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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 2π2\pi when the enclosed flux changes by the normal-electron flux quantum h/eh/e.

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, h/eh/e and h/2eh/2e 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.

Consider source and drain reservoirs connected by upper and lower arms. Let Φ\Phi be the magnetic flux through an oriented effective ring area. For charge qq, the electromagnetic contribution to the relative phase is

ΔφAB=qℏ∮CA⋅dr=qΦℏ.\Delta\varphi_{\mathrm{AB}} = \frac{q}{\hbar} \oint_C \mathbf A\cdot d\mathbf r = \frac{q\Phi}{\hbar}.

Define the positive normal-electron flux quantum

Φ0≡he≈4.1357×10−15 Wb.\Phi_0 \equiv \frac{h}{e} \approx 4.1357\times10^{-15}\,\mathrm{Wb}.

For an electron, q=−eq=-e, so orientation fixes the sign:

ΔφAB=−2πΦΦ0(mod2π).\Delta\varphi_{\mathrm{AB}} = -2\pi\frac{\Phi}{\Phi_0} \pmod{2\pi}.

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

Δφ(E,B)=Δφdyn(E,B)+ΔφAB(B)+Δφint(E,B),\Delta\varphi(E,B) = \Delta\varphi_{\mathrm{dyn}}(E,B) +\Delta\varphi_{\mathrm{AB}}(B) +\Delta\varphi_{\mathrm{int}}(E,B),

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.

In the simplest single-mode picture, write the source-to-drain amplitude as

t=tueiφu+tℓeiφℓ.t = t_u e^{i\varphi_u} +t_\ell e^{i\varphi_\ell}.

Then

T=∣t∣2=Tu+Tℓ+2TuTℓcos⁡Δφ,\begin{aligned} T ={}& \lvert t\rvert^2 \\ ={}& T_u+T_\ell +2\sqrt{T_uT_\ell} \cos\Delta\varphi, \end{aligned}

where Tu=∣tu∣2T_u=\lvert t_u\rvert^2, Tℓ=∣tℓ∣2T_\ell=\lvert t_\ell\rvert^2, and Δφ=φu−φℓ\Delta\varphi=\varphi_u-\varphi_\ell. At zero temperature, one resolved channel contributes

G=e2hT.G = \frac{e^2}{h}T.

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

ttot=∑papexp⁡[iφp(0)+i2πwpΦΦ0],t_{\mathrm{tot}} = \sum_p a_p \exp \left[ i\varphi_p^{(0)} +i2\pi w_p\frac{\Phi}{\Phi_0} \right],

where wp∈Zw_p\in\mathbb Z is the oriented winding number. Conductance contains cross terms between every pair of paths that arrive in the same outgoing channel.

For a nearly sinusoidal trace, an experimental visibility is often defined as

V≡Gmax⁡−Gmin⁡Gmax⁡+Gmin⁡.\mathcal V \equiv \frac{G_{\max}-G_{\min}} {G_{\max}+G_{\min}}.

In the ideal two-path model,

Videal=2TuTℓTu+Tℓ.\mathcal V_{\mathrm{ideal}} = \frac{ 2\sqrt{T_uT_\ell} }{ T_u+T_\ell }.

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.

One additional flux quantum changes the single-winding phase by 2π2\pi:

ΔΦ=Φ0.\Delta\Phi = \Phi_0.

If a perpendicular field threads an effective area AeffA_{\mathrm{eff}}, then

Φ≈BAeff,ΔBh/e=Φ0Aeff.\Phi \approx BA_{\mathrm{eff}}, \qquad \Delta B_{h/e} = \frac{\Phi_0}{A_{\mathrm{eff}}}.

A measured period therefore gives

Aeff=Φ0ΔBh/e,reff=AeffπA_{\mathrm{eff}} = \frac{\Phi_0}{\Delta B_{h/e}}, \qquad r_{\mathrm{eff}} = \sqrt{ \frac{A_{\mathrm{eff}}}{\pi} }

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 rir_i and ror_o, plausible orbital areas span

πri2≲A≲πro2.\pi r_i^2 \lesssim A \lesssim \pi r_o^2.

A broad or split Fourier peak can therefore be geometric rather than evidence for two independent particles or two topological sectors.

