9.2.26

Article: Influence of local strain on the optical probing of a Ni2+ spin in a charged self-assembled epitaxial quantum dot

We investigate the optical and spin properties of semiconductor quantum dots doped with a single Ni²⁺ ion interacting with a charged exciton. The exchange interaction between charge carriers and the localized Ni²⁺ spin—dominated by coupling to holes—provides an efficient optical interface to an individual magnetic atom embedded in a solid-state nanostructure. This makes Ni-doped quantum dots a versatile platform for exploring single-spin physics and carrier–spin interactions at the nanoscale.

Through systematic magneto-optical spectroscopy, we reveal the crucial role played by the local strain environment at the Ni²⁺ site. In positively charged quantum dots dominated by in-plane biaxial strain, the Ni²⁺ ion exhibits three well-defined spin states (𝑆𝑧 = 0, ±1). In this regime, the magneto-optical spectra offer direct access to the local strain anisotropy, enabling the quantum dot itself to function as a sensitive probe of strain on the atomic scale. In contrast, in most dots the presence of lower-symmetry strain leads to strong mixing of all Ni²⁺ spin states, resulting in a much richer optical spectrum with an increased number of allowed transitions.

To interpret the experimental observations, we develop a spin-effective model that explicitly incorporates the orientation and symmetry of the local strain. This model successfully reproduces the key features of the measured spectra and shows that low-symmetry terms in the hole–Ni²⁺ exchange interaction are essential for accurately describing the emission behavior in a magnetic field. Overall, our results establish strain as a powerful control parameter for tailoring the spin structure and optical accessibility of single magnetic ions in quantum dots, opening new perspectives for solotronic and quantum information applications.

Authors: K. E. Połczyńska, S. Karouaz, W. Pacuski and L. Besombes

Physical Review B 112, 245301

Published 1 December, 2025

https://doi.org/10.1103/swfy-f7w8

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9.2.26

Article: Hole to electron crossover in a (Cd,Mn)Te quantum well through surface metallization

We explore how the choice of metal contact can profoundly influence the local charge environment in a CdTe-based quantum well. By depositing ultrathin metallic layers on the sample surface and probing the system with magneto-optical spectroscopy, we demonstrate that surface metallization alone can modify not only the carrier density but even the type of carriers present in the quantum well. While most of the investigated metals preserve the intrinsic p-type character originating from surface states, gold and nickel stand out by inducing a complete conversion to n-type doping.

Using a contactless, all-optical method, we identify the carrier type through the magnetic-field evolution of charged excitons, specifically by tracking singlet–triplet transitions. This approach avoids the need for gated structures or conductive substrates and enables a direct, local probe of carrier gases in low-dimensional systems. Results show that a simple picture based on metal work functions is insufficient to explain the observed behavior. Instead, the decisive role is played by strong metal–semiconductor bonding, which effectively passivates surface states and facilitates electron diffusion from the metal into the quantum well.

These findings highlight surface metallization as a powerful and flexible tool for charge engineering in semiconductor nanostructures. Beyond fundamental insight into surface-induced doping mechanisms, our work opens new possibilities for tailoring carrier properties in quantum wells, with potential applications in spintronics and quantum optoelectronic devices.

Authors: A. Dydniański, M. Raczyński, A. Łopion, T. Kazimierczuk, J. Kasprzak, K.E. Połczyńska, W. Pacuski, P. Kossacki

Solid State Communications 409, 116324

Published 10 January 2026

https://doi.org/10.1016/j.ssc.2026.116324

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14.7.25

Article: Strain-induced speed-up of Mn2+ spin-lattice relaxation in (Cd,Mn)Te/(Cd,Mg)Te quantum wells: A time-resolved optically detected magnetic resonance study

   

Optically detected magnetic resonance (ODMR) is a powerful method for investigating the interactions between localized magnetic spins (such as magnetic ions) and the surrounding carrier gas. In this study, we apply ODMR to a single (Cd,Mn)Te/(Cd,Mg)Te quantum well (QW) containing a hole gas. By simultaneously analyzing ODMR signals associated with both neutral and positively charged excitons, we observe distinct signal characteristics. These differences suggest local fluctuations in carrier gas density, leading to separate populations of Mn²⁺ ions.

Additionally, the shape of the ODMR signal carries information about the effective temperature of the magnetic ions absorbing the microwave radiation. A detailed analysis of this signal provides deeper insights into the interactions between magnetic ions and charge carriers. Within the quantum well, we identify two distinct groups of Mn²⁺ ions, each reaching thermal equilibrium differently due to the presence of the carrier gas.

