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Andreev interferometer device. Blue is Al; gold is Au.

False color SEM image of a device to measure quasiparticle dynamics in a superconductor

False color SEM image of an Al/AlOx/Al Josephson junction fabricated by electron-beam lithography

Differential resistance of a 3 terminal superconductor/normal-metal/superconductor (SNS) device as a function of the two independent phase differences between the superconductors.

Tilted false-color SEM image of a 3 terminal SNS Josephson junction with on chip field coils to independently control the phase differences between the superconductors. Blue is Al, gray is Au.

Quartz tuning fork with an attached etched tungsten wire, used as a force transducer in our home-built scanning probe microscope

Topographic (left) and magnetic force microscopy (MFM) images of permalloy bars fabricated by electron-beam lithography

Recent publications

Kevin Michael Ryan

Transport Studies of Superconductors for Quantum Applications PhD Thesis

2025.

Abstract | Links

@phdthesis{Ryan2025,

title = {Transport Studies of Superconductors for Quantum Applications},

author = {Kevin Michael Ryan},

url = {http://www.nano.northwestern.edu/wp-content/uploads/2025/12/KMRyan_Thesis.pdf},

year  = {2025},

date = {2025-12-02},

urldate = {2025-12-02},

abstract = {In the emerging field of quantum technology, superconducting devices serve many roles and are today the most widely adopted basis for quantum computation. This is because ideally behaved superconductors obey macroscopic quantum phenomena which are relatively easy to manipulate.  However, in the laboratory things are almost never ideal, and the superconducting quantum devices which are hoped to someday yield fully fault tolerant quantum computers are in fact, quite faulty. Chief among their most pressing issues are excitations of the superconductor itself out of its many-body ground state, and other stray defects which can arise during construction of these devices. These issues, known colloquially as quasiparticle poisoning and two-level systems respectively, continue to be the subject of considerable study and will require extensive and detailed examinations if we are going to someday obtain truly competitive quantum computers. 



With this in mind, in this thesis we present studies into both quasiparticles and material defects in superconductors. By taking a transport approach to process development, we have explored the phenomenology of both conventionally prepared superconducting films and those produced via emerging techniques, so as to develop metrics for comparing superconducting devices. Via corroboration with material investigations, we have established compelling connections between inhomogeneity and transport properties with may be important in future developments. In parallel, we explore quasiparticle dynamics in the low temperature regime to better establish how these excitations behave collectively and shed new light on their fundamental behaviors. Such non-equilibrium transport studies of superconductors allow for rich, spectroscopic examination of quasiparticle interactions and may ultimately allow us to answer open questions on both the origin and mitigation of quasiparticle poisoning.},

keywords = {},

pubstate = {published},

tppubtype = {phdthesis}

}

In the emerging field of quantum technology, superconducting devices serve many roles and are today the most widely adopted basis for quantum computation. This is because ideally behaved superconductors obey macroscopic quantum phenomena which are relatively easy to manipulate. However, in the laboratory things are almost never ideal, and the superconducting quantum devices which are hoped to someday yield fully fault tolerant quantum computers are in fact, quite faulty. Chief among their most pressing issues are excitations of the superconductor itself out of its many-body ground state, and other stray defects which can arise during construction of these devices. These issues, known colloquially as quasiparticle poisoning and two-level systems respectively, continue to be the subject of considerable study and will require extensive and detailed examinations if we are going to someday obtain truly competitive quantum computers.

With this in mind, in this thesis we present studies into both quasiparticles and material defects in superconductors. By taking a transport approach to process development, we have explored the phenomenology of both conventionally prepared superconducting films and those produced via emerging techniques, so as to develop metrics for comparing superconducting devices. Via corroboration with material investigations, we have established compelling connections between inhomogeneity and transport properties with may be important in future developments. In parallel, we explore quasiparticle dynamics in the low temperature regime to better establish how these excitations behave collectively and shed new light on their fundamental behaviors. Such non-equilibrium transport studies of superconductors allow for rich, spectroscopic examination of quasiparticle interactions and may ultimately allow us to answer open questions on both the origin and mitigation of quasiparticle poisoning.

http://www.nano.northwestern.edu/wp-content/uploads/2025/12/KMRyan_Thesis.pdf

Austin Marga; Venkat Chandrasekhar

Using Crossed Andreev Reflection to Split Electrons Unpublished

2025.