For particles of charge magnitude ee in a multiply connected normal system, gauge invariance implies periodicity under insertion of h/eh/e 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:

δG(B)=∑n=1∞Ancos⁡(2πnBAeffΦ0+ϕn).\delta G(B) = \sum_{n=1}^{\infty} A_n \cos \left( 2\pi n \frac{BA_{\mathrm{eff}}}{\Phi_0} +\phi_n \right).

The nnth harmonic has field period

ΔBn=Φ0nAeff.\Delta B_n = \frac{\Phi_0}{nA_{\mathrm{eff}}}.

Mesoscopic Aharonov–Bohm ring, flux-periodic magnetoconductance, and harmonic spectrum

Upper and lower arm amplitudes acquire a relative phase 2πΦ/Φ02\pi\Phi/\Phi_0 and recombine at the drain. A field sweep produces oscillatory conductance on top of a generally nonperiodic background. Fourier analysis can reveal an h/eh/e peak and higher winding or time-reversed h/2eh/2e structure, but peak width also records the finite field window and distribution of effective ring areas.

The same field period can arise from different path families, so harmonic labels are not mechanism labels by themselves.

Interference between paths whose winding numbers differ by one gives the h/eh/e 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 h/eh/e contribution even when each individual ring shows it clearly.

A coherent path that circles twice before leaving can interfere with a path whose winding differs by two, producing an h/2eh/2e harmonic. Higher harmonics similarly probe longer dwell paths and are usually more sensitive to dephasing and escape.

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

2ΔφAB=4πΦΦ0,2\Delta\varphi_{\mathrm{AB}} = 4\pi\frac{\Phi}{\Phi_0},

and therefore has period h/2eh/2e. Because static-disorder phases cancel for this time-reversed pair, the ensemble-averaged contribution can survive when the sample-specific h/eh/e term averages toward zero.

This is the ring analogue of the Cooperon channel in weak localization. It is distinct from superconducting h/2eh/2e flux periodicity, where the condensate charge is 2e2e. Observing an h/2eh/2e peak in a normal ring does not by itself imply pairing.

Observed componentRepresentative originDisorder averaging
h/eh/epaths whose winding differs by onephase is generally sample specific
h/2eh/2edouble winding in one samplecan remain sample specific
h/2eh/2etime-reversed diffusive loopssurvives ensemble averaging more robustly
h/2eh/2e in a superconductorcharge-2e2e condensate responsedifferent physical origin

Linear-response reciprocity gives

Gij(B)=Gji(−B)G_{ij}(B) = G_{ji}(-B)

when microscopic reversibility applies and magnetic order or other time-reversal-breaking controls are reversed consistently. In a two-terminal conductor this reduces to

G(B)=G(−B).G(B) = G(-B).

Suppose an isolated first harmonic is written

δG1(B)=A1cos⁡(2πBAeffΦ0+ϕ1).\delta G_1(B) = A_1 \cos \left( 2\pi\frac{BA_{\mathrm{eff}}}{\Phi_0} +\phi_1 \right).

Evenness in BB requires

ϕ1=0orπ(mod2π).\phi_1 = 0 \quad\text{or}\quad \pi \pmod{2\pi}.

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 π\pi 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.

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,

ℓe≪P,\ell_e \ll P,

where PP 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

ℓeversusLϕ:\ell_e \quad\text{versus}\quad L_\phi:

ℓe\ell_e controls momentum randomization, while LϕL_\phi controls phase memory. A ring can be diffusive and phase coherent when

ℓe≪P≲Lϕ.\ell_e \ll P \lesssim L_\phi.

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.

For a one-winding path length PP, a common phenomenological harmonic envelope is

An(T)∼An(0)RT,nResc,nexp⁡(−nPLϕ(T)).A_n(T) \sim A_n^{(0)} R_{T,n} R_{\mathrm{esc},n} \exp \left( -\frac{nP}{L_\phi(T)} \right).

RT,nR_{T,n} describes thermal and energy averaging, while Resc,nR_{\mathrm{esc},n} 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

LTdiff=ℏDkBTL_T^{\mathrm{diff}} = \sqrt{ \frac{\hbar D}{k_{\mathrm B}T} }

in diffusion, or parametrically

LTbal∼ℏvFkBTL_T^{\mathrm{bal}} \sim \frac{\hbar v_F}{k_{\mathrm B}T}

for ballistic energy averaging. Numerical factors depend on the observable and geometry.

At finite temperature, linear conductance averages the energy-dependent transmission:

G(T)=e2h∫dE (−∂f∂E)T(E).G(T) = \frac{e^2}{h} \int dE\, \left( -\frac{\partial f}{\partial E} \right) T(E).

