Aharonov–Bohm Rings
An Aharonov–Bohm ring is an open mesoscopic conductor in which electrons can travel between reservoirs through two or more paths that enclose magnetic flux. The terminal conductance oscillates because the relative phase of those paths changes by when the enclosed flux changes by the normal-electron flux quantum .
The ideal Aharonov–Bohm thought experiment isolates flux from the accessible paths. A laboratory ring is usually placed in a nearly uniform magnetic field, so the field also penetrates the arms and leads. Orbital deflection, Zeeman splitting, weak localization, universal conductance fluctuations, contact motion, and density changes can accompany the flux phase. A periodic trace is therefore a mesoscopic inference problem, not a literal image of two force-free trajectories.
This page owns ring magnetotransport: two-arm interference, conductance harmonics, field-to-area calibration, and mechanisms, two-terminal phase rigidity, ring-specific coherence extraction, and Fourier-analysis controls. Aharonov–Bohm Effect owns the gauge-holonomy and topology derivation. Aharonov–Bohm Effect: First Encounter owns the wave-mechanics and closed-ring spectrum. Quantum Coherence in Conductors owns the generic dephasing kernel and comparison among coherence extractors.
Ring Geometry and Phase
Section titled “Ring Geometry and Phase”Consider source and drain reservoirs connected by upper and lower arms. Let be the magnetic flux through an oriented effective ring area. For charge , the electromagnetic contribution to the relative phase is
Define the positive normal-electron flux quantum
For an electron, , so orientation fixes the sign:
The measured oscillation period is insensitive to that sign. The sign matters when combining magnetic phase with dynamical, spin, valley, or Berry phases under a declared orientation convention.
The total phase difference between two arms is
where the last term can include spin rotation, geometric phase, scattering phases, or a resonant element embedded in one arm. Gate voltage can change path length, wave number, mode occupation, and junction scattering at the same time.
An open ring is not a closed-ring spectrum
Section titled “An open ring is not a closed-ring spectrum”The exactly solvable particle-on-a-ring problem has discrete flux-shifted energies and equilibrium persistent current. A conductance ring is open to reservoirs. Its observable is a scattering probability, broadened by escape and averaged over occupied energies. Both systems are flux periodic, but persistent current and two-terminal conductance are different observables with different ensembles and dissipation ledgers.
Particle on a Ring is the canonical home for the closed spectrum. The open-ring calculation begins with transmission amplitudes.
From Path Amplitudes to Conductance
Section titled “From Path Amplitudes to Conductance”In the simplest single-mode picture, write the source-to-drain amplitude as
Then
where , , and . At zero temperature, one resolved channel contributes
This two-path formula explains the oscillation, but real junctions permit reflection and repeated winding. The amplitude then contains paths labeled by arm sequence, transverse mode, spin, and winding number. A useful schematic expansion is
where is the oriented winding number. Conductance contains cross terms between every pair of paths that arrive in the same outgoing channel.
Visibility is not a pure coherence meter
Section titled “Visibility is not a pure coherence meter”For a nearly sinusoidal trace, an experimental visibility is often defined as
In the ideal two-path model,
Perfect coherence therefore does not guarantee unit visibility: strongly unequal arm amplitudes reduce contrast. Mode averaging, contact reflection, path-length mismatch, finite temperature, detector bandwidth, and background conductance also reduce it. Extracting a phase-coherence length from visibility requires a transport model and control of those non-dephasing factors.
Flux Period and Effective Area
Section titled “Flux Period and Effective Area”One additional flux quantum changes the single-winding phase by :
If a perpendicular field threads an effective area , then
A measured period therefore gives
for a circular-area comparison. The effective area need not equal the area inside a lithographic centerline. Finite arm width provides a distribution of enclosed areas, while depletion, edge states, magnetic focusing, flux focusing, and density-dependent path motion shift the conducting trajectories.
