Ahmet Avsar

@article{ISI:001142010100002,
title = {Highly anisotropic spin transport in ultrathin black phosphorus},
author = {Luke Cording and Jiawei Liu and Jun You Tan and Kenji Watanabe and Takashi Taniguchi and Ahmet Avsar and Barbaros Ozyilmaz},
doi = {10.1038/s41563-023-01779-8},
times_cited = {0},
issn = {1476-1122},
year = {2024},
date = {2024-01-12},
journal = {NATURE MATERIALS},
volume = {23},
number = {4},
publisher = {NATURE PORTFOLIO},
address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY},
abstract = {In anisotropic crystals, the direction-dependent effective mass of carriers can have a profound impact on spin transport dynamics. The puckered crystal structure of black phosphorus leads to direction-dependent charge transport and optical response, suggesting that it is an ideal system for studying anisotropic spin transport. To this end, we fabricate and characterize high-mobility encapsulated ultrathin black-phosphorus-based spin valves in a four-terminal geometry. Our measurements show that in-plane spin lifetimes are strongly gate tunable and exceed one nanosecond. Through high out-of-plane magnetic fields, we observe a fivefold enhancement in the out-of-plane spin signal case compared to in-plane and estimate a colossal spin-lifetime anisotropy of similar to 6. This finding is further confirmed by oblique Hanle measurements. Additionally, we estimate an in-plane spin-lifetime anisotropy ratio of up to 1.8. Our observation of strongly anisotropic spin transport along three orthogonal axes in this pristine material could be exploited to realize directionally tunable spin transport.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000416878100115,
title = {Optospintronics in Graphene \textit{via} Proximity Coupling},
author = {Ahmet Avsar and Dmitrii Unuchek and Jiawei Liu and Oriol Lopez Sanchez and Kenji Watanabe and Takashi Taniguch and Barbaros Ozyilmaz and Andras Kis},
doi = {10.1021/acsnano.7b06800},
times_cited = {0},
issn = {1936-0851},
year = {2017},
date = {2017-11-01},
journal = {ACS NANO},
volume = {11},
number = {11},
pages = {11678-11686},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {The observation of micrometer size spin relaxation makes graphene a promising material for applications in spintronics requiring long-distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphene's intrinsically low spin-orbit coupling strength and optical absorption place an obstacle in their realisation. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of circularly polarized light activates the spin-polarized charge carriers in the WSe2 layer due to its spin-coupled valley-selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 mu m distance, and are finally detected electrically using Co/h-BN contacts in a nonlocal geometry. Polarization-dependent measurements confirm the spin origin of the nonlocal signal. We also demonstrate that such signal is absent if graphene is contacted to bilayer WSe2 where the inversion symmetry is restored.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000411043500027,
title = {van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices},
author = {Ahmet Avsar and Jun Y Tan and Xin Luo and Khoong Hong Khoo and Yuting Yeo and Kenji Watanabe and Takashi Taniguchi and Su Ying Quek and Barbaros Ozyilmaz},
doi = {10.1021/acs.nanolett.7b01817},
times_cited = {0},
issn = {1530-6984},
year = {2017},
date = {2017-09-01},
journal = {NANO LETTERS},
volume = {17},
number = {9},
pages = {5361-5367},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {Because of the chemical inertness of two dimensional (2D) hexagonal-boron nitride (h-BN), few atomic-layer hBN is often used to encapsulate air-sensitive 2D crystals such as black phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping, and contact resistance are not well-known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First-principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (similar to 4.5 k Omega) and are Schottky barrier-free. This allows us to probe high electron mobilities (4,200 cm(2)/(Vs)) and observe insulator metal transitions even under two-terminal measurement geometry.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000409235100023,
title = {Gate-tunable black phosphorus spin valve with nanosecond spin lifetimes},
