Browsing by Department "Department of Electronics and Nanoengineering"

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A 0.35-to-2.6GHz multilevel outphasing transmitter with a digital interpolating phase modulator enabling up to 400MHz instantaneous bandwidth
(2017-03-02) Kosunen, Marko; Lemberg, Jerry; Martelius, Mikko; Roverato, Enrico; Nieminen, Tero; Englund, Mikko; Stadius, Kari; Anttila, Lauri; Pallonen, Jorma; Valkama, Mikko; Ryynänen, Jussi
A4 Artikkeli konferenssijulkaisussa
Advanced wireless radio standards set stringent requirements on the bandwidth, frequency range and reconfigurability of base-station transmitters. Recently, the outphasing concept has shown promise of wide bandwidth while taking advantage of process scaling with extensive exploitation of rail-to-rail signaling. Recent outphasing transmitter designs have often focused on power-amplifier (PA) and power-combiner implementations while omitting the phase modulator [1,2]. Moreover, previously reported transmitters with integrated digital phase modulators have only shown bandwidths up to 40MHz [3,4], although 133MHz has been demonstrated at 10GHz carrier frequency utilizing phase modulators based on conventional IQ-DACs [5]. Thus, digital-intensive outphasing transmitters capable of modulation with hundreds of MHz bandwidth at existing cellular frequency bands have not yet been published. To address the aforementioned challenge, this paper introduces a multilevel outphasing transmitter with four amplitude levels, including the first prototype implementation based on the digital interpolating phase modulator concept [6]. The transmitter is targeted for 5G picocell base stations and has been verified to operate with instantaneous bandwidth up to 400MHz. In addition, the developed phase modulator eliminates the need for complex on-chip frequency synthesizers by introducing digital carrier frequency generation, demonstrated between 0.35 and 2.6GHz, while utilizing a single 1.8GHz reference clock.
A 0.39–3.56-μW Wide-Dynamic-Range Universal Multi-Sensor Interface Circuit
(2020-10-15) Moayer, Mohammad Mehdi; Salomaa, Jarno; Halonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
This paper presents an ultra-low-power, widedynamic- range interface circuit for capacitive and resistive sensors. The circuit is implemented as a switched-capacitor circuit using programmable capacitors to achieve high configurability. The circuit was fabricated using a CMOS 0.18 μm process. Different types of capacitive and resistive sensors were measured using the interface to demonstrate its support for multi-sensor systems with an ultra-low-power budget. Experimental results show that the circuit is able to interface various sensors within the overall capacitance range of 0.6–550 pF and resistance range of 3.7–5100 k , while consuming only 0.39–3.56 μW from a 1.2 V supply. A proximity, gesture, and touch-sensing system is also developed consisting of the designed interface circuit and a sensor element that is able to detect the displacement of an object up to 15 cm from the sensing electrodes consuming only 0.83 μW from a 1.2 V supply.
A 0.6–4.0 GHz RF-Resampling Beamforming Receiver with Frequency-Scaling True-Time-Delays up to Three Carrier Cycles
(2020) Spoof, Kalle; Zahra, Mahwish; Unnikrishnan, Vishnu; Stadius, Kari; Kosunen, Marko; Ryynänen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
True-time-delays (TTDs) enable wideband analog and hybrid beamforming by mitigating the beam squint problem. This letter reports a TTD beamforming receiver supporting delays up to three carrier-frequency cycles. The implementation is the first published work in which the delays scale with the carrier frequency. The scaling enables TTDs for large arrays at low-GHz frequencies where long delays are required due to \lambda _{c}/2 antenna spacing. The delays are implemented through delayed resampling of a passive mixer's discrete-time output. Driving the mixers with pulse-skipped local oscillator (LO) signals allows the delay range to exceed one carrier cycle. A polyphase receiver structure prevents aliasing of noise and unwanted tones caused by LO pulse-skipping. Our prototype implementation demonstrates squint-free beamforming for an-800 MHz instantaneous RF bandwidth. The proposed TTD is efficient for large arrays since the power consumption per antenna is only 5-13-mW across the 0.6-4.0-GHz frequency range. The prototype was implemented in 28-nm FD-SOI CMOS, and the die area including bonding pads is only 1.2 mm2.