Abstract | Links

@unpublished{Marga2025,

title = {Using Crossed Andreev Reflection to Split Electrons},

author = {Austin Marga and Venkat Chandrasekhar},

url = {https://doi.org/10.48550/arXiv.2510.25641

},

year  = {2025},

date = {2025-10-29},

urldate = {2025-10-29},

abstract = {Mesoscopic systems possess shot noise in their currents due to the quantization of the conducting quasiparticles. Measurements of this shot noise are useful to study phenomena that do not manifest themselves in standard conductance or resistance measurements, such as the statistics of the conducting quasiparticles or quantum entanglement via Bell tests. The corresponding particle statistics can be determined via two particle quantum interference experiments, such as the Hong-Ou-Mandel effect which demonstrates a bunching effect for bosons or an anti-bunching effect in fermions. In superconducting proximity junctions, electrons incident on a superconductor can induce holes via crossed Andreev reflection (CAR) in spatially separated normal metal leads, where the resulting hole currents have nontrivial partition noise due to the four terminal configuration. These nonlocally generated currents, using a superconductor as a mesoscopic beam splitter, enable fabrication of mesoscopic analogs to quantum optics interferometers using metallic and superconducting films with multiport geometries.},

keywords = {},

pubstate = {published},

tppubtype = {unpublished}

}

Mesoscopic systems possess shot noise in their currents due to the quantization of the conducting quasiparticles. Measurements of this shot noise are useful to study phenomena that do not manifest themselves in standard conductance or resistance measurements, such as the statistics of the conducting quasiparticles or quantum entanglement via Bell tests. The corresponding particle statistics can be determined via two particle quantum interference experiments, such as the Hong-Ou-Mandel effect which demonstrates a bunching effect for bosons or an anti-bunching effect in fermions. In superconducting proximity junctions, electrons incident on a superconductor can induce holes via crossed Andreev reflection (CAR) in spatially separated normal metal leads, where the resulting hole currents have nontrivial partition noise due to the four terminal configuration. These nonlocally generated currents, using a superconductor as a mesoscopic beam splitter, enable fabrication of mesoscopic analogs to quantum optics interferometers using metallic and superconducting films with multiport geometries.

https://doi.org/10.48550/arXiv.2510.25641

Venkat Chandrasekhar

Tuning the fundamental periodicity of the current-phase relation in multiterminal diffusive Josephson junctions Journal Article

In: Physical Review B, vol. 112, pp. 104508, 2025.

Abstract | Links

@article{Chandrasekhar2025,

title = {Tuning the fundamental periodicity of the current-phase relation in multiterminal diffusive Josephson junctions},

