Single Spin Detection

1
Single Spin Detection

• Carlos Aguilar
• University of Texas Austin
• 7 April 2005

"I think that matter must have a separate reality
independent of the measurements. That is an
electron has spin, location and so forth even
when it is not being measured. I like to think
that the moon is there even if I am not looking
at it. (Albert Einstein)
2
Outline

• Introduction
• Magnetic Resonance Force Microscopy
• Scanning Tunneling Microscopy
• Quantum Dot Electrical Detection
• FET Detection
• Applications Other Detection Technologies
• Questions

3
Introduction

• Spin is a fundamental property of all elementary
  particles and is typically viewed as the
  intrinsic angular momentum.
• Both electrons and nuclei possess spin, and these
  spins precess around the direction defined by an
  applied magnetic field.
• The frequency of precession scales with the
  applied field and is roughly 1,000 times faster
  for electrons.

4
Introduction

• Traditional approaches to spin detection are
  based on electron spin resonance (ESR), where an
  applied static magnetic field B causes the
  spins of a sample to align with the field.
• Some of the spins will align parallel or
  anti-parallel to the field and the difference in
  energy between the two states is called the
  Zeeman energy (i.e. their frequency is the Larmor
  frequency ?L).
• Simply by monitoring this absorption at a given
  frequency as a function of applied magnetic field
  strength, we can extract info about the nature of
  spins in a sample. This approach however,
  requires 1010 electrons for a measurable signal.

5
Introduction

• If the number of spins needed for detection is
  scaled down, the spatial resolution increases.
• Furthermore, if we can further understand and
  control the spin of single particles, the
  potential for series of high-performance
  next-generation devices increases.
• Some of the benefits we can expect to see with
  the exploitation of single particle spins are
  nonvolatility, increased data processing speed,
  decreased electric power consumption and
  increased integration.

Wolf SA. et.al. (2001) Science
6
Introduction

• Over the last decade, there have been innovative
  methods developed to sense and manipulate single
  spins.
• Novel optical methods such as magnetic resonance
  force microscopy and scanning tunneling
  microscopy yield excellent results for single
  spin detection.
• There have also been fantastic strides using
  electrical approaches such as through quantum
  dots and field effect transistors that show
  highly sensitive spin detection capabilities.
• While these methods have shown single electron
  spin sensitivity, the next goal of these devices
  ultimately promise nuclear-spin signals.

7
Magnetic Resonance Force Microscopy

• Magnetic Resonance Force Microscopy (MRFM)
  combines the conventional magnetic resonance
  imaging (MRI) and atomic force microscopy (AFM)
  to detect a magnetic force between a magnetic tip
  and spins in a sample.
• A mass-loaded Si cantilever with an attached
  150-nm wide samariumcobalt magnetic tip senses
  the force from a single electron spin in a sample
  of amorphous silica, which was irradiated with
  gamma rays to produce a low density of free
  unpaired electron spins.

Rugar D et al. (2004) Nature
8
Magnetic Resonance Force Microscopy

• The magnetic tip is mounted above the sample and
  generates a large magnetic field gradient into
  the treated mica.
• In the presence of the field gradient, the
  applied oscillating magnetic field excites
  electrons at a particular depth in the sample at
  their resonant frequency.
• The field gradient also induces spins located at
  different depths beneath the tip to resonate at
  different frequencies, providing selective
  excitation of spins.

Hammel C et al. (2004) Nature
9
Magnetic Resonance Force Microscopy

• The vibration of the cantilever causes the
  resonant slice to sweep back forth and when the
  slice finds a spin, the resonance repeatedly
  flipped the spin of the electron, giving the
  cantilever a slight boost.
• To locate a spin signal, the sample was scanned
  through many independent locations, before a
  strong signal from a well positioned spin was
  found and measured.

10
The Future of MRFM

• MRFM is entirely general and can, in principle,
  be applied to the detection of any magnetic
  moment.
• This technique paves the way for nanoelectronics,
  where atomic-scale characterization and
  single-spin readout of quantum states is crucial.
• MRFM also holds the potential to map 3-D images
  of molecules (e.g. proteins) in situ with high
  resolution.