A 0.9-Nyquist-Band Digital Timing Mismatch Correction for Time-Interleaved ADCs Achieving Delay Tuning Range of 0.12-Sample-Period
(2022) Kempi, Ilia; Jarvinen, Okko; Kosunen, Marko; Unnikrishnan, Vishnu; Stadius, Kari; Ryynanen, Jussi
A4 Artikkeli konferenssijulkaisussa
Time-interleaved analog-to-digital converters (TIADC) require channel matching in terms of offset, gain, and sampling clock skew to achieve best data conversion performance. Conventionally, correction of skew mismatch is realized with analog delay lines, making it challenging for high-speed ADC designs to achieve fine delay resolution over wide tuning range while maintaining low clock jitter. Digital skew correction allows greater flexibility than analog solutions, but is hindered by a significant hardware footprint. This paper demonstrates digital filter-based timing skew correction approach suitable for on-chip implementation. In a 10-bit 8-channel TI-ADC the proposed structure corrects mismatch magnitudes up to 0.12 sample period across 0.9 Nyquist band while requiring only 65% hardware of similar architectures of equivalent performance. The presented digital circuit uses reduced combinational paths and operates at a clock rate of single ADC channel, making it applicable for digitally-assisted high-speed TI-ADCs.
A 1.5-1.9-GHz all-digital tri-phasing transmitter with an integrated multilevel class-D power amplifier achieving 100-MHz RF bandwidth
(2019-06-01) Lemberg, Jerry; Martelius, Mikko; Roverato, Enrico; Antonov, Yury; Nieminen, Tero; Stadius, Kari; Anttila, Lauri; Valkama, Mikko; Kosunen, Marko; Ryynänen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We present a prototype RF transmitter with an integrated multilevel class-D power amplifier (PA), implemented in 28-nm CMOS. The transmitter utilizes tri-phasing modulation, which combines three constant-envelope phase-modulated signals with coarse amplitude modulation in the PA. This new architecture achieves the back-off efficiency of multilevel outphasing, without linearity-degrading discontinuities in the RF output waveform. Because all signal processing is performed in the time domain up to the PA, the entire system is implemented with digital circuits and structures, thus also enabling the use of synthesis and place-and-route CAD tools for the RF front end. The effectiveness of the digital tri-phasing concept is supported by extensive measurement results. Improved wideband performance is validated through the transmission of orthogonal frequency-division multiplexing (OFDM) bandwidths up to 100 MHz. Enhanced reconfigurability is demonstrated with non-contiguous carrier aggregation and digital carrier generation between 1.5 and 1.9 GHz without a frequency synthesizer. For a 20-MHz 256-QAM OFDM signal at 3.5% error vector magnitude (EVM), the transmitter achieves 22.6-dBm output power and 14.6% PA efficiency. Thanks to the high linearity enabled by tri-phasing, no digital predistortion is needed for the PA.
A 1.5-5-GHz Integrated RF Transmitter Front End for Active Matching of an Antenna Cluster
(2020-11) Saleem, Ali; Stadius, Kari; Hannula, Jari-Matti; Lehtovuori, Anu; Kosunen, Marko; Viikari, Ville; Ryynänen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A recently proposed method for realizing frequency-reconfigurable antennas across a wideband is based on adjusting the feed amplitudes and phases of a multiport antenna. In this article, we demonstrate the feasibility of the method, for the first time, with a conjunction of an integrated RF transmitter and a four-element antenna cluster. The implementation performs on-chip amplitude and phase tuning with supply scaling and delay tuning circuits to tune the antenna cluster without requirement of matching network. The antenna cluster is built with four closely spaced antenna elements implemented on a printed circuit board. The transmitter integrated circuit (IC) is implemented in a 28-nm CMOS process with the chip size of 0.85 mm x 0.95 mm, including pads. The proof-of-concept implementation demonstrates tunability across a wideband from 1.5 to 5 GHz.
