What is superconductor - SKKU

What is superconductor ?

Superconductivity (R = 0)

• Current Duration time
1000000000…000 year ( more than one hundred zero’s)

Bulk Superconductors Superconducting Ring

R=0

R-T and XRD of MgB2 Thick Films

Intensity (arbitrary units) MgB2/Al2O3 (0001)

MgB2/Al2O3 (0001)
1.7 µm thick film

RRR = 4

40.5 K

0 90 180 270 360
(degrees)

Tc = 40.5 K, c-axis-oriented epitaxial thick films

M-T and M-H curves of MgB2 supercondcutors

MgB2/Al2O3 (0001)
1.7 µm thick film

Two-step process of MgB2 thin films

Won Nam Kang et al. Science 292, 1521 (2001).

1. Deposition of B film on R-plane Al2O3 by PLD.
2. Put it in sealed Ta/Nb tube together with pure Mg metal.
3. Annealing at 800-900°C for 30 minutes.
4. Quenching within 2-5 minutes.

Comparison between cuprates (HTS) & MgB2

HgBa2Ca2Cu3O10 MgB2

Mg

• Simple crystal structure
• Lower anisotropy ratio (2-3)
• Strongly linked grain boundary
• Easy to synthesis in single phase

HPCVD system

SKKU

HPCVD technique

(Hybrid Physical-Chemical Vapor Deposition)

[Zeng et al., Nature Mat. 1, 1 (2002). Penn State Univ.] B2H6 Gas

Growth mechanism of MgB2 thin films by HPCVD.

Mg + B2H6 = MgB2 + 3H2

Thermal evaporation + Thermal decomposition above 300°C



Chap 1. A survey of superconductivity

 1.1 Introduction
 1.2 Basic properties of superconductors
 1.3 Models of superconductivity
 1.4 Simple superconducting systems

1.2 Basic properties of superconductors

In this Section, we will briefly examine some of those properties that
are important to us.

·▣Physical properties
☞ Perfect conductivity (Zero-resistance)
☞ Perfect diamagnetism (Meissner effect)

▣ The discovery of superconductivity and the science of cryogenics
☞1898, James Dewar : Liquid Hydrogen (20 K)
☞1908, H.K. Onnes : Liquid Helium (4.2 K, 1atm)
☞1911, (Onnes’ s assistant Gilles Holst) : Superconducting state

(Figure 1.1) of solid Hg (mercury)
☞ So far, Hg-based superconductor is the highest Tc superconductor:

HgBa2Ca2Cu3O10 at 135 K (-138℃) and 164 K under high-pressure.

Perfect conductivity (Zero-resistance)

Kamerlingh Onnes reported, now with complete confidence, that
below a critical temperature, the mercury had “passed into a new
state, which may be called the superconducting state.”

Tc: Critical temperature, Jc : Critical current density
T
Hc : Critical magnetic field H (T )  H (1  ( Tc ) 2 )    (1.1)

c c0

where Hc0 is the critical field at T=0 K.

☜ Figure 1.1 The original resistance

versus temperature curve Kamerlingh
Onnes reported to announce the
discovery of superconductivity in
mercury.

Meissner effect (perfect diamagnetism)

In 1933 - Walter Meissner and Robert Ochsenfeld
A material that, when placed in a magnetic field, tries to minimize
the magnetic flux density, B, inside itself, is known as
diamagnetic, Thus a superconductor, which seeks to maintain the
condition B = 0 within itself, is called a perfect diamagnetism.

B  H  4 M (B  0)

The superconductor’s ability to expel flux, now known as the
Meissner effect, is an indication that.

Thus, superconductivity is:
- Perfect conductivity (R = 0)
- Perfect diamagnetism (B = 0)

From the early days of superconductivity research, it was apparent
that an important goal was to find materials with high Tc.

Fig 1.2. An example of this observation.

☜ Figure 1.2 The critical fields and
temperatures of several
superconductors. Each material
remains superconducting for fields
less than the critical tend to have
higher maximum critical fields.

Hc (T )  Hc0 (1  T 2 )
Tc

·In the late 1920s and early 1930s : many series of superconducting
materials were discovered.

