分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: A violation of the null energy condition (NEC) during inflation in a single field inflation model will naturally enhance the amplitude of the parity violation effect (defined by $\Delta\chi$) of inflationary primordial gravitational waves (GWs), provided the inflaton is non-minimally coupled to a gravitational Chern-Simons term. After going through the NEC-violating phase, the universe enters subsequent slow-roll inflation with a higher scale, which results in an enhanced nearly scale-invariant power spectrum (i.e., $P_{\rm T}$) of inflationary primordial GWs in the high-frequency band, while $P_{\rm T}$ remains consistent with observations in the frequency band of the cosmic microwave background. Therefore, the violation of NEC during inflation will amplify the observability (i.e., $P_{\rm T}\cdot\Delta\chi$) of the parity violation effect. For a sufficiently large $P_{\rm T}$, a $\Delta\chi$ as small as a few percent can imprint a significant signal of parity violation in the GWs background, which might be detectable for pulsar timing arrays and space-based detectors in the future.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We investigate the possibility of the enhancement of parity-violation signal in bouncing cosmology. Specifically, we are interested in deciding which phase should generate the most significant parity-violation signals. We find that the dominant contribution comes from the bouncing phase, while the contraction phase has a smaller contribution. Therefore, bouncing cosmology can enhance the parity-violation signals during the bouncing phase. Moreover, since the bouncing phase has the highest energy scale in bouncing cosmology, we can also probe new physics at this scale by studying the parity-violation effect.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the ``beyond Horndeski'' theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and ``beyond Horndeski'' genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the ``beyond Horndeski'' operators.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: A violation of the null energy condition (NEC) during inflation in a single field inflation model will naturally enhance the amplitude of the parity violation effect (defined by $\Delta\chi$) of inflationary primordial gravitational waves (GWs), provided the inflaton is non-minimally coupled to a gravitational Chern-Simons term. After going through the NEC-violating phase, the universe enters subsequent slow-roll inflation with a higher scale, which results in an enhanced nearly scale-invariant power spectrum (i.e., $P_{\rm T}$) of inflationary primordial GWs in the high-frequency band, while $P_{\rm T}$ remains consistent with observations in the frequency band of the cosmic microwave background. Therefore, the violation of NEC during inflation will amplify the observability (i.e., $P_{\rm T}\cdot\Delta\chi$) of the parity violation effect. For a sufficiently large $P_{\rm T}$, a $\Delta\chi$ as small as a few percent can imprint a significant signal of parity violation in the GWs background, which might be detectable for pulsar timing arrays and space-based detectors in the future.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial null energy condition (NEC) violation would imprint a blue-tilted spectrum on gravitational wave background (GWB). However, its implications on the GWB might be far richer than expected. We present a scenario, in which after a slow-roll (NEC-preserving) inflation with Hubble parameter $H\simeq H_{inf1}$, the Universe goes through an NEC-violating period and then enters subsequent slow-roll inflation with a higher $H$ ($=H_{inf2}\gg H_{inf1}$). The resulting primordial gravitational wave spectrum is nearly flat at the cosmic microwave background band, as well as at the frequency $f\sim 1/{\rm yr}$ but with higher amplitude (compatible with the recent NANOGrav result). It is also highlighted that for the multi-stage inflation if the NEC violations happened intermittently, we might have a Great Wall-like spectrum of the stochastic GWB at the corresponding frequency band.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We investigate both the null energy condition (NEC) violating scenario and the $c_T$-diminishing scenario for generating enhanced power spectrum of primordial gravitational waves (GWs) during inflation, where $c_T$ is the propagating speed of primordial GWs. Both of these two scenarios can be realized stably with theories beyond Horndeski, hence can be uniformly implemented within the framework of the effective field theory. We calculate the power spectrum of primordial GWs by assuming that the inflationary Universe undergoes three phases, where the violation of NEC or the diminishment of $c_T$ occurs in the intermediate phase. A template of the spectrum is given for the NEC-violating scenario. We also discuss the underlying relation and discrepancy between these two scenarios with a disformal transformation.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the ``beyond Horndeski'' theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and ``beyond Horndeski'' genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the ``beyond Horndeski'' operators.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We investigate the possibility of the enhancement of parity-violation signal in bouncing cosmology. Specifically, we are interested in deciding which phase should generate the most significant parity-violation signals. We find that the dominant contribution comes from the bouncing phase, while the contraction phase has a smaller contribution. Therefore, bouncing cosmology can enhance the parity-violation signals during the bouncing phase. Moreover, since the bouncing phase has the highest energy scale in bouncing cosmology, we can also probe new physics at this scale by studying the parity-violation effect.
分类: 材料科学 >> 纳米科学和纳米技术 提交时间: 2016-12-23
摘要: A novel double gate AlGaN/GaN HEMT, in which an additional top-gate covers the adjacent regions of normal gate, was proposed and fabricated for the first time to compare the dynamic characteristics of AlGaN/GaN HEMTs with source field plate(SFP) and gate field plate(GFP). During the dynamic characterization, the device was configured in two operation modes, one is the source-field-plate mode (SFP-mode) with the top gate biased at 0V, another is the gate-field-plate mode (GFP-mode) with applying the gate pulse signal on the top gate at the same time. Compared to an AlGaN/GaN HEMT without field plates, both GFP and SFP much improve the dynamic performances. Compared to SFP, the GFP shows better dynamic performances with a ~34% reduction of switch-on delay time and ~6% reduction of dynamic on-resistance. By study of the dynamic characteristics with applying negative voltage on the top gate during off-state, the mechanism differences between GFP and SFP are discussed in detail.
分类: 材料科学 >> 纳米科学和纳米技术 提交时间: 2016-12-22
摘要: This paper presents a fabrication technology of enhancement-mode (E-mode) AlGaN/GaN metal insulator semiconductor high electron-mobility transistors (MIS-HEMTs) using 10 keV fluorine ion implantation. An 8 nm low pressure chemical vapor deposition (LPCVD) silicon nitride layer was deposited on the AlGaN as gate dielectric and energy-absorbing layer that slows down the high energy (10 keV) fluorine ions to reduce the implantation damage. The E-mode MIS-HEMTs exhibit a threshold voltage as high as 3.3 V with a maximum drain current over 200 mA/mm (250 mA/mm for depletion-mode (D-mode) MIS-HEMTs) and a high on/off current ratio of 109. Meanwhile, the E-mode MIS-HEMTs dynamic RON is only 1.53 times than the static RON after off-state VDS stress of 500 V.