A 100–750 MS/s 11-Bit Time-to-Digital Converter With Cyclic-Coupled Ring Oscillator
(2021-03-24) Jarvinen, Okko; Unnikrishnan, Vishnu; Siddiqui, Waqas; Korhonen, Teuvo; Koli, Kimmo; Stadius, Kari; Kosunen, Marko; Ryynanen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
This paper presents the first measured cyclic-coupled ring oscillator (CCRO) time-to-digital converter (TDC). The CCRO realizes a robust true time-domain delay interpolation with sub-gate-delay resolution. The architecture employs real-time quantization to reduce conversion time and hence maximize bandwidth. Furthermore, the CCRO phase progression is encoded with a bubble error suppression logic, thereby building resilience to delay mismatches from circuit/layout imperfections. The prototype circuit implemented in a 28 nm CMOS process demonstrates a combination of high resolution and high sample rate over wide range of sample rates. The TDC achieves its peak figure-of-merit (FoM) of 0.051 pJ/conv.-step at 100 MS/s while delivering 8.38-bit linear resolution and 15.4 ps time resolution, operating from a 0.55 V supply. The TDC demonstrates the highest reported linear resolution of 9.29 bits among converters operating above 100 MS/s, at 125 MS/s and 0.9 V supply, while achieving 4.4 ps time resolution and 0.16 pJ/conv.-step FoM. Further, the real-time quantizing architecture allows fast operation up to 750 MS/s, where the TDC delivers 6-bit linear resolution and 0.48 pJ/conv.-step FoM operating from 0.9 V supply.
172 fs, 24.3 kW peak power pulse generation from a Ho-doped fiber laser system
(2018-10-01) Wang, Mengmeng; Zhang, Hui; Wei, Rongling; Zhu, Zexiu; Ruan, Shuangchen; Yan, Peiguang; Wang, Jinzhang; Hasan, Tawfique; Sun, Zhipei
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We demonstrate a high-peak-power femtosecond fiber laser system based on single-mode holmium (Ho)-doped fibers. 833 fs, 27.7 MHz pulses at 2083.4 nm generated in a passively mode-locked Ho fiber laser are amplified and compressed to near transform-limited 172 fs, 7.2 nJ pulses with 24.3 kW peak power. We achieve this performance level by using the soliton effect and high-order soliton compression. To the best of our knowledge, this is the first demonstration of sub-200 fs pulses, with peak power exceeding 10 kW from a Ho-doped single-mode fiber laser system without using bulk optics compressors.
An 18–28 GHz dual-mode down-converter IC for 5G applications
(2024-02) Naghavi, Saeed; Ryynänen, Kaisa; Zahra, Mahwish; Korsman, Aleksi; Stadius, Kari; Kosunen, Marko; Unnikrishnan, Vishnu; Anttila, Lauri; Valkama, Mikko; Ryynänen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Emerging spectrum trends require a higher integration of 5G New Radio Frequency Range 1 (FR1) and Frequency Range 2 (FR2) bands to enhance the availability of spectrum and spectrum-sharing opportunities. To enable the reception of both FR1 and FR2 bands in a seamless hardware entity, we propose combining homodyne and heterodyne architectures. This necessitates the incorporation of a down-converter module that transfers the incoming signals from FR2 bands down to FR1, ensuring compatibility with an FR1 direct-conversion receiver (DCR) for the final signal reception. The primary focus of this paper is the design and implementation of the required integrated down-converter. The module includes an integrated balun, a low-noise amplifier (LNA) with a bypass mode, a dual-mode mixer, and an intermediate frequency (IF) amplifier. The introduced bypass mode helps to further elevate the linearity performance compared to the nominal mode. The bypass mode is designed for joint communication and sensing operation to avoid the compression of the receiver. This work also incorporates a local oscillator (LO) signal distribution network with phase tuning elements using a mixed-signal approach. The circuit is implemented in a 22-nm CMOS process, and the active die area is 0.6 mm 2 . The measurements demonstrate that the implemented chip can efficiently perform the required frequency conversion over a wide frequency range of 18–28 GHz. Conversion gain of 4.5–7.5 dB, noise figure of 15–19.7 dB, 1 dB compression point (IP1dB) of − 16 to − 10 dBm, and input third-order intercept point (IIP3) of − 5 to 0 dBm are achieved. The measured IP1 dB and IIP3 for the bypass mode are +0.5 to +4.5 dBm and +8.5 to +10 dBm, respectively.
