International Superconductivity Technology Center
Luce Scholars Program
The end goal of my research was to investigate the electron pairing
mechanism responsible for high-temperature superconductivity in copper
oxide superconductors. This study focused on infinite-layer (IL) compounds, which have the
simplest crystalline structure of all superconducting cuprates. I attempted
to fabricate single crystals of infinite-layer compounds ACuO2 (where A=Sr, Ca is a cation) and/or
SrxR1-xCuO2 (where R=Nd, La). Since the heyday of superconductivity
in the late 1980s, researchers have tried to fabricate single crystals of this compound for STM studies to
reveal any atomic-sized defects, charge density modulations, and other fundamental properties that could
explain the underlying mechanism for high-temperature superconductivity. However, mostly because no
phase diagram has been devised that delineates the formation of these compounds, such attempts have
been unsuccessful. Also, high pressures (P) and temperatures (T) are required to
synthesize such compounds. I thoroughly experimented with adjusting parameters such as crucible type,
oxygenation, pressure, temperature, synthesis time, and self-fluxes. Though I too was unable to
produce single crystals, I hope that my thorough data will eventually contribute to the creation of
a P-T-x phase diagram.
Introduction
Superconductivity is a low-temperature phenomenon that is most notably
characterized by resistance-free current conduction and total expulsion
of magnetic flux, known as the Meissner Effect. The applications of
superconductors are vast, including efficient power transmission,
high-speed trains, water purification, medical equipment, and ultrafast
electronics. Superconducting Quantum Interference Devices (SQUIDS) are
sensitive to fields as low as 100 million times weaker than the earth's
magnetic field, making them the most sensitive detectors of magnetic
fields that exist. Furthermore, superconductors are ideal for space
application due to their intrinsic low-temperature functioning ranges.
Such applications include improving the performance of electronics in
space-based systems to controlling the flow of plasma in rockets such as
the Variable Specific Magnetoplasma Rocket (VASIMR) to utilizing the
Meissner effect for the low-noise control of a telescope axis on the
moon.
Superconductors are classified as either low-temperature or
high-temperature superconductors, based on the intrinsic maximum
temperature at which the material exhibits superconducting properties.
The origin of superconductivity in LTS is well-understood and described
by the BCS theory, which pinpoints this origin to interactions between
pairs of electrons, called Cooper pairs, and lattice vibrations.
Because LTS appeared to have a bandgap, such that multiple electrons
assumed the same state similar to Bose-Einstein condensation, Bardeen,
Schraffer, and Cooper derived that these superconductors exhibited
bosonic behavior, leading to eliminate single electrons as the charge
carriers. Coupled pairs of electrons with opposite spin have a total
integral spin, thus can act as bosons. Though Cooper pairing is also a
key element in high-temperature superconducting cuprates (HTSC), the
mechanism for this pairing is one of the many properties of this
phenomenon that are not well understood. Pinpointing the charge
carriers in HTSC, explaining the dynamics of these carriers, and
understanding magnetic interaction of electrons in HTSC through their
spins are a prerequisite for creating a theory to explain the mechanism
of high-temperature superconductivity.
The most promising candidates for studying the origin of the high-Tc
superconductivity are infinite-layer compounds ACuO2 (where A=Sr, Ca is
a cation) and/or SrxR1-xCuO2 (where R=rare earth element) because of
having the simplest crystallographic structure of all superconducting
cuprates. The structure of IL compounds consists of a stack of CuO2
planes separated by cations (in the case of ACuO2) and stoichiometric
alternating arrangements of Sr and rare earth atoms in SrxR1-xCuO2. In
HTSC, it is widely accepted that superconductivity occurs in the CuO2
planes [6]. However, there is evidence that in Ca1-xAxCuO2+z that
other phases in the sample may be responsible for the superconducting
properties [12]. Furthermore, the impurity phases Srn+1CunOxn+1+y are
considered responsible for superconductivity in Sr1-xCaxCuO2 and not the
IL phase itself [8, 2]. Because of this and the uniqueness of IL
compounds as electron-doped HTSC [10], fundamental studies promise to
reveal crucial information on the requirements for superconductivity in
cuprates. This could lead to theories for predicting compounds with
higher transition temperatures and methods for enhancing the
superconducting properties of presently known compounds.
Presently, single-phased polycrystalline forms of infinite-layer
compounds have successfully been fabricated. High pressure and
temperature conditions are necessary for this phase. In general, high
pressure synthesis can either induce superconductivity or enhance the
superconducting properties of compounds. Many phenomena are responsible
for this: structural transitions at high pressure; reduced evaporation
of volatile components; modification of phase diagrams with pressure;
change of solubility of individual components, and change of defect
equilibrium in crystals [5C].
Single crystals are the best medium for studying the crystallographic
and anisotropic physical properties [6] and the electronic coupling
between adjacent CuO2 layers. To date, single crystals of
infinite-layer compounds have not been fabricated. These efforts have
been further hindered by a lack of thorough, consistent information
regarding phases of infinite-layer compounds under a range of
high-pressure and temperature conditions. Though single-phased
polycrystalline samples of infinite-layer compounds with sharp
transitions and high superconducting volume fractions have successfully
been fabricated, a P-T-x phase diagram has not yet been outlined.
