AS-Chap. 3 - 1 R. Gross, A. Marx , F. Deppe, and K. Fedorov Β© Walther-MeiΓner-Institut (2001 - 2017) Chapter 3 Physics of Josephson Junctions: The Voltage State
AS-Chap. 3 - 1
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Chapter 3
Physics of Josephson Junctions:
The Voltage State
AS-Chap. 3 - 2
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For bias currents πΌ > πΌsm
Finite junction voltage
Phase difference π evolves in time: ππ
ππ‘β π
Finite voltage state of the junction corresponds to a dynamic state
Only part of the total current is carried by the Josephson current additional resistive channel capacitive channel noise channel
Key questions
How does the phase dynamics look like?
Current-voltage characteristics for πΌ > πΌsm?
What is the influence of the resistive damping ?
3. Physics of Josephson junctions: The voltage state
AS-Chap. 3 - 3
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At finite temperature π > 0
Finite density of βnormalβ electrons Quasiparticles Zero-voltage state: No quasiparticle current For π > 0 Quasiparticle current = Normal current πΌN Resistive state
High temperatures close to πc
For π β² ππ and 2Ξ π βͺ ππ΅π: (almost) all Cooper pairs are broken up Ohmic current-voltage characteristic (IVC)
πΌN = πΊNπ, where πΊN β‘1
π Nis the normal conductance
Large voltage π > πg =Ξ1+Ξ2
π
External circuit provides energy to break up Cooper pairs Ohmic IVC
For π βͺ πc and |V| < Vg
Vanishing quasiparticle density No normal current
3.1 The basic equation of the lumped Josephson junction3.1.1 The normal current: Junction resistance
AS-Chap. 3 - 4
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For π βͺ πc and V < πg
IVC depends on sweep direction and on bias type (current/voltage) Hysteretic behavior Current bias πΌ = πΌs + πΌN = ππππ π‘. Circuit model
Equivalent conductance GN at T = 0:
Voltage state
Bias current πΌ πΌs π‘ = πΌc sinπ π‘ is time dependent IN is time dependent
Junction voltage π =πΌN
πΊNis time
dependent IVC shows time-averaged voltage π
Current-voltage characteristic
3.1.1 The normal current: Junction resistance
AS-Chap. 3 - 5
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Finite temperature Sub-gap resistance π sg π for π < πg π sg π determined by amount of thermally excited quasiparticles
π π Density of excited quasiparticles
for T > 0 we get
Nonlinear conductance πΊN π, π
Characteristic voltage (πΌcπ N-product)
Note: There may be a frequency dependence of the normal channel Normal channel depends on junction type
3.1.1 The normal current: Junction resistance
AS-Chap. 3 - 6
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If ππ
ππ‘β 0 Finite displacement current
Additional current channel
With π = πΏcππΌs
ππ‘, πΌN = ππΊN, πΌD = πΆ
ππ
ππ‘, πΏs =
πΏc
cos π πΏc and πΊN π, π =
1
π N
Josephson inductance
For planar tunnel junction
πΆ β junction capacitance
3.1.2 The displacement current: Junction capacitance
AS-Chap. 3 - 7
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Equivalent parallel πΏπ πΆ circuit πΏc, π N, C Three characteristic frequencies
Plasma frequency
ππ βπ½c
πΆπ΄, where πΆπ΄ β‘
πΆ
π΄is the specific junction capacitance
π < πp πΌD < πΌs
Inductive πΏc/π N time constant
Inverse relaxation time in the normal+supercurrent system πc follows from πc (2nd Josephson equation) πΌN < πΌc for π < πc or π < πc
Capacitive π NπΆ time constant
πΌD < πΌN for π <1
ππ πΆ
πc = πΌcπ N
Characteristic frequencies
3.1.3 Characteristic times and frequencies
AS-Chap. 3 - 8
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Quality factor
Limiting cases
π½πΆ βͺ 1 Small capacitance and/or small resistance Small π NπΆ time constants (ππ πΆπp βͺ 1)
Highly damped (overdamped) junctions
π½πΆ β« 1 Large capacitance and/or large resistance Large π NπΆ time constants (ππ πΆπp β« 1)
Weakly damped (underdamped) junctions
Stewart-McCumber parameter and quality factor
Stewart-McCumber parameter
(π compares the decay of oscillation amplitudes to the oscillation period)
3.1.3 Characteristic times and frequencies
AS-Chap. 3 - 9