author = {Venkat Chandrasekhar},

url = {https://journals.aps.org/prb/abstract/10.1103/73rl-wmzw},

year  = {2025},

date = {2025-09-18},

urldate = {2025-09-18},

journal = {Physical Review B},

volume = {112},

pages = {104508},

abstract = {Conventional superconductor/insulator/superconductor (SIS) Josephson junctions, devices where two superconductors are separated by a tunnel barrier are technologically important as elements in quantum circuits, particularly with their key role in superconducting qubits. An important characteristic of Josephson junctions is the relation between the supercurrent Is and the phase difference ϕ between them. For SIS junctions, the current-phase relation is sinusioidal and 2π periodic. Other types of Josephson junctions, where the material between the superconductors is a weak link or a normal metal (N) may have non-sinusoidal current-phase relations that are still 2π periodic. We show here that a multi-terminal diffusive SNS Josephson junction with 4 superconducting contacts can show a current phase relation between two of the contacts that is a superposition of 2π and 4π periodic components whose relative strength is controlled by the phase difference between the other two contacts, becoming 2π or 4π periodic for certain values of this phase difference. This tunability might have applications in tailoring the Hamiltonians of superconducting quantum circuits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Conventional superconductor/insulator/superconductor (SIS) Josephson junctions, devices where two superconductors are separated by a tunnel barrier are technologically important as elements in quantum circuits, particularly with their key role in superconducting qubits. An important characteristic of Josephson junctions is the relation between the supercurrent Is and the phase difference ϕ between them. For SIS junctions, the current-phase relation is sinusioidal and 2π periodic. Other types of Josephson junctions, where the material between the superconductors is a weak link or a normal metal (N) may have non-sinusoidal current-phase relations that are still 2π periodic. We show here that a multi-terminal diffusive SNS Josephson junction with 4 superconducting contacts can show a current phase relation between two of the contacts that is a superposition of 2π and 4π periodic components whose relative strength is controlled by the phase difference between the other two contacts, becoming 2π or 4π periodic for certain values of this phase difference. This tunability might have applications in tailoring the Hamiltonians of superconducting quantum circuits.

https://journals.aps.org/prb/abstract/10.1103/73rl-wmzw

Kevin M. Ryan; Venkat Chandrasekhar

Enhanced Quasiparticle Relaxation in a Superconductor via the Proximity Effect Journal Article

In: Applied Physics Letters, vol. 127, pp. 072603, 2025.

Abstract | Links

@article{Ryan2024,

title = {Enhanced Quasiparticle Relaxation in a Superconductor via the Proximity Effect},

author = {Kevin M. Ryan and Venkat Chandrasekhar},

url = {https://doi.org/10.1063/5.0268707

https://arxiv.org/abs/2409.05233v1},

doi = {https://doi.org/10.1063/5.0268707},

year  = {2025},

date = {2025-08-19},

urldate = {2025-08-19},

journal = {Applied Physics Letters},

volume = {127},

pages = {072603},

abstract = {Quasiparticle dynamics have been identified as an important factor in the operational performance of superconductors in quantum computing and sensing applications. In order to study such dynamics, we performed measurements of quasiparticle transport in a superconductor engineered to enhance quasiparticle relaxation. We found that a thin, highly disordered normal metal layer can be used to greatly reduce the relaxation time of quasiparticles in a superconductor, as seen by a large reduction in the quasiparticle charge imbalance in a fully proximitized Cu/Al bilayer wire, without significantly affecting the properties of the superconductor. The enhanced relaxation is likely due to an increased electron–electron scattering rate in the disordered normal metal. Our results provide a positive indication that quasiparticle relaxation can be substantially enhanced via a simple and technologically viable method, and demonstrate that direct measurement of quasiparticle imbalance can be a useful tool for the assessment and engineering of superconductors in various applications at mK temperatures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Quasiparticle dynamics have been identified as an important factor in the operational performance of superconductors in quantum computing and sensing applications. In order to study such dynamics, we performed measurements of quasiparticle transport in a superconductor engineered to enhance quasiparticle relaxation. We found that a thin, highly disordered normal metal layer can be used to greatly reduce the relaxation time of quasiparticles in a superconductor, as seen by a large reduction in the quasiparticle charge imbalance in a fully proximitized Cu/Al bilayer wire, without significantly affecting the properties of the superconductor. The enhanced relaxation is likely due to an increased electron–electron scattering rate in the disordered normal metal. Our results provide a positive indication that quasiparticle relaxation can be substantially enhanced via a simple and technologically viable method, and demonstrate that direct measurement of quasiparticle imbalance can be a useful tool for the assessment and engineering of superconductors in various applications at mK temperatures.

https://doi.org/10.1063/5.0268707
https://arxiv.org/abs/2409.05233v1
doi:https://doi.org/10.1063/5.0268707

Maxwell Wisne; Venkat Chandrasekhar

Supercurrent tuning of the Josephson coupling energy Unpublished

2025.

