La2-xSrxCuO4

La2-xSrxCuO4
Name	Lanthanum strontium copper oxide
Formula	La2‑xSr_xCuO4
Category	Cuprate superconductor
Crystal system	Tetragonal / Orthorhombic
Discovery	1986
Notable for	High‑temperature superconductivity

La2-xSrxCuO4 is a prototypical hole‑doped cuprate that established many experimental and theoretical paradigms for high‑temperature superconductivity. First synthesized and characterized in the mid‑1980s, its tunable composition La2‑xSr_xCuO4 connects an antiferromagnetic Mott insulator to a superconducting dome and to overdoped Fermi‑liquid behavior. The material has been central to studies by groups and institutions such as Bell Labs, IBM, Los Alamos National Laboratory, Stanford University, and Max Planck Institute for Solid State Research.

Introduction

La2‑xSr_xCuO4 occupies a canonical role alongside materials like YBa2Cu3O7 and Bi2Sr2CaCu2O8 in the history of condensed matter research. Its discovery followed and influenced work by teams including those of Georg Bednorz and K. Alex Müller and spurred experimental programs at laboratories such as Argonne National Laboratory and Lawrence Berkeley National Laboratory. The system’s phase diagram has been mapped using probes developed or refined at facilities like Brookhaven National Laboratory and observatories including beamlines at the European Synchrotron Radiation Facility.

Crystal structure and chemistry

The crystal lattice of La2‑xSr_xCuO4 is derived from the K2NiF4 structure type, shared with compounds studied by researchers at University of Cambridge and ETH Zurich. The structure alternates perovskite‑like CuO2 planes and rock‑salt LaO layers; substitution of La3+ by Sr2+ reduces the formal Cu valence and introduces mobile holes. At low temperatures the lattice undergoes a structural transition between tetragonal and orthorhombic symmetries, an effect studied in diffraction experiments at CERN and the National Institute of Standards and Technology crystallography facilities. Chemical control of oxygen content and cation stoichiometry connects synthesis efforts at groups such as MIT and University of Tokyo to thermodynamic analyses advanced by researchers at Columbia University.

Electronic structure and phase diagram

Angle‑resolved photoemission spectroscopy (ARPES) experiments carried out by teams from Stanford University and University of California, Berkeley have mapped the Fermi surface evolution as x increases from the insulating parent to the overdoped regime. The phase diagram displays an antiferromagnetic phase proximate to the Mott insulator, a superconducting dome centered near x≈0.15, and a pseudogap region that has been debated by scholars associated with Princeton University and Harvard University. Quantum oscillation studies performed at HFML and collaborations with Dresden groups have probed the Fermiology in high magnetic fields, while theoretical frameworks from groups led by researchers at Rutgers University and University of Illinois Urbana‑Champaign have applied Hubbard and t‑J models to describe correlations.

Superconductivity and pairing mechanisms

Superconductivity in La2‑xSr_xCuO4 was pivotal to development of unconventional pairing theories proposed by theorists at Institute for Advanced Study and California Institute of Technology. Experimental evidence for d‑wave pairing symmetry emerged from phase‑sensitive Josephson junction experiments performed by teams at University of Wisconsin–Madison and Weizmann Institute of Science, and from tunneling work at Oak Ridge National Laboratory. Competing theoretical scenarios invoking spin‑fluctuation mediated pairing, resonating valence bond (RVB) states championed by proponents at Institute Laue‑Langevin and electron‑phonon coupling modified by strong correlations have all been applied to interpret results measured at institutions such as University of Cambridge.

Magnetic and charge ordering

Neutron scattering by collaborations involving ISIS Neutron and Muon Source and Institut Laue‑Langevin revealed antiferromagnetic order in the parent compound and incommensurate spin fluctuations with doping, observations central to proposals by researchers at Bell Labs and Brookhaven National Laboratory. Static stripe charge order, reported in resonant X‑ray scattering studies at SLAC National Accelerator Laboratory and European Synchrotron Radiation Facility, links La2‑xSr_xCuO4 to stripe phenomena discussed in the literature by groups at University of Groningen and Louisiana State University. Muon spin rotation (μSR) studies from teams at Paul Scherrer Institute and TRIUMF have characterized magnetic volume fractions and coexistence regimes with superconductivity.

Transport and optical properties

Transport measurements—resistivity, Hall effect, and thermopower—by experimentalists at Bell Labs and Argonne National Laboratory demonstrated non‑Fermi‑liquid temperature dependences in the underdoped regime and a crossover to Fermi‑liquid responses in overdoped samples. Optical conductivity and infrared spectroscopy work from groups at University of California, San Diego and University of Pennsylvania mapped spectral-weight transfers linked to strong correlations and the superconducting condensate. High‑field magnetotransport studies undertaken at Los Alamos National Laboratory and National High Magnetic Field Laboratory have elucidated vortex dynamics and normal‑state resistivity under extreme conditions.

Synthesis and growth methods

Single crystals and thin films of La2‑xSr_xCuO4 have been synthesized using techniques developed at IBM Research and Tohoku University: the traveling‑solvent floating zone method, molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and metal‑organic chemical vapor deposition (MOCVD). Thin‑film fabrication for junctions and devices has been pursued at Hitachi and Fujitsu Laboratories, while bulk ceramic synthesis and annealing protocols were refined in laboratories at University of Geneva and University of Oxford. Characterization tools including scanning tunneling microscopy teams from University of Tokyo and electron microscopy centers at Max Planck Institute for Microstructure Physics remain essential for assessing defects, doping homogeneity, and interface quality.

Category:Cuprate superconductors