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Fluctuation/noise Langevin method: include random source fluctuating noise current type of fluctuations: thermal noise, shot noise, 1/f noise
Thermal Noise
Johnson-Nyquist formula for thermal noise (πBπ β« ππ, βπ):
relative noise intensity (thermal energy/Josephson coupling energy):
πΌπ β‘ thermal noise currentπ = 4.2 K πΌπ β 0.15 ΞΌA
(current noise power spectral density)
(voltage noise power spectral density)
3.1.4 The fluctuation current
AS-Chap. 3 - 10
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Shot Noise
Schottky formula for shot noise (ππ β« πBπ π > 0.5 mV@ 4.2 K):
Random fluctuations due to the discreteness of charge carriers Poisson process Poissonian distribution Strength of fluctuations variance π₯πΌ2 β‘ πΌ β πΌ 2
Variance depends on frequency Use noise power:
includes equilibriumfluctuations (white noise)
1/f noise
Dominant at low frequencies Physical nature often unclear Josephson junctions: dominant below about 1 Hz - 1 kHz Not considered here
3.1.4 The fluctuation current
AS-Chap. 3 - 11
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Kirchhoffβs law: πΌ = πΌs + πΌN + πΌD + πΌπΉ
Voltage-phase relation: ππ
ππ‘=
2ππ
β
Basic equation of a Josephson junction
Nonlinear differential equation with nonlinear coefficients
Complex behavior, numerical solution
Use approximations (simple models)
Super-current
Normalcurrent
Displace-ment
current
Noise current
3.1.5 The basic junction equation
AS-Chap. 3 - 12
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Differential equation in dimesionless or energy formulation
β‘ π β‘ πF(π‘)
with
Tilted washboard potential
Resistively and Capacitively ShuntedJunction (RCSJ) model
Mechanical analogGauge invariant phase difference β Particle with mass M and damping π in potential U:
Approximation πΊπ π β‘ πΊ = π β1 = const.π = Junction normal resistance
3.2 The resistively and capacitively shunted junction (RCSJ) model
AS-Chap. 3 - 13
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Normalized time:
Stewart-McCumber parameter:
Plasma frequency
Neglect damping, zero driving and small amplitudes (sinπ β π)
Solution:
Plasma frequency = Oscillation frequency around potential minimum
Finite tunneling probability:Macroscopic quantum tunneling (MQT)
Escape by thermal activation Thermally activated phase slips
Motion of βphase particleβ π in the tilted washboard potential
3.2 The resistively and capacitively shunted junction (RCSJ) model
AS-Chap. 3 - 14
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) The pendulum analog
Plane mechanical pendulum in uniform gravitational field Mass π, length β, deflection angle π Torque π· parallel to rotation axis Restoring torque: ππβ sin π
Equation of motion π· = π© π + π€ π + ππβ sin π
Ξ = πβ2 Moment of inertiaΞ Damping constant
πΌ β π·πΌc β ππβπ·0
2ππ β π€
Cπ·0
2πβ π©
π β π
For π· = 0 Oscillations around equilibrium with
π =π
ββ Plasma frequency πp =
2ππΌc
π·0πΆ
Finite torque (π· > 0) Finite π0 Finite, but constant π0 Zero-voltage stateLarge torque (deflection > 90Β°) Rotation of the pendulum Finite-voltage state
Voltage πβ Angular velocity of the pendulum
Analogies
3.2 The resistively and capacitively shunted junction (RCSJ) model
AS-Chap. 3 - 15
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Underdamped junction
π½πΆ =2ππΌππ
2πΆ
ββ« 1
Capacitance & resistance largeπ large, π smallHysteretic IVC
Overdamped junction
π½πΆ =2ππΌππ
2πΆ
ββͺ 1
Capacitance & resistance smallπ small, π largeNon-hysteretic IVC
(Once the phase is moving, the potential has to be tilt back almost into the horizontal position to stop ist motion)
3.2.1 Under- and overdamped Josephson junctions
(Phase particle will retrap immediately at πΌc because of large damping)
AS-Chap. 3 - 16
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βApplied Superconductivityβ One central question is
βHow to extract information about the junction experimentally?β
3.3 Response to driving sources
Typical strategy
Drive junction with a probe signal and measure response
Examples for probe signals
Currents (magnetic fields) Voltages (electric fields) DC or AC Josephson junctions AC means microwaves!
