CeNS Colloquium

Dr. Hans Werner Schumacher
Physikalisch-Technische Bundesanstalt, Braunschweig

Pumping transport mechanisms have attracted much interest as an alternative means to generate charge and spin currents in the absence of a bias voltage. The current results from periodic modulation of certain system parameters of a nanostructure connected to source and drain leads. Of particular interest has been the quantized regime when the current varies in steps of e·f as a function of the system parameters, where e is the electron charge and f is a modulation frequency. 
Much theoretical and experimental work has been devoted to the adiabatic regime, when the time variation of the parameters is slow compared to characteristic relaxation times of the system [1]. In the non-adiabatic regime the system is driven out of equilibrium and the frequency f becomes another control parameter. Owing to the complex interplay between tunneling rates and frequency experimental realizations have only recently shown quantization [2]. In this talk I will present recent developments in non-adiabatic quantized charge pumping in AlGaAs/GaAs nanowire based devices. The wire is crossed by several metallic top gates, set to negative voltages to define a quantum dot (QD). Single parameter non-adiabatic charge pumping [3,4] is achieved by applying modulation frequencies of f = 0.001 to 1 GHz to one of the outer gates defining the QD, tuning the current into the quantized regime. 
Driving a quantized current by a single modulation parameter, which is only possible in the non-adiabatic regime, is of fundamental practical importance as it enables complex on-chip circuit integration. I will discuss current up-scaling by parallel quantized operation of multiple pumps on a single chip [5] and the realization of a semiconductor quantized voltage source by integration of a pump with a quantum Hall device [6]. Furthermore the serial integration of pumps with charge detector circuits allows in-situ monitoring of single charge errors during pumping operation. 
 

 [1] H. Pothier et al., Europhys. Lett. 17, 249 (1992). 
 [2] M. Blumenthal et al., Nature Physics 3, 343 (2007). 
 [3] B. Kaestner et al., Phys. Rev. B 77, 153301 (2008); 
 [4] V. Kashcheyevs & B. Kaestner, Phys. Rev. Lett. 104, 186805 (2010). 
 [5] P. Mirovsky et al., Appl. Phys. Lett. 97, 252104 (2010). 
 [6] F. Hohls et al. arXiv:1103.1746.