author = {Ahmet Avsar and Jun Y Tan and Marcin Kurpas and Martin Gmitra and Kenji Watanabe and Takashi Taniguchi and Jaroslav Fabian and Barbaros Ozyilmaz},
doi = {10.1038/NPHYS4141},
times_cited = {0},
issn = {1745-2473},
year = {2017},
date = {2017-09-01},
journal = {NATURE PHYSICS},
volume = {13},
number = {9},
pages = {888-+},
publisher = {NATURE PUBLISHING GROUP},
address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND},
abstract = {Two-dimensional materials offer new opportunities for both fundamental science and technological applications, by exploiting the electron's spin. Although graphene is very promising for spin communication due to its extraordinary electron mobility, the lack of a bandgap restricts its prospects for semiconducting spin devices such as spin diodes and bipolar spin transistors. The recent emergence of two-dimensional semiconductors could help overcome this basic challenge. In this letter we report an important step towards making two-dimensional semiconductor spin devices. We have fabricated a spin valve based on ultrathin (similar to 5 nm) semiconducting black phosphorus (bP), and established fundamental spin properties of this spin channel material, which supports all electrical spin injection, transport, precession and detection up to room temperature. In the non-local spin valve geometry we measure Hanle spin precession and observe spin relaxation times as high as 4 ns, with spin relaxation lengths exceeding 6 mu m. Our experimental results are in a very good agreement with first-principles calculations and demonstrate that the Elliott-Yafet spin relaxation mechanism is dominant. We also show that spin transport in ultrathin bP depends strongly on the charge carrier concentration, and can be manipulated by the electric field effect.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000379759800001,
title = {Electronic spin transport in dual-gated bilayer graphene},
author = {Ahmet Avsar and Ivan Jesus Vera-Marun and Jun You Tan and Gavin Kok Wai Koon and Kenji Watanabe and Takashi Taniguchi and Shaffique Adam and Barbaros Ozyilmaz},
doi = {10.1038/am.2016.65},
times_cited = {0},
issn = {1884-4049},
year = {2016},
date = {2016-06-01},
journal = {NPG ASIA MATERIALS},
volume = {8},
publisher = {NATURE PUBLISHING GROUP},
address = {75 VARICK ST, 9TH FLR, NEW YORK, NY 10013-1917 USA},
abstract = {The elimination of extrinsic sources of spin relaxation is key to realizing the exceptional intrinsic spin transport performance of graphene. Toward this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to make a comparative study by separately investigating the roles of the substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a fivefold enhancement in the spin-relaxation time of the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin-relaxation length of similar to 10 mu m in the encapsulated bilayer, with a charge mobility of 24 000 cm(2) Vs(-1). The carrier density dependence on the spin-relaxation time has two distinct regimes; n<4 x 10(12) cm(-2), where the spin-relaxation time decreases monotonically as the carrier concentration increases, and n >= 4 x 10(12) cm(-2), where the spin-relaxation time exhibits a sudden increase. The sudden increase in the spin-relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of the impedance mismatch within the spin channel.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000368936600012,
title = {Enhanced spin-orbit coupling in dilute fluorinated graphene},
author = {Ahmet Avsar and Jong Hak Lee and Gavin Kok Wai Koon and Barbaros Oezyilmaz},
doi = {10.1088/2053-1583/2/4/044009},
times_cited = {34},
issn = {2053-1583},
year = {2015},
date = {2015-12-01},
journal = {2D MATERIALS},
volume = {2},
number = {4},
publisher = {IOP PUBLISHING LTD},
address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND},