For an annulus with inner and outer radii and , plausible orbital areas span
A broad or split Fourier peak can therefore be geometric rather than evidence for two independent particles or two topological sectors.
Byers–Yang periodicity
Section titled “Byers–Yang periodicity”For particles of charge magnitude in a multiply connected normal system, gauge invariance implies periodicity under insertion of flux, including in the presence of interactions when the many-body state and boundary conditions are treated consistently. This fundamental periodicity does not require every measured trace to be a single sinusoid.
The conductance may contain harmonics:
The th harmonic has field period
Upper and lower arm amplitudes acquire a relative phase and recombine at the drain. A field sweep produces oscillatory conductance on top of a generally nonperiodic background. Fourier analysis can reveal an peak and higher winding or time-reversed structure, but peak width also records the finite field window and distribution of effective ring areas.
h/e and h/2e Harmonics
Section titled “h/e and h/2e Harmonics”The same field period can arise from different path families, so harmonic labels are not mechanism labels by themselves.
Sample-specific h/e oscillations
Section titled “Sample-specific h/e oscillations”Interference between paths whose winding numbers differ by one gives the period. In a disordered ring, its phase depends on the particular impurity configuration and Fermi energy. Averaging over many independent disorder realizations, rings, or sufficiently broad gate ranges can suppress the signed contribution even when each individual ring shows it clearly.
Repeated winding
Section titled “Repeated winding”A coherent path that circles twice before leaving can interfere with a path whose winding differs by two, producing an harmonic. Higher harmonics similarly probe longer dwell paths and are usually more sensitive to dephasing and escape.
Altshuler–Aronov–Spivak oscillations
Section titled “Altshuler–Aronov–Spivak oscillations”In a diffusive ring or cylinder, a path and its time reverse accumulate the same random dynamical phase but opposite magnetic phase. Their interference contribution depends on
and therefore has period . Because static-disorder phases cancel for this time-reversed pair, the ensemble-averaged contribution can survive when the sample-specific term averages toward zero.
This is the ring analogue of the Cooperon channel in weak localization. It is distinct from superconducting flux periodicity, where the condensate charge is . Observing an peak in a normal ring does not by itself imply pairing.
| Observed component | Representative origin | Disorder averaging |
|---|---|---|
| paths whose winding differs by one | phase is generally sample specific | |
| double winding in one sample | can remain sample specific | |
| time-reversed diffusive loops | survives ensemble averaging more robustly | |
| in a superconductor | charge- condensate response | different physical origin |
Phase Rigidity and Terminal Geometry
Section titled “Phase Rigidity and Terminal Geometry”Linear-response reciprocity gives
when microscopic reversibility applies and magnetic order or other time-reversal-breaking controls are reversed consistently. In a two-terminal conductor this reduces to
Suppose an isolated first harmonic is written
Evenness in requires
This phase rigidity means that the apparent two-terminal oscillation phase cannot generally slide continuously under a gate sweep while remaining a single linear-response harmonic. Its amplitude can pass through zero and reappear with a jump.
The restriction changes in a multi-terminal interferometer because reciprocity relates different lead permutations rather than forcing one trace to be even. Nonlinear conductance can also contain field-antisymmetric terms. Terminal geometry and bias regime must therefore be reported before interpreting an oscillation phase as the transmission phase of a quantum dot or other embedded object.
Symmetrizing a noisy trace by hand can conceal a real nonlinear or multi-terminal asymmetry. Reciprocity is a physical test to perform on the raw linear-response data, not merely a plotting instruction.
Ballistic and Diffusive Rings
Section titled “Ballistic and Diffusive Rings”In a ballistic or quasiballistic ring, a small number of arm modes and junction scattering amplitudes may dominate. Gate-defined quantum point contacts can tune injection and beam-splitter balance. Semiclassical path lengths and mode conversion then organize the interference.