Abstract | Links

@unpublished{Wisne2025,

title = {Supercurrent tuning of the Josephson coupling energy},

author = {Maxwell Wisne and Venkat Chandrasekhar},

url = {https://doi.org/10.48550/arXiv.2507.14357

},

year  = {2025},

date = {2025-07-18},

abstract = {The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.},

keywords = {},

pubstate = {published},

tppubtype = {unpublished}

}

The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.

https://doi.org/10.48550/arXiv.2507.14357

Akshay A. Murthy; Mustafa Bal; Michael J. Bedzyk; Hilal Cansizoglu; Randall K. Chan; Venkat Chandrasekhar; Francesco Crisa; Amlan Datta; Yanpei Deng; Celeo D. Matute Diaz; Vinayak P. Dravid; David A. Garcia-Wetten; Sabrina Garattoni; Sunil Ghimire; Dominic P. Goronzy; Sebastian Graaf; Sam Haeuser; Mark C. Hersam; Peter Hopkins; Dieter Isheim; Kamal Joshi; Richard Kim; Saagar Kolachina; Cameron J. Kopas; Matthew J. Kramer; Ella O. Lachman; Jaeyel Lee; Peter G. Lim; Andrei Lunin; William Mah; Jayss Marshall; Josh Y. Mutus; Jin-Su Oh; David Olaya; David P. Pappas; Joong-mok Park; Ruslan Prozorov; Roberto Reis; David N. Seidman; Zuhawn Sung; Makariy Tanatar; Mitchell J. Walker; Maxwell Wisne; Jigang Wang; Haotian Wu; Lin Zhou; Shaojiang Zhu; Anna Grassellino; Alexander Romanenko

Identifying Materials-Level Sources of Performance Variation in Superconducting Transmon Qubits Unpublished

2025.

@unpublished{murthy2025identifyingmaterialslevelsourcesperformance,

title = {Identifying Materials-Level Sources of Performance Variation in Superconducting Transmon Qubits},

author = {Akshay A. Murthy and Mustafa Bal and Michael J. Bedzyk and Hilal Cansizoglu and Randall K. Chan and Venkat Chandrasekhar and Francesco Crisa and Amlan Datta and Yanpei Deng and Celeo D. Matute Diaz and Vinayak P. Dravid and David A. Garcia-Wetten and Sabrina Garattoni and Sunil Ghimire and Dominic P. Goronzy and Sebastian Graaf and Sam Haeuser and Mark C. Hersam and Peter Hopkins and Dieter Isheim and Kamal Joshi and Richard Kim and Saagar Kolachina and Cameron J. Kopas and Matthew J. Kramer and Ella O. Lachman and Jaeyel Lee and Peter G. Lim and Andrei Lunin and William Mah and Jayss Marshall and Josh Y. Mutus and Jin-Su Oh and David Olaya and David P. Pappas and Joong-mok Park and Ruslan Prozorov and Roberto Reis and David N. Seidman and Zuhawn Sung and Makariy Tanatar and Mitchell J. Walker and Maxwell Wisne and Jigang Wang and Haotian Wu and Lin Zhou and Shaojiang Zhu and Anna Grassellino and Alexander Romanenko},

url = {https://arxiv.org/abs/2503.14424},

year  = {2025},

date = {2025-01-01},

urldate = {2025-01-01},

keywords = {},

pubstate = {published},

tppubtype = {unpublished}

}

https://arxiv.org/abs/2503.14424

Maxwell Wisne; Yanpei Deng; Hilal Cansizoglu; Cameron Kopas; Josh Mutus; Venkat Chandrasekhar

Transport signatures of phase fluctuations in superconducting qubits Journal Article

In: Materials for Quantum Technology, vol. 4, pp. 046001, 2024.

