Martin Brandt

Experimental Semiconductor Physics

Technical University of Munich

Walter Schoki Institut

Am Coulombwall 4

85748 Garching

Tel. +49 89 289 12758


Research webpage


Research Focus: Dopants and Defects in Semiconductors for Quantum Applications, EPR, ESR

The Brandt group studies the fundamental properties of paramagnetic states in semiconductors for quantum applications. These states include dopants such as phosphorus in silicon, where both the electronic spin as well as the spin of the nucleus can be used as a quantum resource. In isotopically pure 28Si, the latter system has a coherence time of tens of minutes, rendering 28Si:P a unique and highly useful quantum material. The group investigates the interactions between these spins and with their environment and develops methods to tune the spin properties e.g. via strain. For these investigations, the group in particular uses advanced pulsed electron spin resonance techniques.

Approaches to increase the sensitivity of magnetic resonance experiments are a particular focus of the research activity. Spin selection rules govern charge transport processes and allow the readout of quantum states. The Brandt group is spearheading these electrically detected magnetic resonance (EDMR) techniques and uses them to prepare spins at dopants or defects in silicon and other semiconductors and to understand spin-to-charge and spin-to-photon conversion. Both spin-dependent recombination as well as spin-dependent photoexcitation are employed for this. Of particular current interest is the electrical readout of NV- centers in diamond, which the group has recently demonstrated to be highly efficient.

Selected publications

  • Multiple-Quantum Transitions and Charge-Induced Decoherence of Donor Nuclear Spins in Silicon, Phys. Rev. Lett. 118, 246401 (2017)
  • Efficient Electrical Spin Readout of NV? Centers in Diamond, Phys. Rev. Lett. 118, 037601 (2017)
  • Interaction of Strain and Nuclear Spins in Silicon: Quadrupolar Effects on Ionized Donors, Phys. Rev. Lett. 115, 057601 (2015)


Manganese doping for enhanced magnetic brightening and circular polarization control of dark excitons in paramagnetic layered hybrid metal-halide perovskites

T. Neumann, S. Feldmann, P. Moser, A. Delhomme, J. Zerhoch, T. van de Goor, S. Wang, M. Dyksik, T. Winkler, J.J. Finley, P. Plochocka, M.S. Brandt, C. Faugeras, A.V. Stier, F. Deschler

Nature Communications 12, 3489 (2021).

Show Abstract

Materials combining semiconductor functionalities with spin control are desired for the advancement of quantum technologies. Here, we study the magneto-optical properties of novel paramagnetic Ruddlesden-Popper hybrid perovskites Mn:(PEA)2PbI4 (PEA = phenethylammonium) and report magnetically brightened excitonic luminescence with strong circular polarization from the interaction with isolated Mn2+ ions. Using a combination of superconducting quantum interference device (SQUID) magnetometry, magneto-absorption and transient optical spectroscopy, we find that a dark exciton population is brightened by state mixing with the bright excitons in the presence of a magnetic field. Unexpectedly, the circular polarization of the dark exciton luminescence follows the Brillouin-shaped magnetization with a saturation polarization of 13% at 4 K and 6 T. From high-field transient magneto-luminescence we attribute our observations to spin-dependent exciton dynamics at early times after excitation, with first indications for a Mn-mediated spin-flip process. Our findings demonstrate manganese doping as a powerful approach to control excitonic spin physics in Ruddlesden-Popper perovskites, which will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.

DOI: 10.1038/s41467-021-23602-1

Echo Trains in Pulsed Electron Spin Resonance of a Strongly Coupled Spin Ensemble

S. Weichselbaumer, M. Zens, C.W. Zollitsch, M.S. Brandt, S. Rotter, R. Gross, H. Huebl.

Physical Review Letters 125, 137701 (2020).

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We report on a novel dynamical phenomenon in electron spin resonance experiments of phosphorus donors. When strongly coupling the paramagnetic ensemble to a superconducting lumped element resonator, the coherent exchange between these two subsystems leads to a train of periodic, self-stimulated echoes after a conventional Hahn echo pulse sequence. The presence of these multiecho signatures is explained using a simple model based on spins rotating on the Bloch sphere, backed up by numerical calculations using the inhomogeneous Tavis-Cummings Hamiltonian.


Anisotropic Magnetic Resonance in Random Nanocrystal Quantum Dot Ensembles

A.J.S. Almeida, A. Sahu, D.J. Norris, G.N. Kakazei, M.S. Brandt, M. Stutzmann, R.N. Pereira

ACS Omega 5, 11333 (2020).

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Magnetic anisotropy critically determines the utility of magnetic nanocrystals (NCs) in new nanomagnetism technologies. Using angular-dependent electron magnetic resonance (EMR), we observe magnetic anisotropy in isotropically arranged NCs of a nonmagnetic material. We show that the shape of the EMR angular variation can be well described by a simple model that considers magnetic dipole–dipole interactions between dipoles randomly located in the NCs, most likely due to surface dangling bonds. The magnetic anisotropy results from the fact that the energy term arising from the magnetic dipole–dipole interactions between all magnetic moments in the system is dominated by only a few dipole pairs, which always have an anisotropic geometric arrangement. Our work shows that magnetic anisotropy may be a general feature of NC systems containing randomly distributed magnetic dipoles.

DOI: 10.1021/acsomega.0c00279

Thermal characterization of thin films via dynamic infrared thermography

A. Greppmair, N. Galfe, K. Amend, M. Stutzmann, M.S. Brandt

Review of Scientific Instruments 90, 44903 (2019).

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We extend the infrared thermography of thin materials for measurements of the full time response to homogeneous heating via illumination. We demonstrate that the thermal conductivity, the heat capacity, as well as the thermal diffusivity can be determined comparing the experimental data to finite difference simulations using a variety of test materials such as thin doped and undoped silicon wafers, sheets of steel, as well as gold and polymer films. We show how radiative cooling during calibration and measurement can be accounted for and that the effective emissivity of the material investigated can also be measured by the setup developed.

DOI: 10.1063/1.5067400

Measurements and atomistic theory of electron g-factor anisotropy for phosphorus donors in strained silicon

M. Usman, H. Huebl, A.R. Stegner, C.D. Hill, M.S. Brandt, L.C.L. Hollenberg

Physical Review B 98, 35432 (2018).

Show Abstract

This work reports the measurement of electron g-factor anisotropy (|Δg|=|g001−g1¯10|) for phosphorous donor qubits in strained silicon (sSi = Si/Si1−xGex) environments. Multimillion-atom tight-binding simulations are performed to understand the measured decrease in |Δg| as a function of x, which is attributed to a reduction in the interface-related anisotropy. For x<7%, the variation in |Δg| is linear and can be described by ηxx, where ηx≈1.62×10−3. At x=20%, the measured |Δg| is 1.2±0.04×10−3, which is in good agreement with the computed value of 1×10−3. When strain and electric fields are applied simultaneously, the strain effect is predicted to play a dominant role on |Δg|. Our results provide useful insights on the spin properties of sSi:P for spin qubits, and more generally for devices in spintronics and valleytronics areas of research.

DOI: 10.1103/PhysRevB.98.035432

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