11
Outline

• Introduction
• Magnetic Resonance Force Microscopy
• Scanning Tunneling Microscopy
• Quantum Dot Electrical Detection
• FET Detection
• Applications Other Detection Technologies
• Questions

12
Scanning Tunneling Microscopy

• Scanning Tunneling Microscopy (STM) is a common
  technique used for high resolution imaging.
• To detect single spins by STM, a small DC
  magnetic field is applied to a sample in an STM
  system. This field causes all free, unpaired
  electrons to precess at the Larmor frequency.
• Using an STM tip to tunnel into magnetic regions
  of the sample, this spin precession activates
  radio frequency (RF) modulation of the tunnel
  current of the STM. By detecting the RF signal
  with a spectrum analyser, it is possible to
  locate single electronic spins on surfaces.

Durkan C. (2004) Contemp. Phys.
13
Scanning Tunneling Microscopy

• A graphite surface (HOPG) with clusters of
  organic BDPA molecules. A magnetic field is
  applied inducing the free radicals in the
  molecules to precess at the Larmor frequency.
• When the tip of a scanning tunnelling microscope
  is brought close to a cluster, a current flows
  between the tip and the sample. This current is
  modulated at the Larmor frequency detecting the
  modulation effectively measures electronic spin
  in the molecule.

Manoharan HC. (2002) Nature
14
Scanning Tunneling Microscopy

• a) b) are two spectra of different areas of the
  BDPA samples and c) is bare HOPG.
• The fact that there is (a) a modulation of the
  tunnel current at the Larmor frequency when
  tunnelling into a molecule and (b) no evidence of
  any modulation when tunnelling into HOPG
  indicates spin sensitivity.
• The proportionality constant (g-factor) between
  the magnetic field and the Larmor frequency was
  equal (2) to the results of conventional ESR
  experiments.

15
The Future of STM

• STM is a natural platform for sensing single
  spins, as it already provides convenient high
  resolution imaging of single atoms and molecules.
• The technique also holds excellent potential for
  manipulating spins at the single quantum level
  and lends an excellent platform for quantum
  computation.
• Though the physical origin of the spin-sensitive
  signal is very controversial and several theories
  have been published to explain the effect, the
  exciting results suggest that the tool needs
  further investigation.

16
Outline

• Introduction
• Magnetic Resonance Force Microscopy
• Scanning Tunneling Microscopy
• Quantum Dot Electrical Detection
• FET Detection
• Applications Other Detection Technologies
• Questions

17
Quantum Dot Detection of Single Spins

• Single spin detection using quantum dots has been
  realized using both electrical and optical
  techniques.
• In this technique, the spin orientation of a
  single electron (e-) in a quantum dot (QD) is
  measured electrically.
• This system uses spin-to-charge conversion of a
  single e-confined in a QD, and detects the single
  e- charge using a quantum point contact (QPC).
• The QD, which is in close proximity to the QPC,
  acts as a box to trap a single e-, and the QPC
  operates as a charge detector to determine
  whether the dot contains an e- or not.

Elzerman JM et al. (2004) Nature
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Quantum Dot Detection of Single Spins

• The charge on the QD remains constant if the e-
  spin is up, whereas an e- with spin down can
  escape and change the charge on the QD.
• A magnetic field is applied to split the spin up
  and spin down states by the Zeeman energy and the
  spacing between the QDs energy levels is tuned
  so that the only level that can be occupied is
  the ground state.
• Voltages applied to the lettered electrodes
  define a QD next to a QPC, which is sensitive to
  the charge state of the QD.

19
Quantum Dot Detection of Single Spins

• Before the voltage pulse the QD is empty, as both
  the spin up spin down levels are above the
  Fermi level (EF). Then a voltage pulls both
  levels below EF, which now allows for an e- to
  tunnel onto the dot. During twait, the e- is
  trapped on the QD and Coulomb blockade prevents a
  2nd e- from tunneling on the QD.
• After twait, the pulse is reduced to position the
  energy levels in the read-out state. The Coulomb
  blockade lifts and an e- with spin up can tunnel
  onto QD. Therefore, if the QD contains a
  spin-down e-, the current will go up and down
  again.

