独立无液氦超导磁体或“干磁体”


  • 独立室温腔超导磁体,至 18 T,使用GM制冷机,允许竖直或者水平方向使用
  • 57mm低温孔径
  • 千分之一均匀度
  • 可与传统液氦制冷低温插件或无液氦低温插件集成(详见例子)
  • 搭配牛津最新的MercuryiTC温控仪和MercuryiPS磁体电源

低温下的共焦显微光学应用

Confocal microscopy at low temperatures:

3D g-factor mapping of single quantum dots at high magnetic fields by M. Ediger* and R. T. Phillips, Cavendish Laboratory, University of Cambridge.

Confocal microscopy at cryogenic temperatures has become an essential tool for the study of semiconductor quantum dots. Furthermore if a high magnetic field can be applied to the sample and the sample rotated within the field then even greater information can be obtained from magneto-optical spectroscopy.
To gain access to this additional information, they have developed a novel fibre-based confocal microscope [1] to investigate the properties of nanostructures such as InGaAs quantum dots (QDs) via magneto-photoluminescence (PL).
Our design allows us to rotate the samples to arbitrary angles of tilt and rotation with respect to a magnetic field of up to 10 T and at low temperatures, while maintaining focus on a single QD. Modelling the exciton emission [2] we can extract the full 3-dimensional g-factor tensors for the electrons and holes and their exchange parameters. This new method improves upon the first studies of this type [3,4] by allowing dots to be selected in the microscope using
its positioning capability.

Experimental set-up:

The set-up includes two custom-built Oxford Instruments units:
• a 10 T cryogen-free superconducting magnet with a 110 mm room-temperature bore
• a helium bath cryostat with dynamic exchange gas and a 85 mm sample space to accommodate the rotating parts of the confocal microscope .
The microscope cryostat is mounted on a vibration-isolated optical table. The cryostat tail is inserted into the magnet bore with a clearance of about 1mm.
The vacuum can of the magnet system rests on the laboratory floor, so there is no direct coupling between the two cryostat systems.

They found that, even without floating the optical table, the vibration isolation between the closed-cycle cold head of
the magnet and the confocal microscope was so good that we were able to study the same single nanostructure over
days and weeks. The Cryofree® magnet is ready for use after about 48 hours of cooling from room temperature, and
can be left cold for months. The hold-time of the microscope’s cryostat is about 4 days under normal operation and
the helium reservoir can be refilled during experiments without disturbing the optical alignment in the sample space.
In the event of a quench, recovery to normal operating temperature is very quick as there is no liquid helium to boil
off in the magnet cryostat.
The confocal microscope has been designed to minimise the effects of forces related to eddy currents when sweeping
the magnetic field and in fact, we have found that the system remains aligned on the same optical feature with
displacement of less than a micron after a quench.

Experimental results

Figure 1 shows the emission of a neutral exciton of a single InGaAs quantum dot tilted to 45° with respect to a magnetic field of 0 to 10 T at a temperature of 4 K.
The intense upper doublet belongs to the bright exciton states, while the faint lines emerging at about 2 T stem from predominantly dark transitions that only become visible due to a field-induced mixing with the bright states.
If a standard magneto-PL set-up would have been used in Faraday geometry (0° tilt), this mixing would not appear for rotationally symmetric dots. An other obvious feature for tilt angles around 45° is the anti-crossing of the dark and bright states, which in this case happens at about 5 T.
The size of the splitting, as shown in figure 2, obtained from the precise modelling, is dominated by and gives direct access to the in-plane hole g-factor[5]. This is an important parameter for the emerging idea of quantum information processing using long-lived hole spins.

Conclusion

We have developed a technique which is adaptable to a range of different  nanostructures, and gives detailed
information about the shape of wave functions (deduced from diamagnetic shifts), the bright and dark spin states, as well as structural information by probing the 3D confinement properties of the respective nanostructure.

References:
[1] T. Kehoe, M. Ediger, R. T. Phillips, and M. Hopkinson, Rev. Sci. Instr. 81 013906 (2010)
[2] H. W. Van Kesteren, E. C. Cosman, W. A. J. A. Van der Poel, and C. T. Foxon Phys. Rev. B 41 5283 (1990)
[3] A.G. Steffan and R. T. Phillips, physica status solidi a 190 541-545 (2002); Physica E 17 15-18 (2003)
[4] R.T. Phillips, A.G. Steffan, S.R. Newton, T.L. Reinecke and R. Kotlyar physica status solidi b 238 601-606 (2003)
[5] I. Toft and R.T. Phillips, Physical Review B 76 033301 (2007)

 

配合低温系统的整套解决方案

Stand alone Cryogen free magnet used with a full suite of low temperature inserts for Graphene studies:

Dr Enrique Diez from Salamanca University, who works on Graphene, has set-up a unique and flexible cryogen-free suite of three inserts including a 4 K cryostat, 300 mK He-3 insert and a 10 mK dilution refrigerator. All inserts are interchangeable and can be fitted to their 12 T, 55 mm stand-alone Cryogen-free magnet and enables continuous measurements from 10 mK up to 40 K from one cryostat and without the need for liquid cryogen.
Dr Diez quoted: “This set-up gives us complete flexibility in the type of experiments we need to perform. We are able to make measurements from millikelvin temperatures to room temperature continuously without having to change cryostats. Apart from the convenience, it also ensures accuracy of our results. We can also use the magnet cryostat as stand-alone for quantum hall measurements at room temperature. This is particularly useful as graphene, a
new material first created in 2003 by Drs. Geim and Novoselov (Winners of the 2010 Nobel Prize in Physics) from Manchester University, exhibits Quantum Hall plateaus at room temperature.”
Apart from the obvious reduction in operating costs and preservation of helium, a scarce natural resource, dry systems are also very easy to operate, at the touch of a button.
Dr Diez has devoted the last few years to the study of magneto-transport properties of graphene. He looked closely at the temperature dependence of the plateau-plateau and plateau-insulator transition in graphene. He reported the first measurement of the scaling exponent for the plateau-insulator quantum phase transition in graphene.

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