Prototypical experiment
Measure junction IVC Typically done with current bias
Motivation
AS-Chap. 3 - 17
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) Time averaged voltage:2π
Total current must be constant (neglecting the fluctuation source):
where:
πΌ > πΌc Part of the current must flow as πΌN or πΌD
Finite junction voltage π > 0 Time varying πΌs πΌN + πΌD varies in time Time varying voltage, complicated non-sinusoidal oscillations of πΌs,
Oscillating voltage has to be calculated self-consistently Oscillation frequency π = π π·0
π = Oscillation period
3.3.1 Response to a dc current source
AS-Chap. 3 - 18
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For πΌ β³ πΌc
Highly non-sinusoidal oscillations Long oscillation period
π β1
πis small
I/Ic = 1.05
I/Ic = 1.1
I/Ic = 1.5
I/Ic = 3.0
Analogy to pendulum
3.3.1 Response to a dc current source
For πΌ β« πΌc
Almost all current flows as normal current Junction voltage is nearly constant Almost sinusoidal Josephson current
oscillations Time averaged Josephson current almost
zero Linear/Ohmic IVC
AS-Chap. 3 - 19
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) Strong damping
π½πΆ βͺ 1 & neglecting noise current
π < 1 Only supercurrent, π = sinβ1 π is a solution, zero junction voltageπ > 1 Finite voltage, temporal evolution of the phase
Integration using
gives
Periodic function with period
Setting π0 = 0 and using π =π‘
πc
tanβ1 π tan π₯ + πis π-periodic
3.3.1 Response to a dc current source
AS-Chap. 3 - 20
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with
We get for π > 1
and
3.3.1 Response to a dc current source
AS-Chap. 3 - 21
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ππ πΆ =1
π NπΆis very small
Large C is effectively shunting oscillating part of junction voltage π π‘ β π Time evolution of the phase
Almost sinusoidal oscillation of Josephson current
Down to π ββππ πΆ
πβͺ πc = πΌcπ N
Corresponding current βͺ πΌc Hysteretic IVC
Weak damping
π½πΆ β« 1 & neglecting noise current
Ohmic result valid for π N = ππππ π‘.
Real junction IVC determined by voltage dependence of π N = π N V
3.3.1 Response to a dc current source
AS-Chap. 3 - 22
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π½πΆ β 1
Numerically solve IVC General trend
Increasing π½πΆ β Increasing hysteresis
Hysteresis characterized by retrapping current πΌr
πΌr β washboard potential tilt whereEnergy dissipated in advancing to next minimum = Work done by drive current
Analytical calculation possible for π½πΆ β« 1 (exercise class)
Intermediate damping
Numericalcalculation
3.3.1 Response to a dc current source
πΌ r/πΌc
AS-Chap. 3 - 23
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Phase evolves linearly in time:
Josephson current πΌπ oscillates sinusoidally Time average of πΌs is zero
πΌD = 0 sinceππdc
ππ‘= 0
Total current carried by normal current πΌ =πdc
π N
RCSJ model Ohmic IVCGeneral case π = π N π Nonlinear IVC
3.3.1 Response to a dc voltage source
AS-Chap. 3 - 24
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) Response to an ac voltage source
Strong damping π½πΆ βͺ 1
Integrating the voltage-phase relation:
Current-phase relation:
Superposition of linearly increasing and sinusoidally varying phase
Supercurrent πΌs(π‘) and ac voltage π1 have different frequencies Origin Nonlinear current-phase relation
3.3.2 Response to ac driving sources
AS-Chap. 3 - 25
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Fourier-Bessel series identity:π₯π π = πth order Bessel function of the first kind
and:
Ac driven junction π₯ = π1π‘, π =2π
π·0π1and π = π0 +πdcπ‘ = π0 +
2π
π·0πdcπ‘
Frequency πdc couples to multiples of the driving frequency
Imaginary part
Some maths for the analysis of the time-dependent Josephson current
3.3.2 Response to ac driving sources
AS-Chap. 3 - 26
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Ac voltage results in dc supercurrent if πdc β ππ1 π‘ + π0 is time independent
Amplitude of average dc current for a specific step number π
πdc β ππ
πdc β ππ1 π‘ + π0 is time dependent Sum of sinusoidally varying terms Time average is zero Vanishing dc component
3.3.2 Response to ac driving sources
Shapiro steps
AS-Chap. 3 - 27
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Ohmic dependence with sharp current spikes at πdc = ππ Current spike amplitude depends on ac voltage amplitude
πth step Phase locking of the junction to the πth harmonic
Example: π1/2π = 10 GHz
Constant dc current at Vdc = 0 and ππ = ππ1π·0
2πβ π Γ 20 ΞΌV
3.3.2 Response to ac driving sources
AS-Chap. 3 - 28
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Difficult to solve Qualitative discussion with washboard potential
Increase πΌdc at constant πΌ1 Zero-voltage state for πΌdc + πΌ1 β€ πΌc, finite voltage state for πΌdc + πΌ1 > πΌc Complicated dynamics!
