quantum tunnelling


  • [1] Some sources describe the mere penetration of a wave function into the barrier, without transmission on the other side, as a tunneling effect, such as in tunneling into
    the walls of a finite potential well.

  • [14][15][16][17] The latter researchers simultaneously solved the Schrödinger equation for a model nuclear potential and derived a relationship between the half-life of the
    particle and the energy of emission that depended directly on the mathematical probability of tunneling.

  • Substituting the second equation into the first and using the fact that the real part needs to be 0 results in: Quantum tunneling in the phase space formulation of quantum

  • This peculiar property is used in some applications, such as high speed devices where the characteristic tunnelling probability changes as rapidly as the bias voltage.

  • Therefore, the transmission coefficient for a particle tunneling through a single potential barrier is where are the two classical turning points for the potential barrier.

  • [18] In the early days of quantum theory, the term tunnel effect was not used, and the effect was instead referred to as penetration of, or leaking through, a barrier.

  • In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a quantum mechanical phenomenon in which an object such as an electron or atom passes through
    a potential energy barrier that, according to classical mechanics, the object does not have sufficient energy to enter or surmount.

  • [41][42] Dynamical tunneling The concept of quantum tunneling can be extended to situations where there exists a quantum transport between regions that are classically not
    connected even if there is no associated potential barrier.

  • For a rectangular barrier, this expression simplifies to: Faster than light Some physicists have claimed that it is possible for spin-zero particles to travel faster than
    the speed of light when tunnelling.

  • The wave function of a physical system of particles specifies everything that can be known about the system.

  • The square of the absolute value of this wave function is directly related to the probability distribution of the particle positions, which describes the probability that
    the particles would be measured at those positions.

  • [25] The resonant tunnelling diode makes use of quantum tunnelling in a very different manner to achieve a similar result.

  • [26] Tunnel field-effect transistors[edit] Main article: Tunnel field-effect transistor A European research project demonstrated field effect transistors in which the gate
    (channel) is controlled via quantum tunnelling rather than by thermal injection, reducing gate voltage from ≈1 volt to 0.2 volts and reducing power consumption by up to 100×.

  • To start, a classical turning point, is chosen and is expanded in a power series about : Keeping only the first order term ensures linearity: Using this approximation, the
    equation near becomes a differential equation: This can be solved using Airy functions as solutions.

  • However, in certain cases, large isotopic effects are observed that cannot be accounted for by a semi-classical treatment, and quantum tunnelling is required.

  • Away from the potential hill, the particle acts similar to a free and oscillating wave; beneath the potential hill, the particle undergoes exponential changes in amplitude.

  • This is generally attributed to differences in the zero-point vibrational energies for chemical bonds containing the lighter and heavier isotopes and is generally modeled
    using transition state theory.

  • Hence, the probability of a given particle’s existence on the opposite side of an intervening barrier is non-zero, and such particles will appear on the ‘other’ (a semantically
    difficult word in this instance) side in proportion to this probability.

  • This creates a quantum potential well that has a discrete lowest energy level.

  • In particular, the group velocity of a wave packet does not measure its speed, but is related to the amount of time the wave packet is stored in the barrier.

  • Radioactive decay is a relevant issue for astrobiology as this consequence of quantum tunnelling creates a constant energy source over a large time interval for environments
    outside the circumstellar habitable zone where insolation would not be possible (subsurface oceans) or effective.

  • [10] Therefore, problems in quantum mechanics analyze the system’s wave function.

  • It thus follows that evanescent wave coupling can occur if a region of positive M(x) is sandwiched between two regions of negative M(x), hence creating a potential barrier.

  • He assumed a surface potential barrier which confines the electrons within the metal and showed that the electrons have a finite probability of tunneling through or reflecting
    from the surface barrier when their energies are close to the barrier energy.

  • The analysis of a rectangular barrier by means of the Schrödinger equation can be adapted to these other effects provided that the wave equation has travelling wave solutions
    in medium A but real exponential solutions in medium B.

  • When the electric field is very large, the barrier becomes thin enough for electrons to tunnel out of the atomic state, leading to a current that varies approximately exponentially
    with the electric field.

  • [37] Mathematical discussion The Schrödinger equation[edit] The time-independent Schrödinger equation for one particle in one dimension can be written as or where • is the
    reduced Planck’s constant, • m is the particle mass, • x represents distance measured in the direction of motion of the particle, • Ψ is the Schrödinger wave function, • V is the potential energy of the particle (measured relative to any convenient
    reference level), • E is the energy of the particle that is associated with motion in the x-axis (measured relative to V), • M(x) is a quantity defined by V(x) − E which has no accepted name in physics.