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 Lϕ(T)L_\phi(T) either models RTR_T independently, compares harmonics and ring sizes, or uses another coherence observable. Fitting all amplitude loss to exp⁡(−P/Lϕ)\exp(-P/L_\phi) overestimates intrinsic dephasing.

Under the simple common-prefactor model,

An+1An∝exp⁡(−PLϕ).\frac{A_{n+1}}{A_n} \propto \exp \left( -\frac{P}{L_\phi} \right).

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.

Report ring radii and arm width, carrier density, ℓe\ell_e, LϕL_\phi, temperature, excitation bias, lead number, and contact transparency. State whether the transport is ballistic, quasiballistic, diffusive, or resonant.

Reduce the excitation until the trace is bias independent within uncertainty. Compare G(B)G(B) and G(−B)G(-B) before background subtraction. A two-terminal linear trace should be even apart from drift and noise.

Classical magnetoresistance, weak localization, and conductance fluctuations can be larger than the ring oscillation. Detrending must preserve periods near the expected ΔB\Delta B. Report the polynomial, smoothing scale, or high-pass filter and demonstrate robustness against reasonable alternatives.

For uniformly sampled field points BjB_j, transform a declared window of δG(Bj)\delta G(B_j). The finite field span B\mathcal B limits frequency resolution to order

δfB∼1B.\delta f_B \sim \frac{1}{\mathcal B}.

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.

Compare

AFFT=fBΦ0A_{\mathrm{FFT}} = f_B\Phi_0

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 h/eh/e fingerprint while preserving its characteristic period. An array average can enhance the disorder-robust h/2eh/2e component and suppress sample-specific h/eh/e oscillations.

ObservationStrong interpretationEssential control
peak at A/Φ0A/\Phi_0one-winding flux interferencematch area and track gate dependence
peak at 2A/Φ02A/\Phi_0h/2eh/2e harmonicdistinguish double winding, AAS, and pairing
amplitude falls with TTreduced coherent contrastseparate thermal averaging from dephasing
apparent continuous phase shiftpath or scatterer phase evolutioncheck terminal geometry and phase rigidity
split Fourier peakseveral effective areas or beatingvary field window, gate voltage, and detrending
oscillations survive large fieldrobust phase-coherent pathsaudit field penetration, Zeeman, and edge reconstruction

Aharonov–Bohm and Berry phases share the mathematics of a U(1)U(1) holonomy, but their base spaces and physical assumptions differ.

FeatureAharonov–Bohm phaseBerry phase
connectionelectromagnetic A(r)\mathbf A(\mathbf r)eigenstate connection in parameter or momentum space
loopreal-space path around fluxadiabatic path of an eigenspace
basic assumptioncoherent path interferenceadiabatic following
curvaturemagnetic field, possibly excluded from pathsBerry 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 h/eh/e 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.

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 AeffA_{\mathrm{eff}}.

Double winding and Altshuler–Aronov–Spivak interference produce h/2eh/2e 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 00 or π\pi.

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 h/2eh/2e 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.

Starting from two arm probabilities TuT_u and TℓT_\ell, derive Videal\mathcal V_{\mathrm{ideal}}. Evaluate it for Tu=0.64T_u=0.64 and Tℓ=0.16T_\ell=0.16.

Solution

The extrema of

T=Tu+Tℓ+2TuTℓcos⁡ΔφT = T_u+T_\ell +2\sqrt{T_uT_\ell}\cos\Delta\varphi

are

Tmax⁡,min⁡=Tu+Tℓ±2TuTℓ.T_{\max,\min} = T_u+T_\ell \pm 2\sqrt{T_uT_\ell}.

Therefore

Videal=Tmax⁡−Tmin⁡Tmax⁡+Tmin⁡=2TuTℓTu+Tℓ.\mathcal V_{\mathrm{ideal}} = \frac{T_{\max}-T_{\min}} {T_{\max}+T_{\min}} = \frac{2\sqrt{T_uT_\ell}} {T_u+T_\ell}.

For the stated values,

Videal=2(0.64)(0.16)0.80=0.80.\mathcal V_{\mathrm{ideal}} = \frac{2\sqrt{(0.64)(0.16)}}{0.80} = 0.80.

The paths can be fully coherent even though their unequal amplitudes prevent unit visibility.