ππ = ππ1Ξ¦0
2πMotion of phase particle synchronized by ac driving
Simplifying assumption During each ac cycle the phase particle
moves down π minima
Resulting phase change π = π2π
π= ππ1
Average dc voltage π = ππ·0
2ππ1 β‘ ππ
Exact analysis Synchronization of phase dynamics with external ac source for a certain bias current interval
Strong damping π½πΆ βͺ 1 (experimentally relevant)
Kirchhoffβs law (neglecting ID) πΌπ sin π +1
π N
π·0
2π
ππ
ππ‘= πΌdc + πΌ1 sinπ1π‘
3.3.2 Response to ac driving sources
Response to an ac current source
Steps
AS-Chap. 3 - 29
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Experimental IVCs obtained for an underdamped and overdamped Niobium Josephson junction under microwave irradiation
3.3.2 Response to ac driving sources
AS-Chap. 3 - 30
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) Superconducting tunnel junction Highly nonlinear π π
Sharp step at πg =2π₯
π
Use quasiparticle (QP) tunneling current πΌqp π
Include effect of ac source on QP tunneling
Bessel function identity for π1-term Sum of terms Splitting of qp-levels into many levels πΈqp Β± πβπ1Modified density of states!
Tunneling current
Sharp increase of the πΌqp π at π = πg is broken up into many steps of smaller current
amplitude at ππ = πg Β±πβπ1
π
Model of Tien and Gordon: Ac driving shifts levels in electrode up and down
QP energy: πΈqp + ππ1 cosπ1π‘
QM phase factor
3.3.4 Photon-assisted tunneling
AS-Chap. 3 - 31
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Example QP IVC of a Nb SIS Josephson junction without & with microwave irradiation Frequency π1 2π = 230 GHz corresponding to βπ1 π β 950 ΞΌV
Shapiro steps
Appear at ππ = πβ
2ππ1
Amplitude π½π2ππ1
βπ1
Sharp steps
QP steps
Appear at ππ = πβ
ππ1
Amplitude π½πππ1
βπ1
Broadended steps(depending on πΌqp π )
π1 2π = 230 GHz
3.3.4 Photon-assisted tunneling
AS-Chap. 3 - 32
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) Thermal fluctuations with correlation function:
Larger fluctuations
Increase probability for escape out of potential well Escape at rates GnΒ±1
Escape to next minimum Phase change of 2π
πΌ > 0 Gn+1 > Gn-1ππ
ππ‘> 0
Langevin equation for RCSJ model
Equivalent to Fokker-Planck equation:
Normalized force
Normalized momentum
3.4 Additional topic: Effect of thermal fluctuations
Small fluctuations Phase fluctuations around equilibrium
AS-Chap. 3 - 33
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) π(π£, π, π‘) Probability density of finding system at (π£, π) at time π‘
Static solution (ππ
ππ‘= 0)
with:
Boltzmann distribution (πΊ = πΈβπΉπ₯ is total energy, πΈ is free energy)
Constant probability to find system in nth metastable state
statistical average of variable π
3.4 Additional topic: Effect of thermal fluctuations
Small fluctuations
AS-Chap. 3 - 34
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Attempt frequency πA
Weak damping (π½πΆ = ππππ πΆ β« 1) πΌ = 0 πA = πp (Oscillation frequency in the potential well)
πΌ βͺ πΌc πA Β» πp
3.4 Additional topic: Effect of thermal fluctuations
π can change in time
for Ξπ+1 β« Ξπβ1 and πA
Ξπ+1β« 1 πA = Attempt frequency
Amount of phase slippage
Large fluctuations
Strong damping (π½πΆ = ππππ πΆ βͺ 1)
πp β πc (Frequency of an overdamped oscillator)
(underdamped junction)
(overdamped junction)
AS-Chap. 3 - 35
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For πΈJ0 β« πBπ Small escape probability β exp βπ0 πΌ
πBπat each attempt
Barrier height: 2πΈJ0 for πΌ = 0
0 for πΌ β πΌc