Authors: Aleksander Bogucki, Aleksandra Łopion, Karolina Ewa Połczyńska, Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik and Piotr Kossacki

Physical Review B 112, 035407

Published 7 July 2025

https://doi.org/10.1103/q3jm-k7z3

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14.7.25

Article: Impact of the hole gas on optically detected magnetic resonance in (Cd,Mn)⁢Te-based quantum wells

Optically detected magnetic resonance (ODMR) is a powerful method for investigating the interactions between localized magnetic spins (such as magnetic ions) and the surrounding carrier gas. In this study, we apply ODMR to a single (Cd,Mn)Te/(Cd,Mg)Te quantum well (QW) containing a hole gas. By simultaneously analyzing ODMR signals associated with both neutral and positively charged excitons, we observe distinct signal characteristics. These differences suggest local fluctuations in carrier gas density, leading to separate populations of Mn²⁺ ions.

Additionally, the shape of the ODMR signal carries information about the effective temperature of the magnetic ions absorbing the microwave radiation. A detailed analysis of this signal provides deeper insights into the interactions between magnetic ions and charge carriers. Within the quantum well, we identify two distinct groups of Mn²⁺ ions, each reaching thermal equilibrium differently due to the presence of the carrier gas.

Authors: Aleksandra Łopion, Aleksander Bogucki, Mateusz Raczyński, Zuzanna Śnioch, Karolina Ewa Połczyńska, Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik and Piotr Kossacki

Physical Review B 111, 035307

Published 28 January 2025

https://doi.org/10.1103/PhysRevB.111.035307

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1.3.25

Article: Single vanadium ion magnetic dopant in an individual CdTe/ZnTe quantum dot

 

The application of quantum technologies such as spintronics, solotronics, and quantum computing is highly promising when it comes to miniaturization in modern technology. In order to achieve effective devices, there is a need to investigate the spin properties of impurity interacting with the semiconductor lattice and confined carriers. Epitaxial quantum dots (QDs), representing zero-dimensional semiconductor structures, emerge as a model system offering a profound exploration of fundamental interactions in condensed matter. For instance, QDs serve as an invaluable tool for scrutinizing the spin characteristics of individual magnetic ions.

Vanadium is a transitional metal with a nuclear spin 7/2 and 3 electrons on the d shell. It exhibits spin 3/2 in V2+ configuration leading to two possible fundamental states with spin projection ±3/2 or ±1/2. Particular spin configuration is expected to depend on the strain of the crystal lattice in a QD.

In this study, we investigate self-assembled CdTe QDs doped with vanadium within a ZnTe barrier, created through molecular beam epitaxy. Our focus involves the observation of a single quantum dot containing a sole vanadium dopant, and the subsequent measurement of its magneto-optical properties. Through numerical modeling based on experimental data, we discern that the crucial phenomenon explaining the main features in the spectrum is the presence of sheer strain within the quantum dot. Ultimately, our findings lead us to the conclusion that vanadium in this context exhibits a spin of ±1/2, thereby rendering our system a realization of a qubit.

Authors: K. E. Połczyńska, T. Kazimierczuk, P. Kossacki and W. Pacuski

Physical Review B 111, 085428 (2025)

https://doi.org/10.1103/PhysRevB.111.085428

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7.1.25

Article: Carrier localization in defected areas of (Cd, Mn)Te quantum well investigated via Optically Detected Magnetic Resonance employed in the microscale

 

The object of examination is the effect of carrier localization on three properties sensitive to carrier gas density at the micrometer scale: the oscillator strength of charged excitons, local free carrier conductivity, and the Knight shift. The latter two are measured through a micrometer-scale, spatially resolved optically detected magnetic resonance (ODMR) experiment. On the surface of MBE-grown (Cd,Mn)Te quantum wells, we identify defected areas near dislocations. These regions exhibit significantly lower conductivity compared to the pristine areas, while the Knight shift values remain relatively unchanged. This behavior is attributed to carrier localization within the defected regions.

Authors: Amadeusz Dydniański, Aleksandra Łopion, Mateusz Raczyński, Tomasz Kazimierczuk, Karolina Ewa Połczyńska, Wojciech Pacuski and Piotr Kossacki

Solid State Communications, Volume 396, 2025, 115755

Published 19 November 2024

https://doi.org/10.1016/j.ssc.2024.115755

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7.1.25

Article: Spin-lattice relaxation of (Cd,Mn)Te co-doped with Co ions

 

The study investigates the effect of cobalt ion addition on the spin-lattice relaxation rate in the (Cd,Mn)Te system. Measurements using time-resolved spectroscopy of single (Cd,Mn,Co)Te/(Cd,Mg)Te quantum wells under a magnetic field reveal that even a small cobalt admixture significantly reduces the Mn relaxation time. The results indicate that cobalt ions, with their faster spin relaxation, serve as centers that enhance the spin relaxation of manganese ions, resulting in a faster and more multi-exponential relaxation process. This suggests that spin-lattice relaxation is primarily limited by the spin diffusion time to cobalt ions.

Authors: Aleksandra Łopion, Kacper Oreszczuk, Aleksander Bogucki, Karolina Ewa Połczyńska,Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik and Piotr Kossacki

Solid State Communications,Volume 396, 2025, 115753,

Published 19 November 2024

https://doi.org/10.1016/j.ssc.2024.115753

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