A 180-nW static power UWB IR transmitter front-end for energy harvesting applications
(2017-09-28) Haapala, Tuomas; Pulkkinen, Mika; Salomaa, Jarno; Halonen, Kari
A4 Artikkeli konferenssijulkaisussa
This paper presents a versatile, FCC compliant ultra-wideband impulse radio transmitter front-end (TFE) that performs well at a wide range of pulse repetition rates up to 105 MHz. The TFE delivers 2.2 pJ pulses with 6.7 % efficiency at 3.8 GHz center frequency. The leakage power is 180 nW from a 1.2 V supply. The TFE operates robustly with a variety of power sources, including a 6.5 cm2 photovoltaic array in office illumination. Along with the low static power consumption level, this feature makes the TFE suitable for energy harvesting applications. The TFE is fabricated in a 180 nm CMOS process.
180° rotations in the polarization angle for blazars
(2020-04-21) Cohen, M.H.; Savolainen, Tuomas
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Rotations of the electric vector position angle (EVPA) in blazars are often close to an integral multiple of 180°. There are many examples of this in the literature, and we strengthen the evidence by showing that, in the RoboPol monitoring program, nπ rotations occur more frequently than otherwise expected by chance. We explain this using a model consisting of two polarized emission components: a “jet” that is constant in time and a “burst” that is variable. The EVPA of the combination is EVPAjet at both the beginning and the end of the burst, so the net rotation across the burst must be nπ. Several examples of this model are analyzed on the Stokes plane, where the winding number for the Stokes vector of the combination gives the value of n. The main conclusion is that the EVPA rotation can be much larger than the physical rotation of the emission region around the axis of the jet, but this requires the EVPAs of the jet and the burst to be nearly orthogonal. Shock-in-jet calculations can provide a physical model for our toy model and in addition they automatically give the required orthogonality. The model is illustrated with data from the literature on OJ 287. We suggest that the large rapid EVPA rotation seen in OJ 287 might be a phase effect and not representative of a physical rotation.
1D Crystallographic Etching of Few-Layer WS2
(2024-11-12) Li, Shisheng; Lin, Yung Chang; Chiew, Yiling; Dai, Yunyun; Ning, Zixuan; Zhang, Yaming; Nakajima, Hideaki; Lim, Hong En; Wu, Jing; Neitoh, Yasuhisa; Okazaki, Toshiya; Sun, Yang; Sun, Zhipei; Suenaga, Kazu; Sakuma, Yoshiki; Tsukagoshi, Kazuhito; Taniguchi, Takaaki
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Layer number-dependent band structures and symmetry are vital for the electrical and optical characteristics of 2D transition metal dichalcogenides (TMDCs). Harvesting 2D TMDCs with tunable thickness and properties can be achieved through top-down etching and bottom-up growth strategies. In this study, a pioneering technique that utilizes the migration of in situ generated Na-W-S-O droplets to etch out 1D nanotrenches in few-layer WS2 is reported. 1D WS2 nanotrenches are successfully fabricated on the optically inert bilayer WS2, showing pronounced photoluminescence and second harmonic generation signals. Additionally, the modulation of inkjet-printed Na2WO4-Na2SO4 particles to switch between the etching and growth modes by manipulating the sulfur supply is demonstrated. This versatile approach enables the creation of 1D nanochannels on 2D TMDCs. The research presents exciting prospects for the top-down and bottom-up fabrication of 1D-2D mixed-dimensional TMDC nanostructures, expanding their use for electronic and optoelectronic applications.
A 2 GS/s 9-bit Time-Interleaved SAR ADC with Overlapping Conversion Steps
(2022) Tenhunen, Miikka; Spoof, Kalle; Unnikrishnan, Vishnu; Stadius, Kari; Kosunen, Marko; Ryynanen, Jussi
A4 Artikkeli konferenssijulkaisussa
This paper presents a wideband 8-way time-interleaved (TI) 9-bit successive approximation register (SAR) analog-to-digital converter (ADC) with overlapping conversion steps that improve the speed of operation. The ADC generates its clocks using a synchronous counter based circuit which reduces the SAR delay. A common-mode reference based split capacitor array digital-to-analog converter (DAC) is implemented that achieves high speed and low power consumption. Simulation results are presented for the ADC designed in a 22 nm CMOS process. The TI ADC achieves at least 7.7 ENOB at 2 GS/s and consumes a total of 19.8 mW from 0.8 V supplies, resulting in 47.6 fF/conv-step. The single ADC achieves 8.34 ENOB at 250 MS/s, consuming 1.43 mW in total and 17.7 fF/conv-step.