·Until 1950s : A15 materials (named by their crystal structure)
- V3Ga (16 K), Nb3Sn (18 K), Nb3Ge (23 K) at J = 0, H = 0.

·1970s : Chevrel phase materials (ternary compounds)
PbMo6S8(15 K, 60 T) [fig 1.2]

·1986 : Highest Tc superconductor Nb3Ge (23 K)
· By the end of 1986 : K. Alex Műller & J. G. Bednorz (Novel price)

- La1.85Ba0.15CuO4 (35 K)
·1987 : Paul C. W. Chu at UH (Dr. W. N. Kang’s former advisor)

YBa2Cu3O7 (95 K): enough to be cooled by liquid nitrogen ( 77 K)

History of typical superconductors

135K (1993)

SmFeAsO
55K (2008)
LH2 40K (2001)
2008

(LaO)(FeAs) Superconductors



CuO2 2008 H. Hosono
FeAs

Cuprate (청동기) Fe pnictide (철기)

1.3 Models of superconductivity

·In Chapters 2 & 3 : Classical model of superconductivity
- [zero resistance] + [perfect diamagnetism] → 1st and 2nd London Equ.

·In Chapter 4 : Skip

·In Chapter 5 : We develop macroscopic quantum model (MQM).
- type I : does not violate the bulk Meissner effect (B = 0)
- type II : allows flux to enter the bulk of its volume as
a single flux quantum Φ0 = 2.07 x 10-7 G·cm2

·In Chapters 6 & 7 : Type II superconductors are often the materials used
in high field, high power applications.

·In Chapters 8 & 9 : Skip

·In Chapter 10 : Ginzburg – Landau theory

1.4 Simple superconducting systems

Phase diagram of superconductivity ☜ Basically, there three
Jc critical parameters;
Tc, Jc, and Hc.

Tc
Hc

▣ Lumped circuit model of superconducting system

The loop is initially placed in a static magnetic field such that
flux threads the hole. After the system reaches the steady
state, the field is suddenly turned off.
- Induced current in the wire =>Lenz’ s law

  iR  L di  0,   0

dt

L di  iR, di   R dt   iR  L di  0,   0
dt i L dt

To understand the time dependence of the current, it is convenient to
make a lumped circuit model of the system. Notice that the model,
shown in Figure 1.4b.

From elementary circuit theory, we know that the current will decay in
the exponential fashion

I (t)  I0 e(R/ L)t  I0 et / RL   di   R  dt
i L

where the time constant τRL is given by

 RL  L
R

The decay of the current is experimentally observed by measuring the
magnetic field produced by the loop.

I (t)  I0 e(R/ L)t  I0 et / RL

Suppose that the phenomenon observed by Kamerlingh Onnes is
zero dc resistance. If the loop is made out of a superconductor

lim  RL   ㅡ (1. 4)

R0

I (t)  I0 for t  0 ㅡ (1. 5)

A persistent current will flow in the superconducting loop and the
magnetic field it produces will never be observed to decay.

Kamerlingh Onnes did perform this experiment, as have many other
researchers since, and there has never been an observed decay of
the threading flux caused by a nonzero value of resistance.
=> the experimental value of the resistivity is zero.

▣ Persistent currents with superconducting electromagnet?

Consider the magnet design schematically illustrated in Figure 1. 5.
Both the magnet solenoid and the connecting thermal switch are
made from superconducting material.

Figure 1.5 The charging of a superconducting electromagnet that takes
advantage of the flow of persistence currents. Everything
inside the box is made from superconducting material.

·The switch is opened (heating) → T > Tc
·The switch is closed (cooling) → T < Tc

If we now turn off the current source, the superconducting
electromagnet will stay charged because persistent currents will
flow in the closed superconducting loop.

▣ Superconducting memory element.

Since the magnetic field produced by these currents never decays,
we can use this loop as a memory element.
The memory element can store one bit of binary information :
the logical 1 and 0 states respectively represent the cases of whether
a field is or is not produced by the loop.

i  0 (B  0) : state 0
i  0 (B  0) : state 1