  • [43][44] Tunnelling in phase space[edit] The concept of dynamical tunnelling is particularly suited to address the problem of quantum tunnelling in high dimensions (d>1).

  • [23] When a free electron wave packet encounters a long array of uniformly spaced barriers, the reflected part of the wave packet interferes uniformly with the transmitted
    one between all barriers so that 100% transmission becomes possible.

  • In quantum mechanics, a particle can, with a small probability, tunnel to the other side, thus crossing the barrier.

  • Resonance-assisted tunnelling[edit] When is small in front of the size of the regular islands, the fine structure of the classical phase space plays a key role in tunnelling.

  • Tunneling is a consequence of the wave nature of matter, where the quantum wave function describes the state of a particle or other physical system, and wave equations such
    as the Schrödinger equation describe their behavior.

  • From the equations, the power series must start with at least an order of to satisfy the real part of the equation; for a good classical limit starting with the highest power
    of Planck’s constant possible is preferable, which leads to and with the following constraints on the lowest order terms, and At this point two extreme cases can be considered.

  • This diode has a resonant voltage for which a current favors a particular voltage, achieved by placing two thin layers with a high energy conductance band near each other.

  • In the case of an integrable system, where bounded classical trajectories are confined onto tori in phase space, tunnelling can be understood as the quantum transport between
    semi-classical states built on two distinct but symmetric tori.

  • [45] Chaos-assisted tunnelling[edit] Chaos-assisted tunnelling oscillations between two regular tori embedded in a chaotic sea, seen in phase space In real life, most systems
    are not integrable and display various degrees of chaos.

  • The theory predicts that if positively charged nuclei form a perfectly rectangular array, electrons will tunnel through the metal as free electrons, leading to extremely high
    conductance, and that impurities in the metal will disrupt it.

  • Classical mechanics predicts that particles that do not have enough energy to classically surmount a barrier cannot reach the other side.

  • [9] This implies that no solutions have a probability of exactly zero (or one), though it may approach infinity.

  • Tunnel diode[edit] Main article: Tunnel diode A working mechanism of a resonant tunnelling diode device, based on the phenomenon of quantum tunnelling through the potential
    barriers Diodes are electrical semiconductor devices that allow electric current flow in one direction more than the other.

  • The first person to apply the Schrödinger equation to a problem which involved tunneling between two classically allowed regions through a potential barrier was Friedrich
    Hund in a series of articles published in 1927.

  • Classically, the electron would either transmit or reflect with 100% certainty, depending on its energy.

  • Solid-state physics[edit] Electronics[edit] Tunnelling is a source of current leakage in very-large-scale integration (VLSI) electronics and results in a substantial power
    drain and heating effects that plague such devices.

  • Though this probability is still low, the extremely large number of nuclei in the core of a star is sufficient to sustain a steady fusion reaction.

  • The wave packet becomes more de-localized: it is now on both sides of the barrier and lower in maximum amplitude, but equal in integrated square-magnitude, meaning that the
    probability the particle is somewhere remains unity.

  • If, for example, the calculation for its position was taken as a probability of 1, its speed would have to be infinity (an impossibility).

  • Examples include the tunnelling of a classical wave-particle association,[48] evanescent wave coupling (the application of Maxwell’s wave-equation to light) and the application
    of the non-dispersive wave-equation from acoustics applied to “waves on strings”.

  • When a small forward bias is applied, the current due to tunnelling is significant.

  • The reason for this difference comes from treating matter as having properties of waves and particles.

  • By using piezoelectric rods that change in size when voltage is applied, the height of the tip can be adjusted to keep the tunnelling current constant.

  • It is similar to thermionic emission, where electrons randomly jump from the surface of a metal to follow a voltage bias because they statistically end up with more energy
    than the barrier, through random collisions with other particles.

  • To solve this equation using the semiclassical approximation, each function must be expanded as a power series in .

  • As the voltage further increases, tunnelling becomes improbable and the diode acts like a normal diode again before a second energy level becomes noticeable.

  • [29] Quantum tunnelling may be one of the mechanisms of hypothetical proton decay.

  • One interpretation of this duality involves the Heisenberg uncertainty principle, which defines a limit on how precisely the position and the momentum of a particle can be
    simultaneously known.

  • Given the two coefficients on one side of a classical turning point, the two coefficients on the other side of a classical turning point can be determined by using this local
    solution to connect them.


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Photo credit: https://www.flickr.com/photos/ronsaunders47/8050213579/’]