A 2-5.5 GHz Beamsteering Receiver IC With 4-Element Vivaldi Antenna Array
(2020-09) Zahra, Mahwish; Kempi, Ilia; Haarla, Jaakko; Antonov, Yury; Khonsari, Zahra; Miilunpalo, Toni; Ahmed, Nouman; Inkinen, Juha; Unnikrishnan, Vishnu; Lehtovuori, Anu; Viikari, Ville; Anttila, Lauri; Valkama, Mikko; Kosunen, Marko; Stadius, Kari; Ryynänen, Jussi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this article, we present a four-element Vivaldi antenna array and beamsteering receiver IC for the fifth-generation mobile network (5G) new radio (NR). The implemented receiver utilizes a delay-based local-oscillator phase shift technique for accurate beamsteering, and it exhibits 1° to 2.4° phase tuning capability for 2-5 GHz bandwidth accordingly. On-chip delay measurement is performed with pilot signal generation and delay estimation capable of 2-ps accuracy. The IC is fabricated on 28-nm CMOS technology, it occupies an area of 1.4x1.4 mm^2, including bonding pads, and consumes 22.8 mW at 2 GHz for single-receiver path operation. The receiver demonstrates wideband over-the-air reception with the prototype antennas.
A 20-60GHz Digitally Controlled Composite Oscillator for 5G
(2018) Antonov, Yury; Tormanen, Markus; Ryynänen, Jussi; Pärssinen, Aarno; Stadius, Kari
A4 Artikkeli konferenssijulkaisussa
This paper describes a frequency generator supporting over-an-octave tuning range for 5G receiver front-end. Generator is built by composition of smaller-range oscillators multiplexed to the common output that drives a downconversion mixer. Simulated in 28nm CMOS with full physical device models the composite oscillator exhibits a frequency tuning range from 21.5 to 60.7GHz (95.3%) dissipating less then 25.8mW from a 0.9V supply. As a result, it achieves −184dBc/Hz FOM TR.
A 22-nm All-Digital Time-Domain Neural Network Accelerator for Precision In-Sensor Processing
(2024) Mohey, Ahmed M.; Leslin, Jelin; Singh, Gaurav; Kosunen, Marko; Ryynänen, Jussi; Andraud, Martin
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Deep neural network (DNN) accelerators are increasingly integrated into sensing applications, such as wearables and sensor networks, to provide advanced in-sensor processing capabilities. Given wearables' strict size and power requirements, minimizing the area and energy consumption of DNN accelerators is a critical concern. In that regard, computing DNN models in the time domain is a promising architecture, taking advantage of both technology scaling friendliness and efficiency. Yet, time-domain accelerators are typically not fully digital, limiting the full benefits of time-domain computation. In this work, we propose an all-digital time-domain accelerator with a small size and low energy consumption to target precision in-sensor processing like human activity recognition (HAR). The proposed accelerator features a simple and efficient architecture without dependencies on analog nonidealities such as leakage and charge errors. An eight-neuron layer (core computation layer) is implemented in 22-nm FD-SOI technology. The layer occupies 70 × 70 μ m while supporting multibit inputs (8-bit) and weights (8-bit) with signed accumulation up to 18 bits. The power dissipation of the computation layer is 576 μ W at 0.72-V supply and 500-MHz clock frequency achieving an average area efficiency of 24.74 GOPS/mm 2 (up to 544.22 GOPS/mm 2 ), an average energy efficiency of 0.21 TOPS/W (up to 4.63 TOPS/W), and a normalized energy efficiency of 13.46 1b-TOPS/W (up to 296.30 1b-TOPS/W).
