122 iron arsenide
Encyclopedia
The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductor
s. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name “122” comes from their stoichiometry
; the 122s have the chemical formula XFe2As2, where X = Ca, Ba, Sr or Eu. These materials become superconducting under pressure and also upon doping.
The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2. The microscopic description of superconductivity in the 122s is yet unclear.
-induced interactions between electrons responsible for conventional BCS theory
s-wave superconductivity—has recently been expanded by the discovery of high temperature superconductivity (up to Tc = 55 K) in the doped oxypnictide
(1111) superconductors with the chemical composition XOFeAs, where X = La, Ce, Pr, Nd, Sm, Gd, Tb, or Dy. The 122s contain the same iron-arsenide planes as the oxypnictides, but are much easier to synthesize in the form of large single crystals.
There are two different ways in which superconductivity was achieved in the 122s. One method is the application of pressure to the undoped parent compounds. The second is the introduction of other elements (dopants) into the crystal structure in very specific ratios. There are two doping schemes: The first type of doping involves the introduction of holes into the barium
or strontium
varieties; hole doping refers to the substitution of one ion for another with fewer electrons. Superconducting transition temperatures as high as 38 K have been reported upon substitution of the 40% of the Ba2+ or Sr2+ ions with K+. The second doping method is to directly dope the iron-arsenide layer by replacing iron with cobalt. Superconducting transition temperatures up to ~20 K have been observed in this case.
Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the flux method
. The behavior of these materials is interesting by that superconductivity exists alongside antiferromagnetism
. Various studies including electrical resistivity, magnetic susceptibility
, specific heat, NMR
, neutron scattering, X-ray diffraction, Mössbauer spectroscopy, and quantum oscillations have been performed for the undoped parent compounds, as well as the superconducting versions.
. In a nutshell, the flux method uses some solvent in which the starting materials for a chemical reaction are able to dissolve and eventually crystallize into the desired compound. Two standard methods show up in the literature, each utilizing a different flux. The first method employs tin, while the second uses the binary metallic compound FeAs (iron arsenide).
These materials undergo a first-order structural phase transition
into the Fmmm orthorhombic structure below some characteristic temperature T0 that is compound specific. NMR experiments on the CaFe2As2 show that there is a first-order antiferromagnetic magnetic phase transition at the same temperature; in contrast, the antiferromagnetic transition occurs at a lower temperature in the 1111s. The high temperature magnetic state is paramagnetic, while the low temperature state is an antiferromagnetic state known as a spin-density-wave
.
The Tc of 38 K in Ba1−xKxFe2As2 (x ≈ 0.4) superconductor shows the inverse iron isotope effect.
Iron-based superconductor
Iron-based superconductors are chemical compounds with superconducting properties. In 2008, led by recently discovered iron pnictide compounds , they were in the first stages of experimentation and implementation...
s. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name “122” comes from their stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
; the 122s have the chemical formula XFe2As2, where X = Ca, Ba, Sr or Eu. These materials become superconducting under pressure and also upon doping.
The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2. The microscopic description of superconductivity in the 122s is yet unclear.
Overview
Ever since the discovery of high-temperature (High Tc) superconductivity in the cuprate materials, scientists have worked tirelessly to understand the microscopic mechanisms responsible for its emergence. To this day, no theory can fully explain the high-temperature superconductivity and unconventional (non-s-wave) pairing state found in these materials. However, the interest of the scientific community in understanding the pairing glue for unconventional superconductors—those in which the glue is electronic, i.e. cannot be attributed to the phononPhonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
-induced interactions between electrons responsible for conventional BCS theory
BCS theory
BCS theory — proposed by Bardeen, Cooper, and Schrieffer in 1957 — is the first microscopic theory of superconductivity since its discovery in 1911. The theory describes superconductivity as a microscopic effect caused by a "condensation" of pairs of electrons into a boson-like state...
s-wave superconductivity—has recently been expanded by the discovery of high temperature superconductivity (up to Tc = 55 K) in the doped oxypnictide
Oxypnictide
In chemistry, oxypnictides are a class of materials including oxygen, a pnictogen and one or more other elements...
