Research interests

The longstanding progress of conventional semiconductor technology is expected to come to a halt in the next ten to twenty years.  As the size of transistors approach the nanometer scale, severe problems related to miniaturization and energy dissipation will hinder further improvement of conventional devices.  This anticipation is motivating the development of alternative devices that take advantage of the quantum nature at the atomic scale.  A notable example is the quantum computer concept, where each bit is formed by a single atom or a group of a few atoms, and the rules of quantum mechanics dictate the way information is processed.  Another interesting alternative is spintronics, where the spin of the electrons instead of their charge forms the basis for classical memory and logic, promising much lower rates of energy dissipation per device.  Our research addresses several theoretical questions related to the design and optimization of quantum computer and spintronic devices based on semiconductor, superconductor, and magnetic nanostructures. 

Click here for a list of publications.

Our work has been cited over 500 times by other researchers.  Click here for up to date citation statistics.

We are currently interested in the question of electrical control of magnetism in bulk and nanoscale materials.  One promising direction is to study the so called multiferroic materials, which possess coexisting ferroelectric and magnetic phases.  The coupling between electric and magnetic order parameters leads to very rich physics.  For example, the magnetic excitations (magnons) become admixed with electric excitations (optical phonons).  These electromagnon excitations can be observed with optical experiments such as reflectivity, transmitivity, or Raman spectroscopy in the infrared region, revealing the structure of magnetoelectric interactions in these materials.  

Recently, we determined the electromagnon spectra of a class of important multiferroic materials: One that has spiral-like (cycloidal) antiferromagnetic order (The calculated dispersion is shown on the left panel of the figure above).  An important member of this class is bismuth ferrite (BiFeO₃), a rare example of room temperature multiferroic.  Our theoretical results predicted a series of novel 

magneto-dielectric resonances in the far infrared region,  and an important dependence of the magnon spectra on the direction of the ferroelectric moment.  These results were confirmed by a high resolution Raman experiment devised by the University of Paris 7 group - The experimental data is shown on the right panel of the figure. 

This discovery show that the optical response of a multiferroic reveals much more about the magnetic excitations than previously anticipated.  An interesting application is to use the orientation dependence of magnon propagation as an electric switch in spin wave logic devices. 

Here is a list of our recent publications on the physics of Multiferroics:

• R. de Sousa and J.E. Moore, "Optical coupling to spin waves in the cycloidal multiferroic BiFeO₃",
  Phys. Rev. B 77, 012406 (2008);

     

• R. de Sousa and J.E. Moore, "Electrical control of magnon propagation in multiferroic BiFeO₃ films",
  Appl. Phys. Lett. 92, 022514 (2008);

     

• M. Cazayous, Y. Gallais, A. Sacuto, R. de Sousa, D. Lebeugle, and D. Colson, "Possible observation of cycloidal electromagnons in BiFeO₃",
  Phys. Rev. Lett. 101, 037601 (2008);

     

• R. de Sousa and Joel E. Moore, "Comment on Ferroelectrically Induced Weak Ferromagnetism by Design [C. Fennie, PRL 100, 167203 (2008)]",
  Phys. Rev. Lett. 102, 249701 (2009);

     

• Markku P.V. Stenberg and Rogerio de Sousa, "Twin electromagnons and magnetically induced oscillatory polarization in multiferroic RMnO₃",
  Phys. Rev. B 80, 094419 (2009) (Editors' suggestion).
  
  
• P. Rovillain, R. de Sousa, Y. Gallais, A. Sacuto, M.A. Méasson, D. Colson, A. Forget, M. Bibes, A. Barthélémy, and M. Cazayous,
  "Electric-field control of spin waves at room temperature in multiferroic BiFeO₃",
  Nature Materials 9, 975 (2010).

We are currently investigating novel physical phenomena with these materials, with an eye towards applications to spintronic devices.  

Click here for additional information on multiferroics.