20
Outline

• Introduction
• Magnetic Resonance Force Microscopy
• Scanning Tunneling Microscopy
• Quantum Dot Electrical Detection
• FET Detection
• Applications Other Detection Technologies
• Questions

21
FET Detection of Single Spins

• This approach demonstrates the electrical sensing
  of the magnetic resonance spin-flips of a single
  electron paramagnetic spin-centre, formed by a
  defect in the gate oxide of a standard silicon
  transistor.
• When a defect is present, the source/drain
  channel current can experience a random telegraph
  signal, jumping between 2 current values that
  arise from 2 possible trapped electric charge
  states of the defect.
• The spin orientation is converted to electric
  charge, which is measured as a change in the
  source/drain channel current.

Xiao M et al. (2004) Nature
22
FET Detection of Single Spins

• The two available states of the trap were with
  one or two electrons in it. The empty state
  corresponded to a single e-, with lower energy
  (spin-up). By the Pauli principle, a second e-
  could hop onto the defect and switch the trap to
  the filled stateonly if it has opposite spin to
  the first e-.
• The presence of a 2nd e- on the defect site
  decreased the current through the conduction
  channel having the FET act as a sensitive
  electrometer.

23
FET Detection of Single Spins

• The trap studied is a very stable defect because
  the behavior is reproducible over many thermal
  cycles.
• The current through the FET showed switching
  between 2 values as electrons hop onto and off
  the defect site.
• A histogram was plotted to extract the occupation
  probabilities for the empty and filled trap
  states.

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The Future of QD FET Detection

• The ability to control spins electrically offers
  several advantages. In particular, devices could
  be self-contained on a chip without the need for
  lasers and optical tables. And electrical
  implementations could interface naturally with
  conventional electronic circuits.
• The QD FET detection scheme are unique
  spin-filtering approaches and may help further
  our understanding of spins in semiconductors for
  possibly uses in quantum computation.

25
Single Spin Applications and Other Detection
Technologies

• MRFM demonstrates the unique potential to map 3-D
  images of molecules (e.g. proteins) in situ with
  high resolution.
• STM can demonstrate the ability to manipulate
  spins at the single quantum level, which would be
  crucial in quantum computing.
• The electrical sensing techniques offer the
  capacity to directly study spin decoherence and
  possibly serve as platforms for quantum
  computation.
• In addition to presented methods, there are other
  techniques such as optical quantum dot detection,
  nano-SQUIDs, and current detection schemes that
  offer promise into single-spin detection.

26
Primary References

• Wolf S.A. et.al. (2001) Spintronics A Spin-Based
  Electronics Vision for the Future. Science 294,
  1488-1495.
• Rugar D. et.al. (2004) Single Spin Detection by
  Magnetic Resonance Force Microscopy. Nature 430,
  329-332.
• Durkan C. (2004) Detection of Single Electronic
  Spins by Scanning Tunneling Microscopy. Contemp.
  Phys. 45, 1-10.
• Elzerman J.M. et.al. (2004) Single-shot Readout
  of an Individual Electron Spin in a Quantum Dot.
  Nature 430, 431-435.
• Xiao M. et.al. (2004) Electrical Detection of the
  Spin Resonance of a Single Electron in a Silicon
  Field Effect Transistor. Nature 430, 435-439.
• Heinrich AJ. et.al. (2004) Single-Atom Spin-Flip
  Spectroscopy. Science 306, 466-469.

27
Secondary References

• Hammel CP. (2004) Seeing Single Spins. Nature
  430, 300-301.
• Jelezko F. et.al. (2004) Read-out of Single Spins
  by Optical Spectroscopy. J Phys. Cond. Matt.
  1630, R1089-R1104.
• Gywat O. et.al. (2004) Optical Detection of
  Single-Electron Spin Decoherence in a Quantum
  Dot. Phys. Rev. B 6205303.
• Ciorga M. et.al. (2001) Readout of a Single
  Electron Spin Based Quantum Bit by Current
  Detection. Physica E-Low-Dimensional Systems
  Nanostructures 111, 35-40.
• Gallop J. (2003) SQUIDs Some Limits to
  Measurement. Superconductor Science Tech.
  1612, 1575-1582.
• Bandyopadhyay S. (2003) Single-Spin Measurement
  in the Solid-State A Reader for a Spin Qubit.
  Phys Rev B 67193304.