(1111) superconductors with the chemical composition XOFeAs, where X = La, Ce, Pr, Nd, Sm, Gd, Tb, or Dy. The 122s contain the same iron-arsenide planes as the oxypnictides, but are much easier to synthesize in the form of large single crystals.
There are two different ways in which superconductivity was achieved in the 122s. One method is the application of pressure to the undoped parent compounds. The second is the introduction of other elements (dopants) into the crystal structure in very specific ratios. There are two doping schemes: The first type of doping involves the introduction of holes into the barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
or strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
varieties; hole doping refers to the substitution of one ion for another with fewer electrons. Superconducting transition temperatures as high as 38 K have been reported upon substitution of the 40% of the Ba2+ or Sr2+ ions with K+. The second doping method is to directly dope the iron-arsenide layer by replacing iron with cobalt. Superconducting transition temperatures up to ~20 K have been observed in this case.
Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the flux method
Flux method
Flux method is a method of crystal growth where the components of the desired substance are dissolved in a solvent . The method is particularly suitable for crystals needing to be free from thermal strain and it takes place in a crucible made of non-reactive metal such as platinum, tantalum,...
. The behavior of these materials is interesting by that superconductivity exists alongside antiferromagnetism
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
. Various studies including electrical resistivity, magnetic susceptibility
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
, specific heat, NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
, neutron scattering, X-ray diffraction, Mössbauer spectroscopy, and quantum oscillations have been performed for the undoped parent compounds, as well as the superconducting versions.
Synthesis
One of the important qualities of the 122s is their ease of synthesis; it is possible to grow large single crystals, up to ~5×5×0.5 mm, using the flux methodFlux method
Flux method is a method of crystal growth where the components of the desired substance are dissolved in a solvent . The method is particularly suitable for crystals needing to be free from thermal strain and it takes place in a crucible made of non-reactive metal such as platinum, tantalum,...
. In a nutshell, the flux method uses some solvent in which the starting materials for a chemical reaction are able to dissolve and eventually crystallize into the desired compound. Two standard methods show up in the literature, each utilizing a different flux. The first method employs tin, while the second uses the binary metallic compound FeAs (iron arsenide).
Structural and magnetic phase transition
The 122s form in the I4/mmm tetragonal structure. For example, the tetragonal unit cell of SrFe2As2, at room temperature, has lattice parameters a = b = 3.9243 Å and c = 12.3644 Å. The planar geometry is reminiscent of the cuprate high-Tc superconductors in which the Cu-O layers are believed to support superconductivity.These materials undergo a first-order structural phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
into the Fmmm orthorhombic structure below some characteristic temperature T0 that is compound specific. NMR experiments on the CaFe2As2 show that there is a first-order antiferromagnetic magnetic phase transition at the same temperature; in contrast, the antiferromagnetic transition occurs at a lower temperature in the 1111s. The high temperature magnetic state is paramagnetic, while the low temperature state is an antiferromagnetic state known as a spin-density-wave
Spin density wave
Spin-density wave and charge-density wave are names for two similar low-energy ordered states of solids. Both these states occur at low temperature in anisotropic, low-dimensional materials or in metals that have high densities of states at the Fermi level N...
.
Superconductivity
Superconductivity has been observed in the 122s up to a current maxium Tc of 38 K in Ba1−xKxFe2As2 with x ≈ 0.4. Resistivity and magnetic susceptibility measurements have confirmed the bulk nature of the observed superconducting transition. The onset of superconductivity is correlated with the loss of the spin-density-wave state.The Tc of 38 K in Ba1−xKxFe2As2 (x ≈ 0.4) superconductor shows the inverse iron isotope effect.