Gross R., Sidorenko A., Tagirov L. Nanoscale Devices

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Configuring a Bistable Atomic Switch 157

Results and Discussion

After deposition of a silver point contact as described above, an

electrochemical cycling process was started in order to configure an atomic-

scale switch, which allows reproducible bistable switching between an off-

state and a well-defined quantized on-state. As soon as an upper threshold

(4.9 G

) near the desired conductance value for the on-state (5.0 G

) was

exceeded, the gate voltage was changed from a voltage in the deposition

regime (+4 mV) to a voltage in the dissolution regime (−36 mV), the

voltage being changed at a rate of 10 mV/s. As soon as the conductance

dropped below a lower threshold of the source-drain conductance (0.1 G

the gate voltage was changed back to a voltage within the deposition regime

(+4 mV), again at a rate of 10 mV/s. This deposition process was continued

until conductance exceeded the upper threshold of 4.9 G

. At this point, a

new cycle consisting of dissolution of the contact and subsequent

deposition was started.

working electrodes in units of the conductance quantum G

as a function of

time during this cycling process. Figure 2a gives the corresponding voltage

applied to the gate electrode. As seen in Figure 2b, during the first such

dissolution/deposition cycles of each freshly-formed contact, conductance

values strongly vary from cycle to cycle. In most cases, contact formation

resulted in contact conductance values exceeding 20 G

. When dissolving

the contact, conductance immediately drops to zero.

During some of the cycles, however, deposition leads to the formation of

a contact at a significantly lower conductance value. Yet no reproducible

response is observed as a function of the applied gate voltage. Not only the

conductance observed by closing the gap, but also the time needed for

forming and for dissolving a contact varies from cycle to cycle. While

contact conductance values near 5 G

were observed several times, the

behaviour of the contact as a result of the applied gate voltage was still

erratic in the beginning.

Figure 2b shows the conductance of the silver contact between the two gold

158 F.-Q. Xie, Ch. Obermair, Th. Schimmel

0 50 100 150 200 250 300 350

-30

-15

Gate Voltage (mV)

(a)

(b)

Conductance (2e

/h)

Time (s)

Fig. 2. Configuring a bistable atomic-scale switch by repeated electrochemical

cycling. a) Externally applied gate voltage as a function of time. b) Corresponding

change in contact conductance. Only after repeated cycling, regular switching is

observed as a function of the applied gate voltage (see arrow).

Only after 290 seconds from the beginning of the experiment (see black

arrow in Fig. 2b), a sudden transition from an erratic to a regular behaviour

of the contact is observed. Beginning at this point, each of the following

cycles of the gate voltage in Fig. 2a results in a corresponding opening and

closing of the gap in Fig. 2b, the conductance after closing the gap always

being 5 G

. A zoom-in into this sequence of regular switching events of

Configuring a Bistable Atomic Switch 159

Fig. 2 is given in Fig. 3. While Fig. 3a gives the potential applied to the

gate electrode as a function of time, Fig. 3b gives the corresponding

conductance value on the same time scale. Note that each cycle of the gate

voltage results in the corresponding switching of the conductance between

the “source” and “drain” electrodes: The device now reproducibly operates

as an atomic-scale transistor.

This sudden transition from an irregular opening and closing of the

contact to a bistable switching between zero and a well-defined quantized

conductance value was also regularly observed for other electrochemically

deposited silver point contacts, the on-state conductance frequently

exhibiting values which were integer multiples of the conductance quantum

The fact that quantized conductance is observed at values of a few

conductance quanta (e.g. 5 G

for the data shown above) means that the

contact cross-section is still on the atomic scale [18, 19]. The observation

that reversible switching is found between two well-defined conductance

values indicates that the contact switches between two well-defined

configurations on the atomic scale. This could also explain the observed

sudden transition between irregular and regular behaviour of the source-

drain conductance as a function of the gate voltage. As long as no atomic-

scale bistability is formed, each deposition cycle leads to the

electrochemical deposition of a new contact, different contacts having

different conductance values. As soon as a bistable contact configuration

has formed, the variation of the applied gate voltage could lead to a

switching between the two contact configurations even before dissolution

or re-deposition of the contact happens. This switching does not necessarily

involve electrochemical deposition and dissolution, but could also be

induced by changes of local surface forces due to changes of the applied

gate voltage: a variation of the gate voltage will lead to changes of the

electrochemical double layer which, in turn, will change surface forces and

surface tension [20, 21].

160 F.-Q. Xie, Ch. Obermair, Th. Schimmel

300 325 350

-30

-15

Gate Voltage (mV)

(a)

(b)

Conductance (2e

/h)

Time (s)

Fig. 3. Regular switching of a bistable atomic-scale quantum point contact, induced

by the applied gate voltage. a) Gate voltage as a function of time. b) Corresponding

contact conductance in units of the conductance quantum. Fig. 3 represents a

Conclusions

To conclude, the configuration process of an atomic-scale transistor or relay

was studied. The device, which can be controlled by an independent gate

electrode, reproducibly operates at room temperature. Depending on the

voltage applied to an electrochemical gate, an atomic-scale metallic

zoom-in into the data of Fig. 2.