220-240-GHz High-Gain Phase Shifter Chain and Power Amplifier for Scalable Large Phased-Arrays
(2023) Najmussadat, Md; Ahamed, Raju; Varonen, Mikko; Parveg, Dristy; Kantanen, Mikko; Halonen, Kari A.I.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
This paper focuses on the design aspects of the key components for a scalable phased-array system over the 200 GHz frequency range. A high-gain phase shifter chain for 220 to 240 GHz frequency range and a high-gain power amplifier (PA) with a high output power are designed in a 0.13-μm SiGe BiCMOS technology. The phase shifter chain includes a low-noise amplifier (LNA), a vector modulator phase shifter (PS), and a gain-enhancing amplifier. The LNA is a five-stage cascode design. The vector modulator core is realized by two variable gain amplifiers based on the Gilbert cell architecture. A four-stage cascode design is used for the gain-enhancing amplifier. The phase shifter chain shows a measured gain of 18 dB at 230 GHz with a 360° phase tuning range and more than 10 dB of gain control. The chip achieves a minimum measured noise figure of 11.5 dB at 230 GHz and shows a wideband noise characteristic. The complete phase shifter chain chip consumes a dc power of 153 mW and occupies a 1.41 mm2 area.A high-power PA that is critical for a large phased-array system is designed. This paper presents a unique 4-way power combining technique utilizing a differential quadrature coupler. The realized balanced PA occupies an area of 0.67 mm2 and shows a measured peak gain of 21 dB at 244 GHz. The PA consumes 819 mW of dc power and delivers a maximum saturated output power (Psat) of 7.1 dBm at 244 GHz and more than 4.3 dBm of Psat from 230 to 255 GHz.
28-40 GHz variability and polarimetry of bright compact sources in the QUIJOTE cosmological fields
(2021-04) Perrott, Yvette C.; Lopez-Caniego, Marcos; Genova-Santos, Ricardo T.; Rubino-Martin, Jose Alberto; Ashdown, Mark; Herranz, Diego; Lahteenmaki, Anne; Lasenby, Anthony N.; Lopez-Caraballo, Carlos H.; Poidevin, Frederick; Tornikoski, Merja
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We observed 51 sources in the Q-U-I JOint TEnerife (QUIJOTE) cosmological fields that were brighter than 1 Jy at 30 GHz in the Planck Point Source Catalogue (version 1), with the Very Large Array at 28-40 GHz, in order to characterize their high-radio-frequency variability and polarization properties. We find a roughly lognormal distribution of polarization fractions with a median of 2 per cent, in agreement with previous studies, and a median rotation measure (RM) of ≈1110 rad m-2 with one outlier up to ≈64 000 rad m-2, which is among the highest RMs measured in quasar cores. We find hints of a correlation between the total intensity flux density and median polarization fraction. We find 59 per cent of sources are variable in total intensity, and 100 per cent in polarization at 3σ level, with no apparent correlation between total intensity variability and polarization variability. This indicates that it will be difficult to model these sources without simultaneous polarimetric monitoring observations and they will need to be masked for cosmological analysis.
2D electrons and 2D plasmons in AlGaN/GaN nanostructure under highly non-equilibrium conditions
(2020-03-25) Loginov, L. A.; Shalygin, V. A.; Moldavskaya, M. D.; Vinnichenko, M. Ya; Firsov, D. A.; Maremyanin, K. V.; Sakharov, A. V.; Zavarin, E. E.; Arteev, D. S.; Lundin, W. V.; Kauppinen, Christoffer; Suihkonen, Sami
A4 Artikkeli konferenssijulkaisussa
We report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency.
2D proximate quantum spin liquid state in atomic-thin α-RuCl 3
(2019-01-01) Du, Luojun; Huang, Yuan; Wang, Yimeng; Wang, Qinqin; Yang, Rong; Tang, Jian; Liao, Mengzhou; Shi, Dongxia; Shi, Youguo; Zhou, Xingjiang; Zhang, Qingming; Zhang, Guangyu
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Two-dimensional (2D) atomic crystals have made major inroads into condensed-matter physics and give rise to fascinating phenomena due to quantum confinement. Here we report the first Raman scattering study on phonon-magnetic scattering coupling, proximate quantum spin liquid ground state and collective fractionalized excitations in exfoliated α-RuCl 3 atomic layers. Our results uncover that 2D α-RuCl 3 could harbour the unusual magnetic continuum, serving as a hallmark of the 2D proximate quantum spin liquid state and frustrated magnetic interactions. More importantly, our work demonstrates that the unusual magnetic scattering, as compared with bulk, is more obvious in 2D α-RuCl 3, indicating that the frustrated magnetic interactions are enhanced strongly. Such unusual enhancement of frustrated magnetic interactions may be responsible for the gigantic phonon-magnetic scattering coupling of 2D α-RuCl 3 and play a key role in stabilizing the 2D proximate quantum spin liquid state. Our work establishes a firm basis for exploring and understanding the 2D proximate quantum spin liquid and fractionalized excitations based on the atomically thin α-RuCl 3.