in this compound is random polarizability instability driven phenomenon which is correlated with the local octahedral tilt angle. The study supports competition and cancelation between lattice and polar instabilities at global length scale while cooperation between the two at local length scale. The modulation in local structure of the material in temperature interval related to dielectric anomaly has been observed which is not been investigated previously by any structural tool. △ Less

Submitted 25 June, 2019; v1 submitted 25 September, 2018; originally announced September 2018.

Length-scale dependent average structures, piezoelectricity enhancement and depolarization mechanisms in a non-MPB high-performance piezoelectric alloy system PbTiO3-Bi(Zr1/2Ni1/2)O3

Authors: Rishikesh Pandey, Dipak Kumar Khatua, Shekhar Tyagi, Mulualem Abebe, Bastola Narayan, Vasant Sathe, Rajeev Ranjan

Abstract: There is a general perception that large piezoelectric response in ferroelectric alloys requires tuning the system towards a morphotropic phase boundary (MPB), i.e., a composition driven inter-ferroelectric instability. Here we show that high piezoelectric response can be realized even in non-MPB alloy systems. This is demonstrated on (1-x)PbTiO3-(x)Bi(Zr0.5Ni0.5)O3 (PT-BNZ) by a comprehensive stu… ▽ More There is a general perception that large piezoelectric response in ferroelectric alloys requires tuning the system towards a morphotropic phase boundary (MPB), i.e., a composition driven inter-ferroelectric instability. Here we show that high piezoelectric response can be realized even in non-MPB alloy systems. This is demonstrated on (1-x)PbTiO3-(x)Bi(Zr0.5Ni0.5)O3 (PT-BNZ) by a comprehensive study involving electric-field and temperature dependent XRD, Raman spectroscopy, dielectric, piezoelectric and high field electrostrain measurements. We found that poling-field irreversibly suppresses the cubic-like phase at room temperature. Based on our results, we argue that that which appears as MPB, comprising of tetragonal and cubic-like phases on the global scale, is not so actually. The large piezoresponse is due to coexistence of tetragonal regions of long and short-range coherence. The PT-BNZ system is therefore qualitatively different from the conventional MPB systems such as PZT, PMN-PT, and PbTiO3-BiScO3, etc., which exhibits coexisting tetragonal and rhombohedral/monoclinic phases in thermodynamic equilibrium. In the absence of inter-ferroelectric instability as a phenomenon, field induced polarization-rotation and inter-ferroelectric transformation are no longer plausible mechanisms to explain the large piezoelectric response in PT-BNZ. The large piezoelectricity is primarily due to enhanced mobility of the tetragonal domain walls enabled by domain miniaturization. Our study proves that attainment of large piezoelectricity does not require inter-ferroelectric instability as a necessary criterion. △ Less

Submitted 10 January, 2018; originally announced January 2018.

Comments: 30 pages, 10 figures

Journal ref: Phys. Rev. B 97, 224109 (2018)

Evaluation of Coulomb potential in a triclinic cell with periodic boundary conditions

Authors: Sandeep Tyagi

Abstract: Lekner and Sperb's work on the evaluation of Coulomb energy and forces under periodic boundary conditions is generalized that makes it possible to use a triclinic unit cell in simulations in 3D rather than just an orthorhombic cell. The expressions obtained are in a similar form as previously obtained by Lekner and Sperb for the especial case of orthorhombic cell. Lekner and Sperb's work on the evaluation of Coulomb energy and forces under periodic boundary conditions is generalized that makes it possible to use a triclinic unit cell in simulations in 3D rather than just an orthorhombic cell. The expressions obtained are in a similar form as previously obtained by Lekner and Sperb for the especial case of orthorhombic cell. △ Less

Submitted 2 February, 2006; originally announced February 2006.

Logarithmic interaction under periodic boundary conditions: Closed form formulas for energy and forces

Authors: Sandeep Tyagi

Abstract: A method is given to obtain closed form formulas for the energy and forces for an aggregate of charges interacting via a logarithmic interaction under periodic boundary conditions. The work done here is a generalization of Glasser's results [M. L. Glasser, J. Math. Phys. 15, 188 (1974)] and is obtained with a different and simpler method than that by Stremler [M. A. Stremler, J. Math. Phys. 45,… ▽ More A method is given to obtain closed form formulas for the energy and forces for an aggregate of charges interacting via a logarithmic interaction under periodic boundary conditions. The work done here is a generalization of Glasser's results [M. L. Glasser, J. Math. Phys. 15, 188 (1974)] and is obtained with a different and simpler method than that by Stremler [M. A. Stremler, J. Math. Phys. 45, 3584 (2004)]. The simplicity of the formulas derived here makes them extremely convenient in a computer simulation. △ Less

Submitted 30 May, 2005; originally announced May 2005.