Escape probability πA/2π for πΌ β πΌcAfter escape Junction switches to IRN
Experiment
Measure distribution of escape current πΌM Width πΏπΌ and mean reduction π₯πΌc = πΌc β πΌM Use approximation for π0 πΌ and escape rate
πA/2π exp βπ0 πΌ
πBπ
Considerable reduction of Ic when kBT > 0.05 EJ0
3.4.1 Underdamped junctions: Critical current reduction by premature switching
Provides experimental information on real or effective temperature!
AS-Chap. 3 - 36
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) Calculate voltage π induced by thermally activated phase slips as a function of current
Important parameter:
3.4.2 Overdamped junctions: The Ambegaokar-Halperin theory
AS-Chap. 3 - 37
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Finite amount of phase slippage Nonvanishing voltage for πΌ β 0 Phase slip resistance for strong damping (π½πΆ βͺ 1), for U0 = 2EJ0:
πΈJ0
πBπβ« 1 Approximate Bessel function
or
attempt frequency
Attempt frequency is characteristic frequency πc
Plasma frequency has to be replaced by frequency of overdamped oscillator:
Washboard potential Phase diffuses over barrier Activated nonlinear resistance
Modified Bessel function
3.4.2 Overdamped junctions: The Ambegaokar-Halperin theory
Amgegaokar-Halperin theory
AS-Chap. 3 - 38
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epitaxial YBa2Cu3O7 film on SrTiO3 bicrystalline substrate
R. Gross et al., Phys. Rev. Lett. 64, 228 (1990)Nature 322, 818 (1988)
Example: YBa2Cu3O7 grain boundary Josephson junctions Strong effect of thermal fluctuations due to high operation temperature
3.4.2 Overdamped junctions: The Ambegaokar-Halperin theory
AS-Chap. 3 - 39
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Determination of πΌc π close to πc
Overdamped YBa2Cu3O7 grain boundary Josephson junction
R. Gross et al., Phys. Rev. Lett. 64, 228 (1990)
thermally activated
phase slippage
3.4.2 Overdamped junctions: The Ambegaokar-Halperin theory
AS-Chap. 3 - 40
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3.5 Voltage state of extended Josephson junctions
So far
Junction treated as lumped element circuit element Spatial extension neglected
Spatially extended junctions
Specific geometry as as in Chapter 2 Insulating barrier in π¦π§-plane In-plane π΅ field in π¦-direction Thick electrodes β« πL1,2 Magnetic thickness π‘π΅ = π + ππΏ,1 + ππΏ,2 Bias current in π₯-direction
Phase gradient along π§-direction
ππ π§,π‘
ππ§=
2π
π·0π‘π΅π΅π¦ π§, π‘
Expected effects
Voltage state πΈ-field and time-dependence become important Short junction and long junction case
AS-Chap. 3 - 41
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) Neglect self-fields (short junctions)
π© = π©ππ±
Junction voltage π = Applied voltage π0 Gauge invariant phase difference:
Josephson vortices moving in π§-direction with velocity
3.5.1 Negligible Screening Effects
AS-Chap. 3 - 42
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) Long junctions (π³ β« ππ)
Effect of Josephson currents has to be taken into account Magnetic flux density = External + Self-generated field
with B = π0H and D = ν0E:in contrast to static case,
now E/t 0
with πΈπ₯ = βπ/π, π½π₯ = βπ½π sin π and ππ/ππ‘ = 2ππ/Ξ¦0:
consider 1D junction extending in z-direction, B = By, current flow in x-direction
(Josephson penetration depth)
(propagation velocity)
3.5.2 The time dependent Sine-Gordon equation
AS-Chap. 3 - 43
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Time dependentSine-Gordon equation