Literature Review
Polycrystalline forms of infinite-layer compounds have successfully been
fabricated [1, 4, 11, 10, 2, 9, 3, 8], though only two studies have
produced samples with sharp transitions [4, 8]. These studies
investigate a variety of processing conditions. Jung et al [4]
controlled the oxygen content of the sample by sandwiching the
precursors (a calcined mixture of SrCO3, La2CO3, and CuO) between Ti
oxygen getters within an Au capsule. Podlesnyak et al. [8] simply
prevented introducing excess oxygen to the system by using the
precursors SrCuO2 and NdCuO2 within a Pt capsule. Other groups have
been successful by placing the calcined mixture of carbonate powders
directly into the BN capsule [11, 10] because the graphite sleeve heater
provides a reducing environment to remove excess oxygen. The synthesis
pressures range between 2.8GPa and 5GPa and temperatures between 900C
and 1000C.
In some bismuth-based superconductors, single crystals have been
successfully formed under high pressure after long isothermal periods up
to 15 hrs. However, extending the heating times for samples placed
directly into boron-nitride (BN) crucibles will only cause the entire
sample to decompose. Heating samples enclosed in metal capsules for
longer periods will cause the formation of a non-superconducting phase
which will suppress superconductivity in the sample [8]. Studies have
failed to produce single crystals because there is no thorough P-T-x
phase diagram that pinpoints the fragile balance between complete
decomposition and the fabrication of a non-superconducting phase, where
single crystal fabrication could prove successful.
(Note that Karpinski claimed to have produced single crystals of
infinite-layer CaCuO2 [5, 6], but later recanted the claim.)
Methods
There are two main procedures for forming precursors before subjecting
the sample to the extreme environment necessary for synthesis. To
synthesize Sr0.9La0.1CuO2, the first method involves manually grinding
stoichiometric amounts of La2O3, SrCO3, and CuO, calcinations with
intermittent grindings, and then high pressure synthesis. In the second
method, the carbonate powders and CuO are dissolved in nitric acid (in a
reaction that releases carbon dioxide) and then spray dried. Spray
drying forces the solution out of a tiny nozzle at high speeds, the same
method used to produce ground coffee. The latter technique successfully
creates finer grains than manually grinding the powders using a mortar
and pestle. The multiphased resulting samples can then be used for high
pressure synthesis.
A cubic-anvil or inert gas system for applying hydrostatic pressures up
to 5GPa and temperatures up to 1200 C will be required. The former
method requires the sample to be pressed, inserted into a boron-nitride
capsule, which is then inserted into a graphite sleeve heater. This
entire cluster is placed inside a pyrophilite block with molybdenum
electrodes suitable for the cubic-anvil system. Different sample
preparation techniques will be tried. Enclosing the pressed sample in a
metal capsule with or without oxygen getters controls the oxygen content
of the sample, while the BN capsule alone is transparent to the exchange
of oxygen. Additionally, chloride fluxes (ie KCl and NaCl) have been
known to promote crystal growth in other superconductors [13].
Results and Discussion
Once single crystals are successfully grown, 4-circle x-ray diffraction
will be used to determine the crystalline structure. More identifying
information about the sample will be determined from O-J and scanning
electron microscopy (SEM). The former technique can reveal accurate
information about the composition of a sample through analyzing the
kinetic energy of electrons after an Auger process. A SEM will not only
provide a high resolution picture of the sample, but determine the
single crystal size and, in the case of polycrystals, the grain sizes.
Further, SEM can aid in monitoring the success of each fabrication
technique. Observing a progressively increasing grain size indicates
that a given technique is promising for the production of single
crystals, with only minor adjustments in the fabrication environment.
Scanning Tunneling Microscopy (STM) studies of the single crystals will
reveal any atomic-sized defects, charge density modulations, and other
fundamental properties. The information gathered from these analysis
techniques can be used to explain the mechanism for superconductivity in
infinite-layer compounds. Because IL compounds have the simplest
crystalline structures in the class of high-temperature superconducting
cuprates, this information can be extended to determining the origin of
high-temperature superconductivity.
References
[1] G. Er, S. Kikkawa, M. Takahashi, F. Kanamaru, M. Hangyo, K. Kisoda,
S. Nakashima, Physica C 290 (1997) 1-8.
[2] N. Ikeda, Z. Hiroi, M. Azuma, M. Takano, and Y. Bando, Physica C
210 (1993) 367-372.
[3] J. Jorgensen, P. Radaelli, D. Hinks, and J. Wagner Phys. Rev. B 47
(1993) 654-655.
[4] C. Jung, et al Physica C 366 (2002) 299-305.
[5] J. Karpinski, I. Mangelschots, H. Schwer, et al. Physica C 235-240
(1994) 917-918.
[6] J. Karpinski, H. Schwer, I. Mangelschots, K. Conder, A. Morawski, T.
Lada, A. Paszewin Physica C 234 (1994) 10-18.
[7]X J. Markert, K. Mochizuki, and A. Elliott J. of Low Temp. Phys. 105
(1996) 1367-1372.
[8] A. Podlesnyak, A. Mirmelstein, V. Bobrovskii, et al, J.
Superconduct. 13, (1999) 145.
[9] A. Podlesnyak, A. Mirmelstein, V. Bobrovskii, et al. Physica C 258
(1996) 159-168.
[10] M. Smith, A. Manthiram, J. Zhou, J. Goodenough, and J. Markert
Nature 351 (1991) 549-550.
[11] B. Wiedenhorst, H. Berg, R. Gross, B. Freitag, and W. Mader,
Physica C 304, (1998) 147-155.
[12] T. Siegrist, S. Zahurak, D. Murphy and R. Roth Nature 363 (1988)
231.
[13] Growth of Single Crystals of the Bi-Sr-Ca-Cu-O Superconductor from
a KCl Flux, Jpn. J. Appl. Phys., 30(3A) (1991), L349-L351.
A paper that I wrote about London Theory