In a diffusive ring,
where is the circumference. Many elastically scattered paths contribute. Static disorder does not automatically destroy coherence; it fixes a reproducible interference fingerprint. The useful distinction is
controls momentum randomization, while controls phase memory. A ring can be diffusive and phase coherent when
Contacts introduce an escape time and can truncate long winding paths before intrinsic dephasing does. Closing the contacts raises dwell time and harmonic content but can also create resonances or an unintended quantum dot. A complete ring model includes arm disorder, junction scattering, reservoir coupling, and mode number.
Dephasing and Thermal Averaging
Section titled “Dephasing and Thermal Averaging”For a one-winding path length , a common phenomenological harmonic envelope is
describes thermal and energy averaging, while describes loss through contacts. The exponential form is a useful diagnostic, not a universal theorem. Ballistic and diffusive rings weight path times differently; spin, intervalley structure, ring arrays, canonical versus grand-canonical conditions, and nonexponential phase noise can change the prefactor and exponent.
The thermal length is often estimated as
in diffusion, or parametrically
for ballistic energy averaging. Numerical factors depend on the observable and geometry.
Thermal averaging is not dephasing
Section titled “Thermal averaging is not dephasing”At finite temperature, linear conductance averages the energy-dependent transmission:
If the interference phase changes across the Fermi window, contributions at different energies cancel in the measured average even though each electron remains coherent. True dephasing instead reduces phase correlations through inelastic scattering, fluctuating environments, or entanglement with unobserved degrees of freedom.
Changing temperature affects both. A credible extraction of either models independently, compares harmonics and ring sizes, or uses another coherence observable. Fitting all amplitude loss to overestimates intrinsic dephasing.
Why higher harmonics help
Section titled “Why higher harmonics help”Under the simple common-prefactor model,
Higher windings therefore amplify sensitivity to phase memory. But their junction amplitudes, thermal factors, and effective areas need not match. Harmonic ratios become quantitative only after those assumptions are tested.
Measurement and Fourier Workflow
Section titled “Measurement and Fourier Workflow”Establish the regime
Section titled “Establish the regime”Report ring radii and arm width, carrier density, , , temperature, excitation bias, lead number, and contact transparency. State whether the transport is ballistic, quasiballistic, diffusive, or resonant.
Verify linear response and reciprocity
Section titled “Verify linear response and reciprocity”Reduce the excitation until the trace is bias independent within uncertainty. Compare and before background subtraction. A two-terminal linear trace should be even apart from drift and noise.
Separate periodic signal from background
Section titled “Separate periodic signal from background”Classical magnetoresistance, weak localization, and conductance fluctuations can be larger than the ring oscillation. Detrending must preserve periods near the expected . Report the polynomial, smoothing scale, or high-pass filter and demonstrate robustness against reasonable alternatives.
Use a controlled Fourier transform
Section titled “Use a controlled Fourier transform”For uniformly sampled field points , transform a declared window of . The finite field span limits frequency resolution to order
Window functions trade spectral leakage for peak width. Zero padding interpolates the displayed spectrum but does not improve physical resolution. Quote the field range, sampling interval, window, and frequency-to-area conversion.
Test effective area
Section titled “Test effective area”Compare
with the geometric annulus. Track the peak versus gate voltage. Smooth motion can reflect a changing electronic path; abrupt jumps can reflect mode switching or charge rearrangement.
Check reproducibility without averaging away the physics
Section titled “Check reproducibility without averaging away the physics”Repeat sweeps in both directions and at several rates. Warm cycling may rearrange disorder and change the fingerprint while preserving its characteristic period. An array average can enhance the disorder-robust component and suppress sample-specific oscillations.