Interpolation of the Josephson interaction in highly anisotropic superconductors from a solution of the two dimensional sine-Gordon equation

Authors: Yadin Y. Goldschmidt, Sandeep Tyagi

Abstract: In this paper we solve numerically the two dimensional elliptic sine-Gordon equation with appropriate boundary conditions. These boundary conditions are chosen to correspond to the Josephson interaction between two adjacent pancakes belonging to the same flux-line in a highly anisotropic superconductor. An extrapolation is obtained between the regimes of low and high separation of the pancakes.… ▽ More In this paper we solve numerically the two dimensional elliptic sine-Gordon equation with appropriate boundary conditions. These boundary conditions are chosen to correspond to the Josephson interaction between two adjacent pancakes belonging to the same flux-line in a highly anisotropic superconductor. An extrapolation is obtained between the regimes of low and high separation of the pancakes. The resulting formula is a better candidate for use in numerical simulations than previously derived formulas. △ Less

Submitted 25 October, 2004; originally announced October 2004.

Comments: 18 pages, 9 figures

New series representation for Madelung constant

Authors: Sandeep Tyagi

Abstract: A new series representation of the Madelung constant is given. We represent Madelung constant as a sum of an exact term plus an exponentially fast converging series. The remarkable result is that even if the series part is discarded, one obtains Madelung constant correct up to ten good decimal figures. This, to the best of our knowledge, may be the fastest converging series representation of the… ▽ More A new series representation of the Madelung constant is given. We represent Madelung constant as a sum of an exact term plus an exponentially fast converging series. The remarkable result is that even if the series part is discarded, one obtains Madelung constant correct up to ten good decimal figures. This, to the best of our knowledge, may be the fastest converging series representation of the Madelung constant. A few other important identities are also obtained. △ Less

Submitted 20 March, 2005; v1 submitted 17 October, 2004; originally announced October 2004.

Coulomb potentials in two and three dimensions under periodic boundary conditions

Authors: Sandeep Tyagi

Abstract: A method to sum over logarithmic potential in 2D and Coulomb potential in 3D with periodic boundary conditions in all directions is given. We consider the most general form of unit cells, the rhombic cell in 2D and the triclinic cell in 3D. For the 3D case, this paper presents a generalization of Sperb's work [R. Sperb, Mol. Simulation, \textbf{22}, 199-212(1999)]. The expressions derived in thi… ▽ More A method to sum over logarithmic potential in 2D and Coulomb potential in 3D with periodic boundary conditions in all directions is given. We consider the most general form of unit cells, the rhombic cell in 2D and the triclinic cell in 3D. For the 3D case, this paper presents a generalization of Sperb's work [R. Sperb, Mol. Simulation, \textbf{22}, 199-212(1999)]. The expressions derived in this work converge extremely fast in all region of the simulation cell. We also obtain results for slab geometry. Furthermore, self-energies for both 2D as well as 3D cases are derived. Our general formulas can be employed to obtain Madelung constants for periodic structures. △ Less

Submitted 9 September, 2004; v1 submitted 28 July, 2004; originally announced July 2004.

Comments: Generalization of the work done in cond-mat/0405574. To appear in J. Chem. Physics. A few typos have been corrected

Effective way to sum over long range Coulomb potentials in two and three dimensions

Authors: Sandeep Tyagi

Abstract: I propose a method to calculate logarithmic interaction in two dimensions and coulomb interaction in three dimensions under periodic boundary conditions. This paper considers the case of a rectangular cell in two dimensions and an orthorhombic cell in three dimensions. Unlike the Ewald method, there is no parameter to be optimized, nor does it involve error functions, thus leading to the accurac… ▽ More I propose a method to calculate logarithmic interaction in two dimensions and coulomb interaction in three dimensions under periodic boundary conditions. This paper considers the case of a rectangular cell in two dimensions and an orthorhombic cell in three dimensions. Unlike the Ewald method, there is no parameter to be optimized, nor does it involve error functions, thus leading to the accuracy obtained. This method is similar in approach to that of Sperb [R. Sperb, Mol. Simulation, 22, 199 (1999).], but the derivation is considerably simpler and physically appealing. An important aspect of the proposed method is the faster convergence of the Green function for a particular case as compared to Sperb's work. The convergence of the sums for the most part of unit cell is exponential, and hence requires the calculation of only a few dozen terms. In a very simple way, we also obtain expressions for interaction for systems with slab geometries. Expressions for the Madelung constant of CsCl and NaCl are also obtained. △ Less

Submitted 31 August, 2004; v1 submitted 24 May, 2004; originally announced May 2004.