Abstract | Links

@article{Wisne2024,

title = {Transport signatures of phase fluctuations in superconducting qubits},

author = {Maxwell Wisne and Yanpei Deng and Hilal Cansizoglu and Cameron Kopas and Josh Mutus and Venkat Chandrasekhar},

url = {https://iopscience.iop.org/article/10.1088/2633-4356/ad9a47},

doi = {10.1088/2633-4356/ad9a47},

year  = {2024},

date = {2024-12-13},

urldate = {2024-05-30},

journal = {Materials for Quantum Technology},

volume = {4},

pages = {046001},

abstract = {Josephson junctions supply the nonlinear inductance element in superconducting qubits. In the widely used transmon configuration, where the junction is shunted by a large capacitor, the low charging energy minimizes the sensitivity of the qubit to charge noise while maintaining the necessary anharmonicity to qubit states. We report here low-frequency transport measurements on small standalone junctions and identically fabricated capacitively-shunted junctions that show two distinct features normally attributed to small capacitance junctions near zero bias: reduced switching currents and prominent finite resistance associated with phase diffusion in the current–voltage characteristic. Our transport data reveals the existence of phase fluctuations in transmons arising from intrinsic junction capacitance.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Josephson junctions supply the nonlinear inductance element in superconducting qubits. In the widely used transmon configuration, where the junction is shunted by a large capacitor, the low charging energy minimizes the sensitivity of the qubit to charge noise while maintaining the necessary anharmonicity to qubit states. We report here low-frequency transport measurements on small standalone junctions and identically fabricated capacitively-shunted junctions that show two distinct features normally attributed to small capacitance junctions near zero bias: reduced switching currents and prominent finite resistance associated with phase diffusion in the current–voltage characteristic. Our transport data reveals the existence of phase fluctuations in transmons arising from intrinsic junction capacitance.

https://iopscience.iop.org/article/10.1088/2633-4356/ad9a47
doi:10.1088/2633-4356/ad9a47

Maxwell Wisne; Yanpei Deng; Markus Lilja; Pertti Hakonen; Venkat Chandrasekhar

Mapping the topological proximity-induced gap of multiterminal Josephson junctions Journal Article

In: Physical Review Letters, vol. 133, pp. 246601, 2024.

Abstract | Links

@article{Wisne2024b,

title = {Mapping the topological proximity-induced gap of multiterminal Josephson junctions},

author = {Maxwell Wisne and Yanpei Deng and Markus Lilja and Pertti Hakonen and Venkat Chandrasekhar},

url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.246601},

doi = {10.1103/PhysRevLett.133.246601},

year  = {2024},

date = {2024-12-09},

urldate = {2024-08-16},

journal = {Physical Review Letters},

volume = {133},

pages = {246601},

abstract = {Multiterminal Josephson junctions (MTJJs), devices in which a normal metal is in contact with three or more superconducting leads, have been proposed as artificial analogs of topological crystals. The topological nature of MTJJs manifests as a modulation of the quasiparticle density of states (DOS) in the normal metal that may be probed by tunneling measurements. We show that one can reveal this modulation by measuring the resistance of diffusive MTJJs with normal contacts, which shows rich structure as a function of the phase differences φi. Our approach demonstrates a simple yet powerful technique for exploring topological effects in MTJJs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Multiterminal Josephson junctions (MTJJs), devices in which a normal metal is in contact with three or more superconducting leads, have been proposed as artificial analogs of topological crystals. The topological nature of MTJJs manifests as a modulation of the quasiparticle density of states (DOS) in the normal metal that may be probed by tunneling measurements. We show that one can reveal this modulation by measuring the resistance of diffusive MTJJs with normal contacts, which shows rich structure as a function of the phase differences φi. Our approach demonstrates a simple yet powerful technique for exploring topological effects in MTJJs.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.246601
doi:10.1103/PhysRevLett.133.246601

Patrick W. Krantz; Alexander Tyner; Pallab Goswami; Venkat Chandrasekhar

Nonlinear Hall Effect in KTaO3 Two-Dimensional Electron Gases Unpublished

2024.