| Observation | Strong interpretation | Essential control |
|---|---|---|
| peak at | one-winding flux interference | match area and track gate dependence |
| peak at | harmonic | distinguish double winding, AAS, and pairing |
| amplitude falls with | reduced coherent contrast | separate thermal averaging from dephasing |
| apparent continuous phase shift | path or scatterer phase evolution | check terminal geometry and phase rigidity |
| split Fourier peak | several effective areas or beating | vary field window, gate voltage, and detrending |
| oscillations survive large field | robust phase-coherent paths | audit field penetration, Zeeman, and edge reconstruction |
Berry Phase and Topology
Section titled “Berry Phase and Topology”Aharonov–Bohm and Berry phases share the mathematics of a holonomy, but their base spaces and physical assumptions differ.
| Feature | Aharonov–Bohm phase | Berry phase |
|---|---|---|
| connection | electromagnetic | eigenstate connection in parameter or momentum space |
| loop | real-space path around flux | adiabatic path of an eigenspace |
| basic assumption | coherent path interference | adiabatic following |
| curvature | magnetic field, possibly excluded from paths | Berry curvature in the chosen bundle |
Berry Phase in the Aharonov–Bohm Effect owns the detailed comparison.
Spin–orbit coupling can rotate spin around a ring and add an Aharonov–Casher or spin geometric phase. In a topological-insulator nanowire, a surface-state Berry phase can shift which flux values close the finite-size gap. These are additional structures layered on the ordinary electromagnetic phase.
An conductance oscillation alone is not evidence of topological matter: conventional normal-metal and semiconductor rings show the same fundamental period. A topological claim needs the predicted phase or parity pattern, surface or edge-state identification, disorder and density dependence, and consistency with the material’s band topology. Even then, Zeeman energy, bulk conduction, finite thickness, and multiple areas must be modeled.
The word topological also has two meanings to keep separate. The ideal Aharonov–Bohm geometry is multiply connected, so winding classes are topological. The phase itself varies continuously with flux and does not by itself define a topological phase of matter.
Common Mistakes
Section titled “Common Mistakes”Treating uniform-field rings as perfectly field free
Section titled “Treating uniform-field rings as perfectly field free”The field threads the arms and leads, so orbital and Zeeman effects can accompany the flux phase.
Using the lithographic hole as the exact area
Section titled “Using the lithographic hole as the exact area”Finite-width trajectories, depletion, edge motion, and focusing define .
Calling every h/2e peak superconducting
Section titled “Calling every h/2e peak superconducting”Double winding and Altshuler–Aronov–Spivak interference produce structure in normal conductors.
Equating reduced visibility with dephasing
Section titled “Equating reduced visibility with dephasing”Arm imbalance, thermal averaging, mode averaging, escape, and drift also reduce contrast.
Reading a continuous phase from a closed two-terminal trace
Section titled “Reading a continuous phase from a closed two-terminal trace”Linear-response phase rigidity constrains a single harmonic to phase or .
Letting zero padding masquerade as resolution
Section titled “Letting zero padding masquerade as resolution”The physical Fourier resolution is set by the measured field span, not by the number of interpolated frequency points.
Averaging away sample-specific h/e oscillations
Section titled “Averaging away sample-specific h/e oscillations”Disorder or gate averaging can suppress the first harmonic while preserving an ensemble contribution.
Calling an h/e period a topological-material signature
Section titled “Calling an h/e period a topological-material signature”The period is generic electromagnetic interference; topology requires additional phase, state, and material evidence.
Exercises
Section titled “Exercises”1. Derive the ideal visibility
Section titled “1. Derive the ideal visibility”Starting from two arm probabilities and , derive . Evaluate it for and .
Solution
The extrema of
are
Therefore
For the stated values,
The paths can be fully coherent even though their unequal amplitudes prevent unit visibility.
2. Infer an effective radius
Section titled “2. Infer an effective radius”An oscillation has period . Find and the corresponding circular radius.
Solution
Using ,
Then
This radius should be compared with the full conducting annulus, not only the drawn gate centerline.
3. Locate two harmonics
Section titled “3. Locate two harmonics”A ring has . Find the and field periods.
Solution
Since ,
The second harmonic has half that period:
The period identifies the harmonic, not whether its microscopic origin is double winding or an AAS path pair.