Configuring a Bistable Atomic Switch 161

quantum point contact is opened or closed. In this way, the source-drain

conductance can be switched in a controlled manner between an insulating

off-state and a quantized conducting on-state. The device is based on an

atomic-scale silver point contact electrochemically deposited between two

gold electrodes. It is shown that prior to the reversible operation of the

device, electrochemical cycling of the contact is necessary. Our results

indicate that during the repeated electrochemical deposition/dissolution

cycles, a bistable contact configuration is formed, which subsequently

allows reproducible switching between a conducting and a non-conducting

contact configuration by means of an applied gate voltage.

Acknowledgements

The authors thank B. Bruhn, S. Brendelberger and L. Nittler for

experimental support. This work was supported by the Deutsche

Forschungsgemeinschaft within the Center for Functional Nanostructures

(CFN) and by the Research Award of the State of Baden-Württemberg

(Landesforschungspreis).

References

1. Agraït N, Levy Yeyati A, van Ruitenbeek JM (2003). Phys Rep 377:81

2. Agraït N, Rodrigo JG, Vieira S (1993). Phys Rev B 47:12345

3. Pascual JI et al (1993). Phys Rev Lett 71:1852

4. Krans JM, van Ruitenbeek JM, Fisun VV, Yanson IK, de Jongh LJ (1995).

Nature 375:767

5. Scheer E et al (1998). Nature 394:154

6. Li CZ, Tao NJ (1998). Appl Phys Lett 72:894

7. Li CZ, Bogozi A, Huang W, Tao NJ (1999). Nanotechnology 10:221

8. Morpurgo AF, Marcus CM, Robinson DB (1999). Appl Phys Lett 74:2084

9. Li CZ, He HX, Tao NJ (2000). Appl Phys Lett 77:3995

10. Li J, Kanzaki T, Murakoshi K, Nakato Y (2002). Appl Phys Lett 81:123

11. Elhoussine F, Mátéfi-Tempfli S, Encinas A, Piraux L (2002). Appl Phys Lett

81:1681

12. Obermair Ch, Kniese R, Xie F-Q, Schimmel Th (2004). In: Alexandrov AS,

Demsar J, I. Yanson K (eds) Molecular Nanowires and Other Quantum

Objects. Kluwer Academic Publishers, The Netherlands, pp 233-242

13. Eigler DM, Lutz CP, Rudge WE (1991). Nature 352:600

14. Fuchs H, Schimmel Th (1991) Adv Mater 3:112

15. Terabe K, Hasegawa T, Nakayama T, Aono M (2005). Nature 433:47

16. Xie F-Q, Nittler L, Obermair Ch, Schimmel Th (2004). Phys Rev Lett

93:128303

162 F.-Q. Xie, Ch. Obermair, Th. Schimmel

17. Xie F-Q, Obermair Ch, Schimmel Th (2004). Solid State Communications

132:437

18. Brandbyge M, Jacobsen KW, Norskov JK (1997). Phys Rev B 55:2637

19. Cuevas JC, Levy Yeyati A, Martín-Rodero A (1998). Phys Rev Lett 80:1066

20. Bach CE, Giesen M, Ibach H, Einstein TL (1997). Phys Rev Lett 78:4225

21. Friesen C, Dimitrov N, Cammarata RC, Sieradzki K (2001). Langmuir 17:807

Realization of an N-Shaped IVC of Nanoscale

Metallic Junctions Using the Antiferromagnetic

Transition

Yu. G. Naidyuk

, K. Gloos

, I. K. Yanson

Academy of Sciences of Ukraine, 47 Lenin Ave., 61103, Kharkiv, Ukraine

Nano-Science Center, Niels Bohr Institute fAFG, Universitetsparken 5,

DK-2100 Copenhagen, Denmark

Abstract: We have observed at low temperatures (<8 K) hysteretic I(V)

characteristics for sub-µm (∼200 nm) metallic break-junctions based

on the heavy-fermion compound UPd

. Degrading the quality of

the contacts by in situ increasing the local residual resistivity or

temperature rise reduces the hysteresis. We demonstrate that those

hysteretic I(V) curves can be reproduced theoretically by assuming

the constriction to be in the thermal regime. Our calculations show

that such anomalous I(V) curves are due to the sharp increase of

(T)

of UPd

near the Neel temperature T

≈ 14 K. From this point of

view each metal with similar

(T) should produce similar hysteretic

I(V) characteristics. As example we show calculations for the rare-

earth manganite La

0.75

0.25

MnO

, a system with colossal

magnetoresistance. In this way we demonstrate that nano-sized point

contacts can be non-linear devices with N-shaped I(V) characteristics,

tunnel diodes or Gunn diodes as amplifiers, generators, and switching

units. Their characteristic response time is estimated to be less than

1ns for the investigated contacts.

Key words: point contacts, negative differential resistance, UPd

, La

0.75

0.25-

MnO

163

R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications, 163–170.

