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Two-beam interference
Two-beam interference is the modulation of light intensity when two mutually coherent beams are superposed, giving cosine fringes in interferometers such as Young's double slit and the Michelson interferometer, the basis of LIGO's gravitational-wave detection.
Two-body problem
The two-body problem, or gravitational two-body problem, asks how two masses move under their mutual gravitational attraction; it is the one gravitational problem with a closed-form solution.
Two-body problem in general relativity
The two-body problem in general relativity, also called the relativistic two-body problem, seeks to predict the motion and gravitational radiation of two bodies from the nonlinear Einstein field equations.
Two-channel Kondo effect
The two-channel Kondo effect, or 2CK, occurs when a single spin-1/2 impurity is equally coupled to two independent electron reservoirs, so competing screening channels leave a fragile non-Fermi-liquid state, first realized in quantum dots in 2007.
Two-child policy
Two-child policy: China's 2016 universal two-child policy (全面两孩), replacing the one-child policy, and Vietnam's 1988 two-child limit, abolished 2025; covers birth effects, sex ratio, and the 2021 three-child shift.
Two-Colour
Two-Colour is a two-player graph theory game by Tahmina Begum (2020) in the Ludii General Game System: players place coloured pieces on graph vertices, scoring a point when the coloured vertex has no same-coloured neighbour; highest score wins.
Two-dimensional amorphous carbon
Two-dimensional amorphous carbon (2DAC), best known as monolayer amorphous carbon (MAC), first synthesized free-standing in 2020, is a carbon material one to a few atomic layers thick lacking long-range order.
Two-dimensional antiferromagnet
A two-dimensional antiferromagnet is an atomically thin magnetic material whose neighboring moments align in opposite directions, giving zero net magnetization and making it a spintronics platform.
Two-dimensional cell culture
Two-dimensional cell culture is the bench method of growing anchorage-dependent cells as a flat monolayer on treated polystyrene, the default for growing animal and human cells for over a century.
Two-dimensional chromatography
Two-dimensional chromatography is a separation technique in which parts or all of a sample are subjected to a second separation step, often a second column with different characteristics.
Two-dimensional electron gas
A two-dimensional electron gas (2DEG), or two-dimensional electron system, is electrons confined to a plane within a solid, known for the quantum Hall effect and high-mobility transistors.
Two-dimensional electron gas at GdTiO3/SrTiO3
The two-dimensional electron gas at the GdTiO3/SrTiO3 interface is a sheet of mobile electrons with density about 3 × 10^14 cm−2, the highest in oxide heterostructures.
Two-dimensional electronic spectroscopy
Two-dimensional electronic spectroscopy, or 2DES, is an ultrafast optical technique that maps energy flow between electronic states in molecules, widely used to study photosynthesis, photovoltaics, and quantum dots.
Two-dimensional electronic–vibrational spectroscopy
Two-dimensional electronic–vibrational spectroscopy (2DEV) is an ultrafast four-wave mixing technique that correlates electronic and vibrational transitions, developed at UC Berkeley and first demonstrated on a laser dye in 2014.
Two-dimensional ferroelectricity
2D ferroelectricity is switchable spontaneous electric polarization in atomically thin layered materials, once thought impossible and confirmed experimentally between 2015 and 2021 for memory devices.
Two-dimensional ferroelectricity in SnTe
Two-dimensional ferroelectricity in SnTe is the confirmed presence of switchable, spontaneous in-plane electric polarization in tin telluride films as thin as a single unit cell, first observed in 2016.
Two-dimensional ferromagnetism
Two-dimensional ferromagnetism is the persistence of ferromagnetic order in materials one or a few atomic layers thick, first demonstrated in 2017 in chromium germanium telluride and chromium triiodide using polar magneto-optical Kerr microscopy.
Two-dimensional gas chromatography
Two-dimensional gas chromatography (GC×GC) is an analytical technique separating volatile compounds on two coupled columns of different selectivity, introduced in 1991 and used for complex mixtures like petroleum.
Two-dimensional gel electrophoresis
Two-dimensional gel electrophoresis is a protein separation technique that resolves complex protein mixtures by isoelectric point, then molecular mass, producing spot maps of several thousand proteins.
Two-dimensional ice
Two-dimensional ice is crystalline water ice whose hydrogen-bond network is confined to one or a few molecular layers, forming square, pentagonal, and other structures absent from bulk ice.
Two-dimensional ice phases in graphene nanocapillaries
Two-dimensional ice phases in graphene nanocapillaries are crystalline forms of water forced into flat layers between graphene sheets, including the square ice imaged in 2015.
Two-Dimensional Immunoelectrophoresis
Two-dimensional immunoelectrophoresis, or crossed immunoelectrophoresis (CIE, X-IEP), is a gel-based immunoassay in which proteins separated by first-dimension electrophoresis enter an antibody-containing gel at right angles, forming quantifiable precipitation peaks; described by Ressler in 1960 and refined by Laurell.
Two-dimensional infrared spectroscopy
Two-dimensional infrared spectroscopy is an ultrafast technique that correlates infrared absorption frequencies across two dimensions to reveal molecular vibrations, their couplings, and their dynamics, first demonstrated in 1998.
Two-dimensional liquid chromatography
Two-dimensional liquid chromatography (2D-LC) is an analytical technique that applies two independent liquid-phase separations to one sample, used for complex mixtures in proteomics and pharmaceuticals since 1978.
Two-dimensional magnetic recording
Two-dimensional magnetic recording (TDMR) is a hard disk drive recording technology, proposed by Roger Wood at HGST in 2009, that increases areal density through shingled writing and two-dimensional readback.
Two-dimensional magnetism
Two-dimensional magnetism is the study of magnetic moment arrangements in atomically thin van der Waals crystals such as CrI3, first shown experimentally in 2016 and pursued for spintronic devices.
Two-dimensional magnetism in CrI3
Two-dimensional magnetism in CrI3 is the intrinsic ferromagnetic order in atomically thin chromium triiodide, first reported in 2017 with a Curie temperature of 45 K.
Two-dimensional materials
Two-dimensional materials are crystalline substances one or a few atomic layers thick, with strong in-plane bonds and weak bonding between planes; graphene, isolated in 2004, is the best-known example.
Two-dimensional multiferroics
Two-dimensional multiferroics are van der Waals materials one to a few atomic layers thick in which two or more ferroic orders, usually ferromagnetism and ferroelectricity, coexist and couple, with proposed uses in low-power memory and spintronics.
Two-dimensional NMR spectroscopy
Two-dimensional NMR spectroscopy is a family of nuclear magnetic resonance experiments spreading signals over two frequency axes, proposed by Jean Jeener in 1971 and developed by Richard Ernst.