An h/eh/e oscillation has period ΔB=25 mT\Delta B=25\,\mathrm{mT}. Find AeffA_{\mathrm{eff}} and the corresponding circular radius.

Solution

Using Φ0=4.1357×10−15 Wb\Phi_0=4.1357\times10^{-15}\,\mathrm{Wb},

Aeff=Φ0ΔB≈4.1357×10−152.5×10−2≈1.65×10−13 m2.A_{\mathrm{eff}} = \frac{\Phi_0}{\Delta B} \approx \frac{4.1357\times10^{-15}} {2.5\times10^{-2}} \approx 1.65\times10^{-13}\,\mathrm{m^2}.

Then

reff=Aeffπ≈2.30×10−7 m=230 nm.r_{\mathrm{eff}} = \sqrt{\frac{A_{\mathrm{eff}}}{\pi}} \approx 2.30\times10^{-7}\,\mathrm m = 230\,\mathrm{nm}.

This radius should be compared with the full conducting annulus, not only the drawn gate centerline.

A ring has Aeff=0.50 μm2A_{\mathrm{eff}}=0.50\,\mu\mathrm m^2. Find the h/eh/e and h/2eh/2e field periods.

Solution

Since 0.50 μm2=5.0×10−13 m20.50\,\mu\mathrm m^2=5.0\times10^{-13}\,\mathrm{m^2},

ΔBh/e=Φ0Aeff≈8.27 mT.\Delta B_{h/e} = \frac{\Phi_0}{A_{\mathrm{eff}}} \approx 8.27\,\mathrm{mT}.

The second harmonic has half that period:

ΔBh/2e=Φ02Aeff≈4.14 mT.\Delta B_{h/2e} = \frac{\Phi_0}{2A_{\mathrm{eff}}} \approx 4.14\,\mathrm{mT}.

The period identifies the harmonic, not whether its microscopic origin is double winding or an AAS path pair.

Show that G(B)=G(−B)G(B)=G(-B) forces ϕ=0\phi=0 or π\pi for

δG=Acos⁡(2πBAeff/Φ0+ϕ)\delta G = A\cos \left( 2\pi BA_{\mathrm{eff}}/\Phi_0+\phi \right)

when A≠0A\neq0.

Solution

Let θ=2πBAeff/Φ0\theta=2\pi BA_{\mathrm{eff}}/\Phi_0. Evenness requires

cos⁡(θ+ϕ)=cos⁡(−θ+ϕ)\cos(\theta+\phi) = \cos(-\theta+\phi)

for all θ\theta. Their difference is

−2sin⁡ϕsin⁡θ.-2\sin\phi\sin\theta.

It vanishes for every θ\theta only if sin⁡ϕ=0\sin\phi=0, hence

ϕ=0orπ(mod2π).\phi = 0 \quad\text{or}\quad \pi \pmod{2\pi}.

If AA passes through zero during a gate sweep, the fitted phase can jump by π\pi without violating reciprocity.

Explain why an ensemble of disordered rings can lose its average h/eh/e signal while retaining an h/2eh/2e component.

Solution

The h/eh/e 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 h/2eh/2e.

6. Estimate a coherence length from harmonics

Section titled “6. Estimate a coherence length from harmonics”

Assume

An=Cexp⁡(−nPLϕ)A_n = C \exp \left( -\frac{nP}{L_\phi} \right)

with the same prefactor CC for two harmonics. A ring has P=1.5 μmP=1.5\,\mu\mathrm m and A2/A1=0.25A_2/A_1=0.25. Find LϕL_\phi.

Solution

The ratio is

A2A1=exp⁡(−PLϕ)=0.25.\frac{A_2}{A_1} = \exp \left( -\frac{P}{L_\phi} \right) = 0.25.

Thus

Lϕ=Pln⁡4≈1.5 μm1.386≈1.08 μm.L_\phi = \frac{P}{\ln4} \approx \frac{1.5\,\mu\mathrm m}{1.386} \approx 1.08\,\mu\mathrm m.

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 h/eh/e amplitude falls rapidly with temperature. Give four measurements that help decide whether LϕL_\phi is shrinking or energy averaging is dominant.

Solution

Useful controls include measuring rings with different circumference, comparing several harmonic ratios, extracting an independent LϕL_\phi 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 −∂f/∂E-\partial f/\partial E before assigning the remaining decay to dephasing.

A nanowire shows an h/eh/e 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 h/eh/e 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.

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