π = velocity of TEM mode in the junction transmission line
Example: ν β 5 β 10, 2πL
πβ 50 β 100 π β 0.1π
Reduced wavelength For f = 10 GHz Free space: 3 cm, in junction: 1 mm
with the Swihart velocity
Other form of time-dependent Sine-Gordon equation
3.5.2 The time dependent Sine-Gordon equation
AS-Chap. 3 - 44
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Mechanical analogue
Chain of mechanical pendula attached to a twistablerubber ribbon
Restoring torque πJ2 π
2π
ππ§2
Short junction w/o magnetic field 2/z2 = 0 Rigid connection of pendula Corresponds to single pendulum
Time-dependent Sine-Gordon equation:
3.5.2 The time dependent Sine-Gordon equation
AS-Chap. 3 - 45
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) Simple case 1D junction (W βͺ Ξ»J), short and long junctions
Short junctions (L βͺ Ξ»J) @ low damping
Neglect z-variation of π
Equivalent to RCSJ model for πΊN = 0, πΌ = 0Small amplitudes Plasma oscillations(Oscillation of π around minimum of washboard potential)
Long junctions (L β« Ξ»J)
Solution for infinitely long junction Soliton or fluxon
π = π at π§ = π§0 + π£π§π‘goes from 0 to 2π for ββ β π§ β β Fluxon (antifluxon: β β π§ β ββ)
3.5.3 Solutions of the time dependent SG equation
AS-Chap. 3 - 46
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Pendulum analog Local 360Β° twist of
rubber ribbon
Applied current Lorentz forceMotion of phase twist (fluxon)
Fluxon as particle Lorentz contraction for vz β c
Local change of phase difference Voltage
Moving fluxon = Voltage pulse
Other solutions: Fluxon-fluxon collisions, β¦
π = π at π§ = π§0 + π£π§π‘goes from 0 to 2π for ββ β π§ β β Fluxon (antifluxon: β β π§ β ββ)
3.5.3 Solutions of the time dependent SG equation
AS-Chap. 3 - 47
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Linearized Sine-Gordon equation
π1 = Small deviation Approximation
Substitution (keeping only linear terms):
π0 solves time independent SGE π2π0
ππ§2= πJ
β2sin π0
π0 slowly varying π0 β ππππ π‘.
3.5.3 Solutions of the time dependent SG equation
Josephson plasma waves
AS-Chap. 3 - 48
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π < πp,J
Wave vector k imaginary No proagating solutionπ > πp,J
Mode propagation Pendulum analogue Deflect one pendulum Relax Wave like excitationπ = πp,J
Infinite wavelength Josephson plasma wave Analogy to plasma frequency in a metal Typically junctions πp,J β 10 GHz
For very large πJ or very small I
Neglectsin π
πJ2 term Linear wave equation Plane waves with velcoity π
=πp2
4π2cosπ0
3.5.3 Solutions of the time dependent SG equation
Solution:
Dispersion relation Ο(k) :
Josephson plasma frequency
(small amplitude plasma waves)
Plane waves
AS-Chap. 3 - 49
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) Interaction of fluxons or plasma waves with oscillating Josephson current
Rich variety of interesting resonance phenomena Require presence of π©ππ±
Steps in IVC (junction upconverts dc drive)
For π΅ex > 0
Spatially modulated Josephson currrent density moves at π£π§ = π π΅π¦π‘π΅ Josephson current can excite Josephson plasma waves
On resonance, em waves couple strongly to Josephson current if π = ππ
Eck peak at frequency:
Corresponding junction voltage:
3.5.4 Resonance phenomena
Flux-flow steps and Eck peak
AS-Chap. 3 - 50
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Alternative point of view
Lorentz force Josephson vortices move at π£π§ =π
π΅π¦π‘π΅