abstract = {The preservation and manipulation of a spin state mainly depends on the strength of the spin-orbit interaction. For pristine graphene, the intrinsic spin-orbit coupling (SOC) is only in the order of few mu eV, which makes it almost impossible to be used as an active element in future electric field controlled spintronics devices. This stimulates the development of a systematic method for extrinsically enhancing the SOC of graphene. In this letter, we study the strength of SOC in weakly fluorinated graphene devices. We observe high non-local signals even without applying any external magnetic field. The magnitude of the signal increases with increasing fluorine adatom coverage. From the length dependence of the non-local transport measurements, we obtain SOC values of similar to 5.1 meV and similar to 9.1 meV for the devices with similar to 0.005% and similar to 0.06% fluorination, respectively. Such a large enhancement, together with the high charge mobility of fluorinated samples (mu similar to 4300 cm(2) V-1 s(-1)-2700 cm(2) V-1 s(-1)), enables the detection of the spin Hall effect even at room temperature.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000353867000079,
title = {Air-Stable Transport in Graphene-Contacted, Fully Encapsulated Ultrathin Black Phosphorus-Based Field-Effect Transistors},
author = {Ahmet Avsar and Ivan J Vera-Marun and Jun You Tan and Kenji Watanabe and Takashi Taniguchi and Antonio Castro H Neto and Barbaros Oezyilmaz},
doi = {10.1021/acsnano.5b00289},
times_cited = {0},
issn = {1936-0851},
year = {2015},
date = {2015-04-01},
journal = {ACS NANO},
volume = {9},
number = {4},
pages = {4138-4145},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {The presence of direct bandgap and high mobility in semiconductor few-layer black phosphorus offers an attractive prospect for using this material in future two-dimensional electronic devices. However, creation of barrier-free contacts which is necessary to achieve high performance in black phosphorus-based devices is challenging and currently limits their potential for applications. Here, we characterize fully encapsulated ultrathin (down to bilayer) black phosphorus field effect transistors fabricated under inert gas conditions by utilizing graphene as source drain electrodes and boron nitride as an encapsulation layer. The observation of a linear I-SD-V-SD behavior with negligible temperature dependence shows that graphene electrodes lead to barrier-free contacts, solving the issue of Schottky barrier limited transport in the technologically relevant two-terminal field-effect-transistor geometry. Such one-atom-thick conformal source-drain electrodes also enable the black phosphorus surface to be sealed, to avoid rapid degradation, with the inert boron nitride encapsulating layer. This architecture, generally applicable for other sensitive two-dimensional crystals, results in air-stable, hysteresis-free transport characteristics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000348086100051,
title = {van der Waals Force: A Dominant Factor for Reactivity of Graphene},
author = {Jong Hak Lee and Ahmet Avsar and Jeil Jung and Jun You Tan and K Watanabe and T Taniguchi and Srinivasan Natarajan and Goki Eda and Shaffique Adam and Antonio Castro H Neto and Barbaros Oezyilmaz},
doi = {10.1021/nl5036012},
times_cited = {7},
issn = {1530-6984},
year = {2015},
date = {2015-01-01},
journal = {NANO LETTERS},
volume = {15},
number = {1},
pages = {319-325},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000342841200001,
title = {Giant spin Hall effect in graphene grown by chemical vapour deposition},
author = {Jayakumar Balakrishnan and Gavin Kok Wai Koon and Ahmet Avsar and Yuda Ho and Jong Hak Lee and Manu Jaiswal and Seung-Jae Baeck and Jong-Hyun Ahn and Aires Ferreira and Miguel A Cazalilla and Antonio Castro H Neto and Barbaros Oezyilmaz},
doi = {10.1038/ncomms5748},
times_cited = {0},
issn = {2041-1723},
year = {2014},
date = {2014-09-01},
journal = {NATURE COMMUNICATIONS},
volume = {5},
publisher = {NATURE PUBLISHING GROUP},
address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND},
abstract = {Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20 meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle similar to 0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000291322600029,
title = {Toward Wafer Scale Fabrication of Graphene Based Spin Valve Devices},
author = {Ahmet Avsar and Tsung-Yeh Yang and Sukang Bae and Jayakumar Balakrishnan and Frank Volmer and Manu Jaiswal and Zheng Yi and Syed Rizwan Ali and Gernot Guentherodt and Byung Hee Hong and Bernd Beschoten and Barbaros Oezyilmaz},
doi = {10.1021/nl200714q},
times_cited = {2},
issn = {1530-6984},
year = {2011},
date = {2011-06-01},
journal = {NANO LETTERS},
volume = {11},
number = {6},
pages = {2363-2368},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}