4. Prove phase rigidity for one harmonic
Section titled “4. Prove phase rigidity for one harmonic”Show that forces or for
when .
Solution
Let . Evenness requires
for all . Their difference is
It vanishes for every only if , hence
If passes through zero during a gate sweep, the fitted phase can jump by without violating reciprocity.
5. Why can h/2e survive averaging?
Section titled “5. Why can h/2e survive averaging?”Explain why an ensemble of disordered rings can lose its average signal while retaining an component.
Solution
The phase contains sample-specific dynamical phases fixed by each disorder realization, so its signed contribution averages toward zero across many statistically independent rings or configurations. For a path paired with its time reverse, the static-disorder phase is the same and cancels in the relative phase, while the magnetic phase doubles. This disorder-robust pair produces an Altshuler–Aronov–Spivak contribution with period .
6. Estimate a coherence length from harmonics
Section titled “6. Estimate a coherence length from harmonics”Assume
with the same prefactor for two harmonics. A ring has and . Find .
Solution
The ratio is
Thus
This estimate fails if the two harmonics have different junction, thermal, area, or escape prefactors.
7. Separate thermal averaging from dephasing
Section titled “7. Separate thermal averaging from dephasing”The amplitude falls rapidly with temperature. Give four measurements that help decide whether is shrinking or energy averaging is dominant.
Solution
Useful controls include measuring rings with different circumference, comparing several harmonic ratios, extracting an independent from weak localization or conductance correlations, measuring electron temperature, varying arm-length imbalance, and acquiring energy or gate-correlation data to determine how rapidly the interference phase changes across the Fermi window. A model should convolve the energy-dependent transmission with before assigning the remaining decay to dephasing.
8. Audit a topological-ring claim
Section titled “8. Audit a topological-ring claim”A nanowire shows an magnetoconductance peak. Why is that insufficient to establish topological surface transport, and what additional evidence is needed?
Solution
Ordinary metallic and semiconductor rings also show interference. A stronger case requires the predicted flux-dependent phase or parity pattern, gate evolution tied to surface-state occupancy, independent evidence for the relevant surface or edge bands, robustness and disorder trends consistent with the proposed model, and exclusion of bulk channels, Zeeman-driven changes, multiple effective areas, and ordinary universal conductance fluctuations. The period is necessary for the proposed interference picture but not diagnostic of band topology by itself.
Connections
Section titled “Connections”- Aharonov–Bohm Effect owns electromagnetic gauge holonomy, winding, Stokes-theorem subtleties, and the precise topological statement.
- Aharonov–Bohm Effect: First Encounter derives the phase in wave mechanics and introduces the flux-shifted closed ring.
- Particle on a Ring develops the exact closed-system spectrum, twisted boundary conditions, and persistent-current interpretation.
- Quantum Coherence in Conductors owns dephasing mechanisms, coherence lengths, thermal averaging, and comparison among weak localization, fluctuations, and ring harmonics.
- Weak Localization treats the broad ensemble of time-reversed diffusive loops and its nonperiodic low-field cusp.
- Universal Conductance Fluctuations owns the reproducible aperiodic background, its covariance, symmetry crossover, and disorder-averaging controls.
- What Is Mesoscopic Physics? supplies the wavelength, mean-free-path, thermal, coherence, dwell, and geometry hierarchy.
- Quantum Point Contacts develops tunable beam splitters, transmission, partition noise, and detector backaction.
- Conductance Quantization owns the Landauer channel formula, terminal conventions, and contact resistance.
- Berry Phase in the Aharonov–Bohm Effect compares electromagnetic and eigenstate holonomies.
- Dephasing Versus Dissipation separates loss of phase information from energy relaxation in open-system language.
- Topological Insulators owns bulk topology and anomalous surface states.
Further Reading
Section titled “Further Reading”- S. Washburn and R. A. Webb, “Aharonov–Bohm Effect in Normal Metal: Quantum Coherence and Transport,” Advances in Physics 35, 375–422 (1986), doi:10.1080/00018738600101921.
