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Quantum materials is a term used in condensed matter physics, which deals with macroscopic and microscopic physical properties of matter. Quantum materials are defined as solids that have exotic physical properties, which arise from the quantum mechanical properties of their electrons.
All materials exhibit some quantum properties, but quantum materials have unique properties such as quantum fluctuations, coherence, entanglement and specific topological behavior and various analytical techniques can be used to measure their properties.
Types and Uses
The types of quantum materials vary from superconductors and graphene to topological insulators, Weyl semimetals, quantum spin ices and spin liquids. Quantum materials include systems that contain organics, metals and oxides. The materials have a variety of applications that include magnetic field sensors, energy-related technologies, low-power memory modules, high-density storage devices and quantum computing.
The uses of quantum materials range from superconductors to multiferroics, topological insulators, colossal magnetoresistance compounds and nano-structures such as quantum dots.
Many quantum materials get their properties from the reduced dimensionality of their 2D sheets and the confinement of electrons. Quantum materials have strongly correlated electron systems, especially transition metal oxides with partially filled d or f electron shells.
Examples of strongly correlated phenomena includes superconductivity in copper oxides, iron-based superconductors, heavy fermions, colossal magnetoresistance, spin-charge ordering, Mott insulators and the Kondo effect.
Imaging Techniques - Spectroscopy
Spectroscopy is one of the main analytical techniques used to measure the properties of quantum materials, and includes Raman spectroscopy, infra-red (IR) spectroscopy, tip-enhanced spectroscopy, angle-resolved photoemission spectroscopy (ARPES) and Mössbauer spectroscopy.
Both Raman and IR spectroscopy are types of vibrational spectroscopy. Raman spectroscopy works by the detection of inelastic scattering of monochromatic light from a laser. IR uses the infrared region of the electromagnetic spectrum and works by measuring how much light is absorbed by the bonds of a vibrating molecule. They can be used to look at the intrinsic ferroelectric domains of nanocrystals. Raman and IR spectroscopy can also be combined with microscopy to yield more information about a material, and an advanced version of Raman spectroscopy called tip-enhanced Raman spectroscopy is often used in quantum material analysis.
Angle-resolved photoemission spectroscopy is a type of photoemission spectroscopy, and it works by looking photoemission of electrons from a sample from illumination with X-rays. ARPES provides information on valence electrons of a material, including the direction, speed and scattering process of the electrons. As well as this, ARPES can visualize band dispersions and Fermi surfaces. A particular area of interest that ARPES has been used for is to advance the knowledge of iron-based superconductors and topological insulators.
Mössbauer spectroscopy is a type of spectroscopy that uses the Mössbauer effect. The Mössbauer effect describes the recoilless resonant absorption and emission of gamma radiation by atomic nuclei in solids. Mössbauer spectroscopy works by exposing a material to a beam of gamma radiation, and the intensity of the beam being transmitted through the sample is then measured by a detector. One of the main applications of Mössbauer spectroscopy is in the field of geology for looking at redox ratios, spin and valence states and the coordination polyhedral of atoms in a crystal structure.
Imaging Techniques - Microscopy
A common way of imaging quantum materials is Transmission electron microscopy (TEM). TEM works by sending a beam of electrons through a thin sample and capturing the electrons that have passed through to create a detailed two-dimensional image. It is used to study the interior of a sample.
Another type of electron microscope used to image quantum materials is a scanning tunneling microscope (STM). STM is a type of scanning probe microscopy that scans the surface of samples with a probe to measure fine surface shapes and properties and generate an image at an atomic level. STM uses the principles of quantum tunneling, and the metal tips with single apical atoms attached to a tube where the current flows. The arrangement of individual atoms on the surfaces of metals has been observed using STM.
Ground state depletion (GSD) nanoscopy is a technique that can be used to look at layered van der Waals materials and single photon emitters from materials such as hexagonal boron nitride. GSD enables direct imaging with spatial resolutions that are beyond 10 nm and it removes the requirement for post image acquisition processing used in other techniques such as photo-activated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM).
Imaging Techniques - Spectrometry
A special type of spectrometer known as the BIFROST spectrometer is being developed by the Technical University Denmark (DTU) for the European Spallation Source (ESS) in Lund Sweden. The construction project has run since 2016 and expects to run to 2022, with user programs to be available from 2023. The new spectrometer is an indirect time-of-flight spectrometer. It looks at the physics of magnetism and superconductivity using neutron spectroscopy, which is the main tool for investigating low-energy dispersive dynamics in single crystals.