B. Verkin Institute for Low Temperature Physics and Engineering, National

i.e. with negative differential resistance, that could serve like Esaki

164 Yu. G. Naidyuk, K. Gloos, I. K. Yanson

Introduction

Point-contact (PC) spectroscopy is widely used to study the interaction of

conduction electrons with elementary excitations or quasiparticles in

conducting solids [1-5]. On the other hand, PC investigations can shed light

on peculiarities of the electronic transport in nano-scale devices at ultrahigh

current densities. The latter matters for mesoscopic or nanoscale physics,

and especially for applied research, where electronic devices have already

reached this sub-µm scale.

Electron transport in nanostructures can be distinguished by basically

three different current regimes, depending on the relationship between the

elastic l

and the inelastic l

mean free path of electrons, and the

constriction or point-contact diameter d (for a review see [1], Chapter 3).

The constrictions are either ballistic l

, l

d, diffusive

<< d <<

el in

ll , or thermal l

, l

<< d. A priori it is not trivial to

determine the specific current regime, which is important to further

characterize the nano-object: first, because the nano-sized object needs a

well-defined geometry; second, to evaluate the electronic mean free path,

especially the inelastic one, can be rather speculative. To investigate the

non-linear I(V) curves using PC spectroscopy seems to be the best method

to solve this problem.

Experiment and Discussion

Here we present experiments on PCs between two pieces of the heavy-

fermion compound UPd2Al3, using the technique of mechanically-

controllable break junctions (see Fig. 1, Refs. [1], Chapter 4.1.5 and [6]).

For further details of experimental setup and sample preparation see also

[7, 8]).

UPd2Al3 becomes antiferromagnetic (AFM) at T

≈ 14 K [9]. We have

observed

huge non-linearities of the PC resistances and even hysteretic

I(V) characteristics below T

(Fig. 2). We derived the contact size and the

residual resistivity in the PC region as described in [10]. We found that the

very

short elastic mean-free path in the constriction l

<< d points to at

least

the diffusive regime of electron transport through the PC.

Considering also the small inelastic mean-free path in UPd2Al3, reflected

by the steep

(T) rise with temperature around the AFM transition (see

Fig. 3 inset), we applied the thermal model developed in Refs. [11-15]. In

this

case the excess electron energy eV dissipates within the constriction,

Realization of an N-Shaped IVC of Nanoscale Metallic Junctions 165

increasing the temperature inside the contact when a bias voltage V is

applied. As a result I(V) is governed by the resistivity

(T) via [14,15]:

(1 )

()

IV Vd

−

∫

Fig. 1. Break-junction set-up sketch.

where the temperature T in the center of the constriction is set by

= T

bulk

+ V

/4L

and L is the Lorenz number.

The calculated according to the above mentioned equation I(V) curve at

T = 1 K in Fig. 3 has maximum at around 2.5 mV, resulting in a hysteresis

for up- and downward sweeps when the junction is driven by a current

source. Figure 3 shows that the theoretical I(V) describe well the

experimental data, including the width of the hysteresis, using d = 200 nm

and

= 10 µΩcm, where

is the additional residual resistance:

(T) =

bulk

(T). These are the only two adjustable parameters which

have been derived independently from the measured contact resistance R(T)

in [10]. This agreement strongly supports our interpretation that local

thermal effects at the PC determine the behavior of our UPd

break-

junction conductivity.

The UPd

junctions presented here are non-linear devices. Their N-

shaped I(V) characteristics have a negative differential resistance at very

SCREW+PIEZO

SOLDERING

SAMPLE

BENDING BEAM

ISOLATOR

COUNTER SUPPORT

166 Yu. G. Naidyuk, K. Gloos, I. K. Yanson

-15 -10 -5 0 5 10 15

-2

-1

(a)

T=0.1K

Current (mA)

Voltage (mV)

Fig. 2. I(V) curves of a UPd

break junction with R

= 0.66 Ω at the indicated

temperatures. Solid (dashed) lines correspond to sweeps with increasing

(decreasing) current. The hysteretic loops become smaller when the temperature

high current densities up to 5×10

A/m

. Those devices could be applied –

in principle – like an Esaki tunnel diode or a Gunn diode [16, 17] as

amplifiers, generators, or switching units. Of practical interest is therefore

the possible minimum response time. We estimate [10] a thermal relaxation

time

≈ 100 ps for a d = 100 nm wide contact. This is three orders of

magnitude larger than for a standard tunnel diode, but it could be reduced

by using smaller contacts as long as they remain in the thermal regime.

Obviously UPd

is not such a unique material for creating N-shaped

I(V) curves – each metal with a similar

(T) should also produce similar

I(V) characteristics. This can be expected for many materials which order

magnetically, since their resistivity typically increases steeply when the

magnetic order is destroyed by thermal fluctuations.

An example for this behavior is the well known rare-earth manganite

0.75

0.25

MnO

, a system with colossal magnetoresistance [19, 20] as

rises and vanish above ~5 K.

Gross R., Sidorenko A., Tagirov L. Nanoscale Devices - Fundamentals and Applications

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