Increase driving force Increase π£π§ Maximum possible speed is π£π§ = π Further increase of πΌ does not increase π) Flux flow step in IVC
πffs = ππ΅π¦π‘π΅ = ππ·
πΏ=
πp
2π
πJ
πΏπ·0
π·
π·0
Corresponds to Eck voltage
Traveling current wave only excites traveling em wave of same direction Low damping, short junctions Em wave is reflected at open end Eck peak only observed in long junctions at medium damping when
the backward wave is damped
3.5.4 Resonance phenomena
AS-Chap. 3 - 51
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Standing em waves in junction βcavityβ at ππ = 2πππ = 2π π
2πΏπ =
π π
πΏπ
Fiske steps at voltages for L Β» 100 ΞΌmfirst Fiske step Β» 10 GHz(few 10s of ΞΌV)
Interpretation
Wave length of Josephson current density is 2π
π
Resonance condition πΏ = π
2πππ =
π
2π β ππΏ = ππ or π· = π
π·0
2
where maximum Josephson current of short junction vanishes Standing wave pattern of em wave and Josephson current match Steps in IVC
Influence of dissipation
Damping of standing wave pattern by dissipative effects Broadening of Fiske steps Observation only for small and medium damping
π/2-cavity
ππ = πππ£ph πΏ
π£ph = Phase velocity
3.5.4 Resonance phenomena
Fiske steps
AS-Chap. 3 - 52
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) Fiske steps at small damping and/or small magnetic field
Eck peak at medium damping and/or medium magnetic field
For π β πEck and π β ππ πΌπ = πΌc sin π0π‘ + ππ§ + π0 β 0 πΌ = πΌN π = π/π N π
3.5.4 Resonance phenomena
AS-Chap. 3 - 53
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Motion of trapped flux due to Lorentz force (w/o magnetic field) Junction of length L, moving back and forth
Phase change of 4π in period π =2πΏ
π£z
At large bias currents (π£π§ β π)
πzfs = πβ
2π=4π
π
β
2π=
4π
2πΏ/ π
β
2π=
β
2π
π
πΏ=πp
π
πJπΏπ·0
For π fluxons
ππ,zfs = πππ§ππ ππ,zfπ = 2 Γ Fiske voltage ππ (fluxon has to move back and forth)
πffs = ππ,zfs for π· = ππ·0 (introduce n fluxons = generate n flux quanta)
Example:IVCs of annular Nb/insulator/PbJosephson junction containing adifferent number of trapped fluxons
3.5.4 Resonance phenomena
Zero field steps
AS-Chap. 3 - 54
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Voltage state: (Josephson + normal + displacement + fluctuation) current = total current
RCSJ-model (πΊπ π = ππππ π‘.)
Motion of phase particle in the tilted washboard potential
Equivalent LCR resonator, characteristic frequencies:
Equation of motion for phase difference π:
Quality factor:
ππ
ππ‘=2ππ
β
Summary (Voltage state of short junctions)
π½πΆ = Stewart-McCumber parameter
AS-Chap. 3 - 55
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IVC for strong damping and π½πΆ βͺ 1
Driving with π π‘ = πdc + π1 cosπ1π‘
Shapiro steps at ππ = ππ·0
2ππ1
with amplitudes πΌs π = πΌπ π₯π2ππ1
π·0π1
Photon assisted tunneling
Voltage steps at ππ = ππ·0
ππ1 due to nonlinear
QP resistance
Summary (Voltage state of short junctions)
Effect of thermal fluctuations
Phase-slips at rate Ξπ+1 =πA
2πexp β
π0
kBπ
Finite phase-slip resistance π p even below πΌc
Premature switching
AS-Chap. 3 - 56
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) β’ voltage state of extended junctionsw/o self-field:
β’ with self-field: time dependent Sine-Gordon equation
characteristic velocity of TEM mode in the junction transmission line
Summary
characteristic screening length
Prominent solutions: plasma oscillations and solitons
nonlinear interactions of these excitations with Josephson current: flux-flow steps, Fiske steps, zero-field steps