Silicon is the material of choice in the current microelectronics industry.  Therefore, the development of quantum computing and spintronic devices based on silicon nanostructures has the advantages of being easily integrated into existing technology and being compatible with large scale fabrication techniques.  We are currently interested in spin-dependent effects in silicon nanostructures, particularly effects leading to the measurement of the spin state of a single donor impurity in a silicon device.  This work is in collaboration with Dr. Thomas Schenkel from the Lawrence Berkeley National Laboratory, and Prof. Jeffrey Bokor from the Dept. of Electrical Engineering at U.C. Berkeley.  Click here to see a recent research news article from Science@Berkeley Lab.

Silicon combines many special features that make its electronic spin a promising basis for classical and quantum logic devices. Properties such as weak spin-orbit coupling, extremely long spin coherence times, and the possibility of removing all nuclear spins make silicon and related structures prominent candidates for the realization of spin-based applications. However, these same features also make the spin degree of freedom extremely hard to detect: Optical methods such as Faraday and Kerr rotation are inapplicable to silicon and related materials, whose spin-selective optical transitions are extremely weak and ineffective.  Recently, we proposed the scattering of conduction electrons off neutral donor impurities as way to interrogate the spin degree of freedom in silicon transistors.  The figure above shows a silicon transistor doped with three different species of neutral donor impurities.  Each donor species has a distinct set of electron spin resonance frequencies.  At resonance, the source-drain current gets modulated - The amplitude of this modulation is directly proportional to the local carrier (conduction electron) spin polarization, allowing spatially resolved mapping of carrier spin polarization in a silicon device.  This scheme may become an important tool for the study of spin injection in silicon devices.

Our theory led to a physical optimization of this effect.  Surprisingly, we show that it is possible to detect the spin resonance of a single donor impurity in a relatively large transistor of area 0.1 μm².


Look at our recent paper:

• R. de Sousa, C.C. Lo, and J. Bokor, "Spin-dependent scattering in a silicon transistor",
  Phys. Rev. B 80, 045320 (2009).

Noise and decoherence in metallic nanostructures

We are investigating the physical origin of noise and decoherence affecting quantum bits based on metallic and superconducting nanostructures.  Our goal is to control the imperfections inherent to these devices in order to allow the development of large scale fault tolerant quantum hardware. 

Recently, we developed a microscopic model for the noise on single electron tunnelling devices arising due to the presence of trapping-centres in the gate electrodes.  The noise spectrum resulting from coupling to such trapping-centres is found to show quite different behavior depending on the relative energy of the trapping-centre and the Fermi level. When the trap energy level is close to the Fermi sea and has a linewidth greater than kT, it results in an Ohmic noise spectrum, whereas when the linewidth is less than kT the Lorentzian form expected from a semiclassical limit is obtained (In the semiclassical limit, trapping-centre noise is well described by a random telegraph noise model).  Multiple trap levels above the Fermi level are shown to lead to a staircase noise spectrum that can be used to probe the energetics of the trapping-centres, allowing identification of individual trapping-centres coupled to a tunnelling device.  For details, see our papers:

• R. de Sousa, K.B. Whaley, F.K. Wilhelm, and J. von Delft,
  "Ohmic and step noise from a single trapping-center hybridized with a Fermi sea"
  Phys. Rev. Lett. 95, 247006 (2005).

• R. de Sousa, K.B. Whaley, T. Hecht, J. von Delft, and F.K. Wilhelm,
  "Microscopic model of critical current noise in Josephson junction qubits: Subgap resonances and Andreev bound states"
  Phys. Rev. B 80, 094515 (2009).

Another interesting problem is the origin and control of magnetic flux noise in Superconducting Quantum Interference Devices (SQUIDs).  SQUIDs are the most sensitive detectors of magnetic field, with applications ranging from detecting neuron activity in the brain (Magneto Encephalography or MEG, a major tool in neuroscience) to finding short-circuit faults in microchips.  SQUIDs are also being used as implementations of quantum bits (qubits) in superconductor-based quantum computers.  

Therefore it is very important to understand  and to control the ultimate source of magnetic noise in SQUIDs.  We believe paramagnetic dangling bonds at the semiconductor/metal-oxide interface in the SQUID are the main cause for this noise.  Recently, we showed that the interactions of these spins with lattice vibrations at the amorphous interface lead to magnetic noise with 1/f frequency dependence.  Here is our recent publication,

• R. de Sousa, "Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory"
  Phys. Rev. B 76, 245306 (2007).