- A. G. Aronov and Yu. V. Sharvin, “Magnetic Flux Effects in Disordered Conductors,” Reviews of Modern Physics 59, 755–779 (1987), doi:10.1103/RevModPhys.59.755.
- Y. Imry, Introduction to Mesoscopic Physics, 2nd ed., Oxford University Press, 2002.
- S. Datta, Electronic Transport in Mesoscopic Systems, Cambridge University Press, 1995.
References
Section titled “References”- Y. Aharonov and D. Bohm, “Significance of Electromagnetic Potentials in the Quantum Theory,” Physical Review 115, 485–491 (1959), doi:10.1103/PhysRev.115.485.
- N. Byers and C. N. Yang, “Theoretical Considerations Concerning Quantized Magnetic Flux in Superconducting Cylinders,” Physical Review Letters 7, 46–49 (1961), doi:10.1103/PhysRevLett.7.46.
- B. L. Altshuler, A. G. Aronov, and B. Z. Spivak, “The Aharonov–Bohm Effect in Disordered Conductors,” JETP Letters 33, 94–97 (1981).
- M. Büttiker, Y. Imry, R. Landauer, and S. Pinhas, “Generalized Many-Channel Conductance Formula with Application to Small Rings,” Physical Review B 31, 6207–6215 (1985), doi:10.1103/PhysRevB.31.6207.
- R. A. Webb, S. Washburn, C. P. Umbach, and R. B. Laibowitz, “Observation of Aharonov–Bohm Oscillations in Normal-Metal Rings,” Physical Review Letters 54, 2696–2699 (1985), doi:10.1103/PhysRevLett.54.2696.
- A. Yacoby, M. Heiblum, D. Mahalu, and H. Shtrikman, “Coherence and Phase Sensitive Measurements in a Quantum Dot,” Physical Review Letters 74, 4047–4050 (1995), doi:10.1103/PhysRevLett.74.4047.
- A. Yacoby, R. Schuster, and M. Heiblum, “Phase Rigidity and Oscillations in a Single-Ring Aharonov–Bohm Experiment,” Physical Review B 53, 9583–9586 (1996), doi:10.1103/PhysRevB.53.9583.
- R. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, and H. Shtrikman, “Phase Measurement in a Quantum Dot via a Double-Slit Interference Experiment,” Nature 385, 417–420 (1997), doi:10.1038/385417a0.
- A. E. Hansen, A. Kristensen, S. Pedersen, C. B. Sørensen, and P. E. Lindelof, “Mesoscopic Decoherence in Aharonov–Bohm Rings,” Physical Review B 64, 045327 (2001), doi:10.1103/PhysRevB.64.045327.
- G. Seelig and M. Büttiker, “Charge-Fluctuation-Induced Dephasing in a Gated Mesoscopic Interferometer,” Physical Review B 64, 245313 (2001), doi:10.1103/PhysRevB.64.245313.
- R. Leturcq et al., “Magnetic Field Symmetry and Phase Rigidity of the Nonlinear Conductance in a Ring,” Physical Review Letters 96, 126801 (2006), doi:10.1103/PhysRevLett.96.126801.
- A. G. Aronov and Y. B. Lyanda-Geller, “Spin-Orbit Berry Phase in Conducting Rings,” Physical Review Letters 70, 343–346 (1993), doi:10.1103/PhysRevLett.70.343.
- H. Peng et al., “Aharonov–Bohm Interference in Topological Insulator Nanoribbons,” Nature Materials 9, 225–229 (2010), doi:10.1038/nmat2609.
- J. H. Bardarson, P. W. Brouwer, and J. E. Moore, “Aharonov–Bohm Oscillations in Disordered Topological Insulator Nanowires,” Physical Review Letters 105, 156803 (2010), doi:10.1103/PhysRevLett.105.156803.