Comments: To appear in Phy. Rev. E

Flux melting in BSCCO: Incorporating both electromagnetic and Josephson couplings

Authors: Sandeep Tyagi, Yadin Y. Goldschmidt

Abstract: Multilevel Monte Carlo simulations of a BSCCO system are carried out including both Josephson as well as electromagnetic couplings for a range of anisotropies. A first order melting transition of the flux lattice is seen on increasing the temperature and/or the magnetic field. The phase diagram for BSCCO is obtained for different values of the anisotropy parameter $γ$. The best fit to the experi… ▽ More Multilevel Monte Carlo simulations of a BSCCO system are carried out including both Josephson as well as electromagnetic couplings for a range of anisotropies. A first order melting transition of the flux lattice is seen on increasing the temperature and/or the magnetic field. The phase diagram for BSCCO is obtained for different values of the anisotropy parameter $γ$. The best fit to the experimental results of D. Majer {\it et al.} [Phys. Rev. Lett. {\bf 75}, 1166 (1995)] is obtained for $γ\approx 250$ provided one assumes a temperature dependence $λ^2(0)/λ^2(T)=1-t$ of the penetration depth with $t=T/T_c$. Assuming a dependence $λ^2(0)/λ^2(T)=1-t^2$ the best fit is obtained for $ γ\approx 450$. For finite anisotropy the data is shown to collapse on a straight line when plotted in dimensionless units which shows that the melting transition can be satisfied with a single Lindemann parameter whose value is about 0.3. A different scaling applies to the $γ=\infty$ case. The energy jump is measured across the transition and for large values of $γ$ it is found to increase with increasing anisotropy and to decrease with increasing magnetic field. For infinite anisotropy we see a 2D behavior of flux droplets with a transition taking place at a temperature independent of the magnetic field. We also show that for smaller values of anisotropy it is reasonable to replace the electromagnetic coupling with an in-plane interaction represented by a Bessel function of the second kind ($K_0$), thus justifying our claim in a previous paper. △ Less

Submitted 14 April, 2004; originally announced April 2004.

Comments: 12 figures, revtex4

Effects of columnar disorder on flux-lattice melting in high-temperature superconductors

Authors: Sandeep Tyagi, Yadin Y. Goldschmidt

Abstract: The effect of columnar pins on the flux-lines melting transition in high-temperature superconductors is studied using Path Integral Monte Carlo simulations. We highlight the similarities and differences in the effects of columnar disorder on the melting transition in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) and the highly anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO) at magnetic fields such that the me… ▽ More The effect of columnar pins on the flux-lines melting transition in high-temperature superconductors is studied using Path Integral Monte Carlo simulations. We highlight the similarities and differences in the effects of columnar disorder on the melting transition in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) and the highly anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO) at magnetic fields such that the mean separation between flux-lines is smaller than the penetration length. For pure systems, a first order transition from a flux-line solid to a liquid phase is seen as the temperature is increased. When adding columnar defects to the system, the transition temperature is not affected in both materials as long as the strength of an individual columnar defect (expressed as a flux-line defect interaction) is less than a certain threshold for a given density of randomly distributed columnar pins. This threshold strength is lower for YBCO than for BSCCO. For higher strengths the transition line is shifted for both materials towards higher temperatures, and the sharp jump in energy, characteristic of a first order transition, gives way to a smoother and gradual rise of the energy, characteristic of a second order transition. Also, when columnar defects are present, the vortex solid phase is replaced by a pinned Bose glass phase and this is manifested by a marked decrease in translational order and orientational order as measured by the appropriate structure factors. For BSCCO, we report an unusual rise of the translational order and the hexatic order just before the melting transition. No such rise is observed in YBCO. △ Less

Submitted 6 February, 2003; originally announced February 2003.

Comments: 32 pages, 13 figures, revtex