Abstract | Links

@unpublished{Krantz2024b,

title = {Nonlinear Hall Effect in KTaO3 Two-Dimensional Electron Gases},

author = {Patrick W. Krantz and Alexander Tyner and Pallab Goswami and Venkat Chandrasekhar},

url = {https://arxiv.org/abs/2411.09161},

year  = {2024},

date = {2024-11-14},

abstract = {The observation of a Hall effect, a finite transverse voltage induced by a longitudinal current, usually requires the breaking of time-reversal symmetry, for example through the application of an external magnetic field or the presence of long range magnetic order in a sample. Recently it was suggested that under certain symmetry conditions, the presence of finite Berry curvatures in the band structure of a system with time-reversal symmetry but without inversion symmetry can give rise to a nonlinear Hall effect in the presence of a probe current. In order to observe the nonlinear Hall effect, one requires a finite component of a so-called Berry dipole along the direction of the probe current. We report here measurements of the nonlinear Hall effect in two-dimensional electron gases fabricated on the surface of KTaO3 with different surface crystal orientations as a function of the probe current, a transverse electric field and back gate voltage. For all three crystal orientations, the transverse electric field modifies the nonlinear Hall effect. We discuss our results in the context of the current understanding of the nonlinear Hall effect as well as potential experimental artifacts that may give rise to the same effects.},

keywords = {},

pubstate = {published},

tppubtype = {unpublished}

}

The observation of a Hall effect, a finite transverse voltage induced by a longitudinal current, usually requires the breaking of time-reversal symmetry, for example through the application of an external magnetic field or the presence of long range magnetic order in a sample. Recently it was suggested that under certain symmetry conditions, the presence of finite Berry curvatures in the band structure of a system with time-reversal symmetry but without inversion symmetry can give rise to a nonlinear Hall effect in the presence of a probe current. In order to observe the nonlinear Hall effect, one requires a finite component of a so-called Berry dipole along the direction of the probe current. We report here measurements of the nonlinear Hall effect in two-dimensional electron gases fabricated on the surface of KTaO3 with different surface crystal orientations as a function of the probe current, a transverse electric field and back gate voltage. For all three crystal orientations, the transverse electric field modifies the nonlinear Hall effect. We discuss our results in the context of the current understanding of the nonlinear Hall effect as well as potential experimental artifacts that may give rise to the same effects.

https://arxiv.org/abs/2411.09161

Venkat Chandrasekhar

Current dependence of the low bias resistance of small capacitance Josephson junctions Journal Article

In: Physics Letters A, vol. 507, pp. 129493, 2024.

Abstract | Links

@article{Chandrasekhar2024,

title = {Current dependence of the low bias resistance of small capacitance Josephson junctions},

author = {Venkat Chandrasekhar},

url = {https://doi.org/10.1016/j.physleta.2024.129493},

year  = {2024},

date = {2024-04-10},

journal = {Physics Letters A},

volume = {507},

pages = {129493},

abstract = {The dc current-voltage characteristics of small Josephson junctions reveal features that are not observed in larger

junctions, in particular, a switch to the finite voltage state at current values much less than the expected critical current

of the junction and a finite resistance in the nominally superconducting regime. Both phenomena are due to the increased

sensitivity to noise associated with the small capacitance of the Josephson junction and have been extensively studied

a few decades ago. Here I focus on the current bias dependence of differential resistance of the junction at low current

bias in the nominally superconducting regime, using a quantum Langevin equation approach that enables a physically

transparent incorporation of the noise environment of the junction. A similar approach might be useful in modeling the

sensitivity of superconducting qubits to noise in the microwave regime.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

The dc current-voltage characteristics of small Josephson junctions reveal features that are not observed in larger
junctions, in particular, a switch to the finite voltage state at current values much less than the expected critical current
of the junction and a finite resistance in the nominally superconducting regime. Both phenomena are due to the increased
sensitivity to noise associated with the small capacitance of the Josephson junction and have been extensively studied
a few decades ago. Here I focus on the current bias dependence of differential resistance of the junction at low current
bias in the nominally superconducting regime, using a quantum Langevin equation approach that enables a physically
transparent incorporation of the noise environment of the junction. A similar approach might be useful in modeling the
sensitivity of superconducting qubits to noise in the microwave regime.

https://doi.org/10.1016/j.physleta.2024.129493

Patrick W Krantz; Alexander Tyner; Pallab Goswami; Venkat Chandrasekhar

Intrinsic magnetism in KTaO3 heterostructures Journal Article

In: Applied Physics Letters, vol. 124, pp. 093102, 2024.

Abstract | Links

@article{Krantz2024,

title = {Intrinsic magnetism in KTaO3 heterostructures},

author = {Patrick W Krantz and Alexander Tyner and Pallab Goswami and Venkat Chandrasekhar},

url = {https://pubs.aip.org/aip/apl/article/124/9/093102/3267521/Intrinsic-magnetism-in-KTaO3-heterostructures},

doi = {doi: 10.1063/5.0189956},

year  = {2024},

date = {2024-02-27},

urldate = {2024-02-27},

journal = {Applied Physics Letters},

volume = {124},

pages = {093102},

abstract = {There has been intense recent interest in the two-dimensional electron gases (2DEGs) that form at the surfaces and interfaces of KTaO 3

(KTO), with the discovery of superconductivity at temperatures significantly higher than those of similar 2DEGs based on SrTiO 3 (STO).

Like STO heterostructures, these KTO 2DEGs are formed by depositing an overlayer on top of appropriately prepared KTO surfaces. Some

of these overlayers are magnetic, and the resulting 2DEGs show signatures of this magnetism, including hysteresis in the magnetoresistance

(MR). Here, we show that KTO 2DEGs fabricated by depositing AlOx on top of KTO also show hysteretic MR, indicative of long-range mag-

netic order, even though the samples nominally contain no intrinsic magnetic elements. The hysteresis appears in both the transverse and

longitudinal resistance in magnetic fields both perpendicular to and in the plane of the 2DEG. The hysteretic MR has different characteristic

fields and shapes for surfaces of different crystal orientations and vanishes above a few Kelvin. Density functional theory (DFT) calculations

indicate that the magnetism likely arises from Ta 4+ local moments created in the presence of oxygen vacancies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

There has been intense recent interest in the two-dimensional electron gases (2DEGs) that form at the surfaces and interfaces of KTaO 3
(KTO), with the discovery of superconductivity at temperatures significantly higher than those of similar 2DEGs based on SrTiO 3 (STO).
Like STO heterostructures, these KTO 2DEGs are formed by depositing an overlayer on top of appropriately prepared KTO surfaces. Some
of these overlayers are magnetic, and the resulting 2DEGs show signatures of this magnetism, including hysteresis in the magnetoresistance
(MR). Here, we show that KTO 2DEGs fabricated by depositing AlOx on top of KTO also show hysteretic MR, indicative of long-range mag-
netic order, even though the samples nominally contain no intrinsic magnetic elements. The hysteresis appears in both the transverse and
longitudinal resistance in magnetic fields both perpendicular to and in the plane of the 2DEG. The hysteretic MR has different characteristic
fields and shapes for surfaces of different crystal orientations and vanishes above a few Kelvin. Density functional theory (DFT) calculations
indicate that the magnetism likely arises from Ta 4+ local moments created in the presence of oxygen vacancies.

https://pubs.aip.org/aip/apl/article/124/9/093102/3267521/Intrinsic-magnetism-in[...]
doi:doi: 10.1063/5.0189956

Department of Physics and Astronomy &
Graduate Program in Applied Physics
Northwestern University
2145 Sheridan Road
Evanston, IL 60208 USA

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